Behavioral Ecology - recent issues

Mimicry in coral reef fish: how accurate is this deception in terms of color and luminance?

Cheney, K. L., Marshall, N. J. — 2009-05-06

Batesian and aggressive mimics are considered to be under selective pressure to resemble their models, whereas signal receivers are under selection to discriminate between mimics and models. However, the perceptual ability of signal receivers to discriminate between mimics and models is rarely studied. Here we examined 15 model–mimic coral reef fish pairs using nonsubjective methods to judge the accuracy of mimics in terms of color and luminance. We then investigated the potential ability of fish with various visual systems to discriminate between model and mimic colors using theoretical vision models. We found the majority of mimics closely resembled models in terms of color and luminance from a nonsubjective perspective. However, fish that have potentially trichromatic (3 distinct cone photoreceptors) visual systems with ultraviolet sensitivity had a much better capacity to discriminate between models and mimics compared with fish with midrange sensitivity or dichromatic (2 cone photoreceptors) fish. The spectral reflectance of color patches reflected by models and mimics became more similar with an increase in depth, indicating that signal receivers may be more likely to distinguish mimics from models in habitats located closer to the surface. There was no such change in luminance contrast with depth. The selection pressure on mimics to accurately resemble their model is therefore predicted to vary depending on the visual system of the signal receiver and the light environment.

The effect of social facilitation on vigilance in the eastern gray kangaroo, Macropus giganteus

2009-05-06

The relevance of vigilance activity to predator detection has been demonstrated in numerous studies. However, few studies have investigated the effect of one group member being vigilant on the probability of others being vigilant in group-forming prey species. Thus, we studied vigilance activity of eastern gray kangaroos Macropus giganteus that still experience occasional predation. We video recorded the behavior of all group members simultaneously and investigated the probability of a focal group member being vigilant (or nonvigilant) in relation to other individuals' vigilant and nonvigilant behaviors. Our results show that the decision of an individual to exhibit a vigilant posture depended on what it and other group members had been doing (scanning or foraging) at the preceding second and on group size. The probability of an individual being vigilant was positively affected by the proportion of companions that were vigilant at the previous second, confirming the existence in this species of a tendency for synchronization of individual vigilance. Group size affected individuals' vigilance in 3 ways. First, individuals were more likely to be vigilant if the proportion of their group mates that was vigilant was high, and this was strengthened with increasing group size. Second, the effect of the individual's own vigilance state (vigilant or not) at the previous second also increased with group size. Third, the probability of an individual being vigilant decreased with group size. These findings increase our understanding of the much-studied relationship between vigilance and group size.

Maternal effects on offspring social status in spotted hyenas

East, M. L., Honer, O. P., Wachter, B., Wilhelm, K., Burke, T., Hofer, H. — 2009-05-06

Social status is an important phenotypic trait that determines fitness-relevant parameters. In many mammalian societies, offspring acquire a social position at adulthood similar to that held by their mother ("rank inheritance") and thus obtain fitness benefits associated with this status. Mothers may influence the rank of their offspring at adulthood in at least three distinct ways. Firstly, the direct genetic inheritance of maternal traits that influence resource holding potential might predispose offspring to obtain a rank similar to that held by their mother. Secondly, the prenatal maternal environment might influence offspring rank if fetal exposure to maternal androgens is related to maternal status and affects offspring competitiveness. Thirdly, maternal behavioral support, a component of the postnatal maternal environment, may help offspring dominate individuals subordinate to their mother, thereby assisting offspring to acquire a rank similar to that of their mother. Here, we simultaneously test predictions derived from these three potential maternal effects on offspring rank acquisition at adulthood, using cases of offspring adoption in the spotted hyena Crocuta crocuta. We demonstrate that the rank of adopted offspring at adulthood was similar to that of their surrogate mother and that the competitive ability of offspring at adulthood was best explained by postnatal maternal behavioral support.

Modeling the role of competition and cooperation in the evolution of katydid acoustic synchrony

Nityananda, V., Balakrishnan, R. — 2009-05-06

The precise timing of individual signals in response to those of signaling neighbors is seen in many animal species. Synchrony is the most striking of the resultant timing patterns. One of the best examples of acoustic synchrony is in katydid choruses where males produce chirps with a high degree of temporal overlap. Cooperative hypotheses that speculate on the evolutionary origins of acoustic synchrony include the preservation of the species-specific call pattern, reduced predation risks, and increased call intensity. An alternative suggestion is that synchrony evolved as an epiphenomenon of competition between males in response to a female preference for chirps that lead other chirps. Previous models investigating the evolutionary origins of synchrony focused only on intrasexual competitive interactions. We investigated both competitive and cooperative hypotheses for the evolution of synchrony in the katydid Mecopoda "Chirper" using physiologically and ecologically realistic simulation models incorporating the natural variation in call features, ecology, female preferences, and spacing patterns, specifically aggregation. We found that although a female preference for leading chirps enables synchronous males to have some selective advantage, it is the female preference for the increased intensity of aggregations of synchronous males that enables synchrony to evolve as an evolutionarily stable strategy.

Modeling rule-based behavior: habitat selection and the growth-survival trade-off in larval cod

Kristiansen, T., Jorgensen, C., Lough, R. G., Vikebo, F., Fiksen, O. — 2009-05-06

Environmental variation can cause significant fluctuations in the survival of larval fish and plankton. Understanding these fluctuations is critical for developing more accurate fisheries models, which are needed for both scientific and socioeconomic research. Growth, survival, and dispersal of marine planktonic larvae rely strongly on their behavior. Larval fish change their vertical positioning due to strong vertical gradients in light, temperature, predation pressure, and prey availability. Here, we explore how various behavioral rules predict vertical distribution, growth, and survival of larval cod (Gadus morhua) in a numerical model. The rules determine the trade-offs between larval growth, feeding rate, and predation rate, including their dependence on gut fullness and body mass. We evaluated the survival through size classes for different rules and random behavior and compared model predictions with observed larval distribution patterns. The rules predicted the correct average depth position with larval size, but failed to predict the timing of the observed vertical distribution pattern. However, model simulations revealed significant increases in survival for larval and juvenile cod with active behavior compared with larvae with random behavior. Behavior was important across all sizes of fish, and this study illustrates the value or added information of incorporating behavior in biophysical models.

Superb fairy-wren males aggregate into hidden leks to solicit extragroup fertilizations before dawn

2009-05-06

Female superb fairy-wrens Malurus cyaneus initiate extragroup fertilizations by forays to the territory of preferred males, just before sunrise, 2–4 days before egg laying. Over a prolonged breeding season, males advertise their availability to foraying females by singing during the dawn chorus. Here, we show that 1) males commence dawn advertisement at the same time of the year regardless of their quality or status; 2) subordinate males advertise by singing in close proximity to the dominant, or by using the dominant's song perch, despite inevitable punishment; 3) low-quality dominants and their helpers sing from the boundary of their own territory, which increases their proximity to attractive neighboring dominants; 4) each spatial cluster of males use a common dialect of a song that is implicated in extragroup choice, despite the ability of individual males to sing several dialects; and 5) there is leakage of paternity to lower-quality helpers and neighbors as a result of their "satellite" behavior. Collectively, these data suggest that Wagner's hidden lek hypothesis (Wagner RA, 1998. Hidden leks: sexual selection and the clustering of avian territories. In: Parker PG, Burley NT, editors. Avian reproductive tactics: female and male perspectives. Ornithological Monographs No. 49. Allen Press. p. 123–145) can be extended to birds that defend year-round all-purpose territories and that mating induced by parasitic behavior of low-quality satellites can be one explanation for polyandry in birds.

Species and population differences in social recognition between fishes: a role for ecology?

Ward, A.J.W., Webster, M.M., Magurran, A.E., Currie, S., Krause, J. — 2009-05-06

The social organization of animals is reliant on recognition. However, the precision and specificity with which an individual animal recognizes another in a social context, and the sensory mechanisms that it employs, may vary both within and between species. Differences in the ecology and in the mating systems of species may drive the evolution of different recognition abilities, ranging from individual-specific recognition to more general forms of recognition. We examined social recognition in two important model species in behavioral ecology, the guppy (Poecilia reticulata) and the three-spined stickleback (Gasterosteus aculeatus). We found that guppies were capable of individual recognition of conspecifics, as well as being able to differentiate between groups of conspecific based on cues relating to resource use and habitat use. By contrast, sticklebacks showed no ability to recognize individuals in a social context after prior interactions. Nonetheless, two out of three populations of sticklebacks demonstrated general recognition abilities, based on cues relating to resource use. We discuss the potential relationship between social recognition mechanisms and the ecological and life-history parameters of species and populations.

Offspring development mode and the evolution of brood parasitism

Dearborn, D. C., MacDade, L. S., Robinson, S., Dowling Fink, A. D., Fink, M. L. — 2009-05-06

In the evolution of interspecific social parasites, the shift from facultative to obligate brood parasitism is thought to be driven by the cost of parental investment. Accordingly, avian brood parasites with precocial young are almost exclusively facultative parasites, whereas those with altricial young are almost exclusively obligate parasites. Surprisingly, then, North American cuckoos (Coccyzus spp.) have altricial young but are described as facultative brood parasites. Because little is known about parasitism by Coccyzus cuckoos, we explored the potential importance of heterogeneric brood parasitism to their reproductive strategy. In contrast to the existing set of anecdotal reports of cuckoos parasitizing songbirds, we found no evidence of cuckoo parasitism in 10 197 songbird nests, despite spatial and temporal overlap between cuckoos and potential hosts and despite varied food availability. Experiments revealed a lack of egg-rejection behavior in some of the most common potential hosts, suggesting that parasitic eggs would be accepted if laid and that we would detect cuckoo parasitism if it occurred regularly. We propose that reports of Coccyzus cuckoos parasitizing songbirds stem from errant attempts to parasitize other cuckoos. This resolves a theoretical paradox about interspecific parasitism and mode of offspring development, as we suggest that Coccyzus cuckoos have not evolved to parasitize other species.

The distribution of unequal predators across food patches is not necessarily (semi)truncated

Smallegange, I. M., van der Meer, J. — 2009-05-06

Game theoretical studies on contest behavior suggest that in foraging predators, interference through loss of foraging time is strongest between equal competitors. However, this phenomenon has not been incorporated into mechanistic models of interference. Instead, such models currently assume that individuals suffer most from dominant competitors, resulting in (semi)truncated, ideal free distributions (IFDs) of animals. Here, we develop a mechanistic interference model for 2 types of competitors: subordinates and dominants. The assumptions are that subordinates suffer interference through loss of foraging time from dominants but not vice versa. Time loss is greatest when 2 equal searchers interfere. A striking property of this 2-phenotype interference model is that dominants are most superior at intermediate values of the parameters prey density, handling time, and searching efficiency. This is because there the proportion of interfering subordinates relative to interfering dominants was highest. As the interference area for equal searchers increases, the difference in interference between dominants and subordinates diminishes. The IFD of the model is a mixed one with a larger share of dominants on the better patch but where the range of feeding rates exhibited by dominants and subordinates is the same for each patch. This contrasts with the (semi)truncated IFD predicted from other mechanistic interference models. We illustrate the generality of the model assumptions on interference and suggest that our modeling framework is applicable to many predator–prey systems.

Testosterone increases UV reflectance of sexually selected crown plumage in male blue tits

Roberts, M. L., Ras, E., Peters, A. — 2009-05-06

A central assumption of models of sexual selection, including the immunocompetence handicap hypothesis, is that the male sex hormone testosterone (T) is responsible for the expression of male sexual signaling; however, this has been questioned for colorful avian plumage. In this experiment, we manipulated T in juvenile male blue tits (Cyanistes caeruleus) during the molt and measured crown ultraviolet (UV) chroma (a sexually selected trait) immediately after molt and in the following spring during the breeding season, as well as recording preening behavior during spring. We found that males that were implanted with T during the molt had higher crown UV chroma than control males (C-males) in the subsequent breeding season but not immediately after molt. We also found that testosterone-treated males preened more than C-males during the spring but not during the preceding molt. These results suggest not only that T influences plumage coloration during the mate attraction period, possibly by increasing preening behavior, but also that exogenous T administered during the juvenile molt may have organizational effects in the subsequent breeding season. Because our study supports the assumption that T enhances the expression of male sexually selected plumage coloration, the results indicate that T could enforce costliness, and therefore honesty, of male plumage color as a signal of quality to females.

Interactive effect of starting distance and approach speed on escape behavior challenges theory

Cooper, W. E., Hawlena, D., Perez-Mellado, V. — 2009-05-06

Escape theory predicts flight initiation distance (FID, predator-to-prey distance when escape begins) based on fixed functions relating costs and benefits of fleeing to distance between a prey and an approaching predator. Theory accurately predicts effects of costs for fixed functions and changes in functions due to changes in predator behavior approach. Less obvious is how the effect of starting distance (predator-to-prey distance when approach begins) on FID can be explained when predator behavior does not change during approach. We simulated predators to study effects of starting distance on FID in Balearic lizards (Podarcis lilfordi). Starting distance and approach speed affected FID interactively. It increased as starting distance increased during faster, but not slower, approaches. Because risk functions are considered fixed for a given approach speed, we must explain why FID varies with starting distance, why only for rapid approach, and how risk is assessed. Because prey approached slowly assess risk as small until the predator is very close, approach from greater distance has little effect on risk curves. Because continued rapid approach suggests that the predator has detected the prey and is attacking, not merely approaching, risk varies with starting distance. Theoretical difficulty in explaining the effect of starting distance on FID disappears if risk curves vary among starting distances at faster approach speeds, but each curve is fixed. This might occur if prey use a temporal rule of thumb assigning increasing risk as duration of rapid approach increases.

Mating behavior in the seed beetle Acanthoscelides obtectus selected for early and late reproduction

Seslija, D., Lazarevic, J., Jankovic, B., Tucic, N. — 2009-05-06

Lines of Acanthoscelides obtectus that had been selected for either early- or late-life fitness components were compared with respect to early-life mating behavior of both females and males. Early-life mating frequencies and mating speed of both sexes, as well as female remating rates, were substantially higher in the late- than in the early-reproducing lines. These findings do not corroborate the hypothesis that selection for increased age at reproduction (and hence increased longevity) should result in reduced early-life mating efforts. We suggest that cryptic sexual selection within the late-age selection regime and relaxed sexual selection within the early-age selection regime may be the most important contributors to the pattern of mating behavior we see in the present study.

Condition-dependent resource value affects male-male competition in the blue-black grassquit

Santos, E. S.A., Maia, R., Macedo, R. H. — 2009-05-06

Male–male competition frequently can be resolved without overt aggression through the use of behavioral and phenotypic traits that signal body condition, dominance status, and fighting ability. In this study, we used male blue–black grassquits (Volatinia jacarina) to examine the relationship between male dominance status and behavioral, body condition, and ornamental traits in intrasexual agonistic encounters over a food resource. We found an association between body condition and winning, where winners were lighter than losers. This pattern was explained by low amounts of aggression exhibited by losers when they were heavy. In addition to being lighter, winners were, on average, up to 5 times more aggressive than the heavier losers. There were no associations between ornamental characteristics and dominance status, which suggests that male blue–black grassquits do not exhibit a badge signaling body condition and fighting ability, and we propose that such ornamental characteristics may be more functional in mate-choice contexts. However, the amount of male nuptial plumage of winners predicted the aggressiveness of their opponents, suggesting a social cost for sustaining this ornament.

What decision rules might pink-footed geese use to depart on migration? An individual-based model

2009-05-06

Decisions taken during migration can have a large effect on the fitness of birds. Migration must be accurately timed with food availability to allow efficient fueling but is also constrained by the optimal arrival date at the breeding site. The decision of when to leave a site can be driven by energetics (sufficient body stores to fuel flight), time-related cues (internal clock under photoperiodic control), or external cues (temperature, food resources). An individual based model (IBM) that allows a mechanistic description of a range of departure decision rules was applied to the spring migration of pink-footed geese (Anser brachyrhynchus) from wintering grounds in Denmark to breeding grounds on Svalbard via 2 Norwegian staging sites. By comparing predicted with observed departure dates, we tested 7 decision rules. The most accurate predictions were obtained from a decision rule based on a combination of cues including the amount of body stores, date, and plant phenology. Decision rules changed over the course of migration with the external cue decreasing in importance and the time-related cue increasing in importance for sites closer to breeding grounds. These results are in accordance with descriptions of goose migration, following the "green-wave": Geese track the onset of plant growth as it moves northward in spring, with an uncoupling toward the end of the migration if time is running out. We demonstrate the potential of IBMs to study the possible mechanisms underlying stopover ecology in migratory birds and to serve as tools to predict consequences of environmental change.

How important is sex for females of a haplodiploid species under local mate competition?

Steiner, S., Ruther, J. — 2009-05-06

Virgin females of Hymenoptera are still able to reproduce by laying unfertilized eggs that develop into male offspring (arrhenotokous haplodiploidy). Therefore, these constrained females may face a conflict between searching hosts and obtaining immediate fitness or investing time and energy in finding a mate enabling them to produce offspring of either sex. We studied this conflict in Nasonia vitripennis Walker (Pteromalidae), a pupal parasitoid of numerous fly species, by investigating olfactory preferences of constrained females for the male sex pheromone and host odor, respectively. According to Godfray's constrained model, females of haplodiploid species gain only little advantage from mating when population sex ratio is at equilibrium. However, in species with local mate competition like N. vitripennis, females are predicted to invest comparatively more time and energy in mating activities. In behavioral 2-choice experiments, virgin females were innately attracted to both male pheromone deposits and the odor of host puparia. When given the choice between these 2 stimuli, virgin females preferred the pheromone even when they were allowed to associate host odor with oviposition success prior to the bioassay. Immediately after mating, however, the olfactory preference of females switched to host odor. We conclude that constrained females of N. vitripennis are much more motivated to invest in mate finding than in host finding as predicted by the constrained model. The results are discussed with respect to the ecology of N. vitripennis.

Why some species of birds do not avoid inbreeding: insights from New Zealand robins and saddlebacks

Jamieson, I. G., Taylor, S. S., Tracy, L. N., Kokko, H., Armstrong, D. P. — 2009-05-06

When dispersal options are limited and encounters with relatives are likely, individuals need to recognize and avoid mating with kin to avoid the fitness costs of close inbreeding. New Zealand robins and saddlebacks are genetically monogamous and possess life-history traits that predict they should show zero tolerance of close inbreeding. However, of 11 population-years of pedigree data, there was evidence of inbreeding avoidance in only 1 year. We also found no indication that incestuous pairings were avoided or that individuals were choosing genetically dissimilar mates based on microsatellite DNA analysis. Furthermore, a review of the literature revealed that inbreeding avoidance via kin recognition is common in cooperatively breeding birds, but pair-breeding birds such as robins and saddlebacks mate randomly with respect to relatedness. A model that incorporates encounter rates with close kin for various degrees of mate-searching effort shows that inbreeding avoidance is beneficial at intermediate to high levels of encounter rates with close kin (as found in cooperative breeders), but that random mating is more beneficial at low or extremely high encounter rates. We conclude that random mating normally results in such low rates of close inbreeding that it exerts negligible selection pressure to evolve kin recognition. Consequently, many threatened species are unlikely to have a natural "built-in" mechanism for avoiding close inbreeding, and the assumption of random mating built into many population viability models may be appropriate.

Theory successfully predicts hiding time: new data for the lizard Sceloporus virgatus and a review

Cooper, W. E. — 2009-05-06

Economic hypotheses predict that prey in refuges after short-term encounters with predators decide how long to hide before emerging (hiding time) based on cost of emerging (predation risk) and cost of not emerging. Here, I report tests of predictions of these hypotheses for several cost of emerging and cost of remaining in refuge factors in the lizard Sceloporus virgatus by approaching lizards to elicit refuge entry, review tests of the predictions to evaluate the success of current models in predicting hiding time, and note parallels between models predicting hiding time and flight initiation distance (distance separating predator from prey when escape begins). Hiding times were longer under greater risk implied by faster, more direct, and repeated approaches and by greater proximity of the predator to the refuge during hiding. Lizards responded to costs of refuge use by emerging sooner when food was visible outside and when the refuge was substantially cooler than the lizard. In addition, lizards were more likely to enter refuges when approached rapidly and when cloudiness precluded effective thermoregulation by basking. Review of findings for lizards and other taxa revealed strong support for the predictions of cost-benefit models that hiding time increases with cost of emerging and decreases with cost of staying in refuge. Examination of parallel predictions by escape theory and refuge use theory emphasizes their fundamental similarity and led to identification of an untested prediction of refuge use theory.

Behavioral dominance between female color morphs of a Lake Victoria cichlid fish

2009-05-06

Species that exhibit genetic color polymorphism are suitable for studying the evolutionary forces that maintain heritable phenotypic variation in nature. Male color morphs often differ in behavioral dominance, affecting the evolution of color polymorphisms. However, behavioral dominance among female color morphs has received far less attention. We studied a polymorphic population of the cichlid fish Neochromis omnicaeruleus from Lake Victoria, in which 3 distinct female color morphs coexist, black-and-white blotched (WB), orange blotched (OB), and plain (P) color morphs. First, we investigated dominance relationships among female morphs using triadic and dyadic encounters in the laboratory. In triadic encounters, both WB and OB females dominated plain, whereas WB females dominated OB females. Dominance of WB over OB was confirmed using dyadic encounters. In a second experiment, blotched (WB or OB) and plain full-sib sisters were bred by crossing a blotched and a plain parent. In dyadic encounters, WB female morphs dominated their plain sisters, suggesting that dominance of WB females is a pleiotropic effect of color or that genes coding for color and those influencing behavioral dominance are genetically linked, explaining the association between color and behavioral dominance despite gene flow. We conclude that behavioral dominance asymmetries exist among female color morphs of the fish N. omnicaeruleus, and discuss possible mechanisms that may account for the tight association between color and behavioral dominance.

The cost of infidelity to female reed buntings

Suter, S. M., Bielanska, J., Rothlin-Spillmann, S., Strambini, L., Meyer, D. R. — 2009-05-06

Females of many socially monogamous bird species accept or even actively seek copulations outside the social pair bond. It has been shown that females profit from extrapair fertilization by increased offspring quality, but extrapair mating may also induce costs to females. We measured parental food provisioning and paternity in the reed bunting, Emberiza schoeniclus, a species with biparental brood care and high levels of extrapair paternity (EPP). We found a negative relationship between the proportion of EPP in broods and paternal care across the local population. Individual males adjusted food provisioning to the amount of EPP between sequential broods. Females did compensate for low male food provisioning. The cost of extrapair fertilizations to females is increased nestling mortality due to the social partner's reduction of brood care. If the mixed mating system of the reed bunting is at an evolutionary equilibrium, we expect that genetic fitness for males as well as for females is maximized. Any costs incurred by extrapair mating should therefore not exceed the benefits for both sexes.

Hotshots, hot spots, and female preference: exploring lek formation models with a bower-building cichlid fish

Young, K. A., Genner, M. J., Joyce, D. A., Haesler, M. P. — 2009-05-06

In many animals, males congregate in leks that females visit for the sole purpose of mating. We observed male and female behavior on 3 different-sized leks of the bower-building cichlid fish Nyassachromis cf. microcephalus to test predictions of 3 prominent lek models: the "hotshot," "hot spot," and "female preference" models. In this system, we were able to refine these predictions by distinguishing between indirect mate choice, by which females restrict their set of potential mates in the absence of individual male assessment, and direct mate choice, by which females assess males and their territories through dyadic behavioral interactions. On no lek were males holding central territories favored by indirect or direct mate choice, contrary to the prediction of the hotshot model that leks form because inferior males establish territories surrounding hotshot males preferred by females. Average female encounter rate of males increased with lek size, a pattern typically interpreted as evidence that leks form through female preference for lekking males, rather than because males congregate in hot spots of high female density. Female propensity to engage in premating behavior once courted did not increase with lek size, suggesting female preference for males on larger leks operated through indirect choice rather than direct choice based on individual assessment. The frequency of male–male competitive interactions increased with lek size, whereas their foraging rate decreased, implying a cost to males maintaining territories on larger leks. Together these data most strongly support the female preference model, where females may benefit through indirect mate choice for males able to meet the competitive cost of occupying larger leks.

Long-term growth and movement in relation to food supply and social status in a stream fish

Hansen, E. A., Closs, G. P. — 2009-05-06

Long-term patterns of residency, growth, and movement among drift-feeding fish in relation to their social status and macroinvertebrate drift were determined. Each month, over 21 months, we quantified macroinvertebrate drift, fish abundance, and biomass, recorded individual fish lengths, weights, and social rank, and monitored individual diel activity of fish residing in different pools. Despite marked variation in food supply between pools, average growth rates within each pool were not significantly different. However, size and growth of the largest (dominant) fish (i.e., ones that had priority access to preferred feeding positions and times) was positively related to food supply. In contrast, the distribution and growth of smaller subordinate fish were only weakly related to the distribution of resources. Over the 21 months of study, growth rates of the first-ranked fish were high and they rarely relocated to a new pool. Second- and third-ranked fish exhibited comparatively low growth rates and relocated to new pools more frequently than fish of any other ranks (higher or lower). Movement of these fish from their original pool of residency did not always result in increased growth rates. Overall, our study demonstrates that short-term behavioral responses to spatial and temporal resource availability and intraspecific competition (i.e., social status) interact to determine long-term patterns of distribution, movement, and growth.

A trait-based approach to understand the evolution of complex coalitions in male mammals

Olson, L. E., Blumstein, D. T. — 2009-05-06

Coalitions occur when multiple individuals cooperate against a common opponent or for a common goal. Coalition formation is a complex behavior, typically described in highly social and cognitively complex species. Surprisingly, we know little about the social and environmental factors that may select for the evolution of coalitions. We studied the evolution of coalitionary behavior by first redefining it in a continuous way that acknowledges variation in the degree to which animals collaboratively work toward a common goal. We then examined the evolutionary association of coalition complexity with 3 social factors (estrous duration, group size, and presence of a dominance hierarchy) and 3 environmental factors (habitat type, diurnality, and diet type). We found that estrous duration, group size, and dominance hierarchy were significantly correlated with coalition complexity and thus conclude that social factors are relatively more important in the evolution of complex coalitions than are environmental factors. From these results, we infer that complex coalitions may be the product of social factors that reduce female monopolizability and encourage the aggregation of multiple males.

How site fidelity leads to individual differences in the foraging activity of harvester ants

Beverly, B. D., McLendon, H., Nacu, S., Holmes, S., Gordon, D. M. — 2009-05-06

We examined how differences in activity among individual foragers of the red harvester ant, Pogonomyrmex barbatus, could arise from site fidelity. Using observations of individually marked foragers, we found that each day most foragers made a few foraging trips, whereas only a few foragers made many trips. To determine whether only particular individuals are capable of high foraging activity, we removed the foragers that made the most foraging trips on 1 day and examined the frequency distribution of foraging the subsequent day. The most active foragers were replaced by other individuals. We then examined site fidelity of foragers. Though foraging trails extend up to 20 m from the nest, observations of marked individuals showed that on successive trips, a forager returns to sites within about 0.5 m. Foraging trip duration depended on search time and not on the distance from the nest of the final destination. Thus, the more food available, the shorter the search time and the shorter the trip. Because foragers return to the same site over and over within a day, a forager making many short trips to a high-quality patch can make more foraging trips per day. Thus, variation in patch quality, rather than individual variation in foraging ability, could produce the observed distribution of trip number. These results show that regulation of foraging in harvester ants does not require any individuals to show others a particular location with abundant food. Instead, a decentralized system of interactions tunes the numbers foraging to current food availability.

Sexual coercion in Panorpa scorpionflies?--The function of the notal organ reconsidered

Kock, D., Engels, S., Fritsche, C., Sauer, K. P. — 2009-05-06

Conflict between the sexes over the occurrence and timing of copulation is a common feature of sexual reproduction. In Panorpa scorpionflies (Mecoptera, Panorpidae), sexual conflict occurs over mating duration, and it has been suggested that the notal organ—a clamp-like structure on the males’ abdomen—is an adaptation to coerce mating duration, enabling males to prolong mating against female interests. However, as recent studies suggest that female scorpionflies have considerable control over mating decisions, we question this hypothesis and reexamined the function of the notal organ with respect to prolonged mating. We manipulated the notal organs of male Panorpa vulgaris and compared copulation durations achieved by males with functional and nonfunctional notal organs. There were no effects on the overall copulation duration or the period of time a copulation continues after the last nuptial gift has been delivered. We therefore reject the hypothesis that the notal organ is a male adaptation to extend copulation duration against female interests.

Sex allocation and mate choice of selfed and outcrossed Schistocephalus solidus (Cestoda)

Schjorring, S. — 2009-05-06

Animals that inbreed regularly under natural conditions may provide valuable information about the evolutionary response of mate choice to an increase in a population's rate of inbreeding. I studied how an individual's inbreeding status affects its criteria of mate choice, as well as its own attractiveness, in a parasite, the hermaphroditic cestode Schistocephalus solidus, which inbreeds under natural conditions. Specifically, I tested whether a cestode's inbreeding status and allocation to reproductive tissue affect its attractiveness to selfed and outcrossed individuals. In a simultaneous choice situation, outcrossed cestodes strongly preferred an outcrossed mating partner over a selfed one, whereas selfed cestodes showed no preference with respect to the partner's inbreeding status. Both selfed and outcrossed cestodes were attracted to partners with a large combined amount of male and female reproductive tissue and with the potential to produce large eggs. I discuss how assortative mating with respect to inbreeding status may have consequences for the maintenance of a genetic load in the population as well as for the maintenance of selfing.

Conflict over parental care in house sparrows: do females use a negotiation rule?

Lendvai, A. Z., Barta, Z., Chastel, O. — 2009-05-06

Ho do parents resolve their conflict over parental care? The classical "sealed-bid" model of biparental care suggested that parents use a fixed best effort given the partner's effort. Alternatively, parents may "negotiate" their actual effort until the efforts of both partners settle down to limiting values, but in this case, the resulting efforts will not be the best responses to one another. Consequently, under the best response scenario, the response of 1 parent to the removal of its mate can be predicted from the response to a reduction in its partner's effort, whereas the "negotiation" model predicts that such an extrapolation will underestimate the effort of a parent caring alone. We tested this prediction in free-living house sparrows (Passer domesticus). We experimentally manipulated the males’ parental care as follows: males’ care in group 1) was reduced by using a capture–handling–release stress protocol, 2) stopped by removing the male, and 3) left as control. In response to these manipulations, control females kept their feeding rate constant, whereas male-stressed-released females showed a moderate increase of feeding rate. When this response was extrapolated to zero male effort, their effort was still significantly lower than the observed effort of male-removed females. These results suggest that females may use the negotiation rule to determine their actual parental effort.

Examining potential benefits of group living in a sawfly larva, Perga affinis

Fletcher, L. E. — 2009-05-06

Although defense may serve as a primary selective force behind the evolution and maintenance of social behavior, numerous other benefits play a role as well. Larvae of the Australian sawfly, Perga affinis, live in colonies and retain their gregarious lifestyle through pupation. To evaluate selective pressures maintaining social behavior in P. affinis, I investigated potential benefits of group living. These included predation protection, immune function, thermoregulation, foraging facilitation, and pupation success where I compared treatments between individuals versus groups and between groups of different sizes. I found no evidence of predation on larvae; however, the mortality risk was significantly higher for single versus grouped larvae, suggesting that other grouping benefits are important. Immune function was not a benefit as hemolymph from single versus grouped larvae did not differ in its ability to limit bacterial growth. Investigating thermoregulation revealed that grouped larvae attained significantly higher temperatures than single individuals and that large groups reached higher temperatures than small groups. Larvae grew significantly faster at higher temperatures, indicating that the increased heat absorption and retention capacity of large groups may speed up development. Group size did not affect foraging facilitation (measured via weight gain) except during the last time period, where individuals in large groups gained significantly more weight. Weight gain in the last instar may be critical for pupation as high larval weight significantly increased the odds of pupation success. Thus, the group size effect on weight gain during the last instar provides an adaptive explanation for the observed ontogenetic increase in colony size and coalescence.

Fleeing and hiding under simultaneous risks and costs

Cooper, W. E. — 2009-05-06

Prey evaluate risk implied by a predator during its approach, intrinsic prey features, and environmental factors to weigh risk against costs. Most studies of escape and hiding time in refuge focus on a single factor, but prey that ignore other factors may soon be dead. Two hypotheses make different predictions about additivity versus interaction of risk–cost factors. The risk-sensitivity hypothesis predicts that multiple factors have additive effects, but optimality theory suggests that interaction is more common. In the only previous study, flight initiation distance (FID = predator–prey distance when escape begins) was affected interactively by 2 risk factors (predator approach speed and directness) but additively by a risk and an escape cost (approach speed and food presence). I predicted that effects of approach speed and other risk factors interact because differences in risk are likely to increase more rapidly with increase in approach speed than other factors. FID by lizards that I approached increased interactively with approach speed and directness in the Sceloporus virgatus and speed and predator persistence in Sceloporus jarrovii. During indirect approaches, fewer S. virgatus fled, and they ran shorter distances. A review of effects of multiple risks and costs on FID and hiding time revealed far more frequent interactive than additive effects between 2 risk factors, as predicted by optimality theory. Additive and interactive effects were equally common between a risk and a cost factor. Findings for some factors differ between FID and hiding time and in other ways. I propose hypotheses to explain some differences.

Reproductive skew and the evolution of conflict resolution: a synthesis of transactional and tug-of-war models

Buston, P. M., Zink, A. G. — 2009-05-06

The formation of animal societies is a major transition in evolution. It is challenging to understand why societies are stable, given the reproductive conflicts inherent within them. Reproductive skew theory provides a compelling explanation for how and why reproductive conflicts are resolved. Indeed, some have suggested that skew theory represents a general theory of social evolution. Lamentably, skew theory is composed of many independent models, with the generality of each model being restricted by its assumptions. Here, we tackle this problem, using Hamilton's rule to predict the conditions under which assumptions of major classes of skew models (transactional and tug-of-war) apply. First, building on transactional models, we define the amount of reproduction that individuals can negotiate based on the threat of group dissolution (the "outside option") and determine conditions under which groups will be stable (free of group dissolution). Second, building on tug-of-war models, we define the amount of reproduction that individuals can negotiate based on the threat of costly competition (the "inside option") and determine conditions under which groups will be tranquil (free of costly competition). Finally, synthesizing transactional and tug-of-war approaches, we determine the conditions under which individuals will negotiate based on outside rather than inside options. Simply, individuals will negotiate using their outside option when it is greater than their inside option and vice versa. We conduct a post hoc test of all predictions in one simple animal society - the clown anemonefish, Amphiprion percula. The product is a more general and demonstrably testable model of reproductive skew, which should help to refocus the debate surrounding the utility of reproductive skew theory as a general theory of social evolution.

Boldness and behavioral syndromes in the bluegill sunfish, Lepomis macrochirus

Wilson, A. D.M., Godin, J.-G. J. — 2009-03-17

In recent years, evidence for interindividual variation in "personality" within animal populations has been accumulating. Personality is defined as consistency in an individual's behavioral responses over time and/or across situations. One personality trait that has potentially far-reaching implications for behavioral ecology, and may provide insight into the mechanisms by which consistent behavioral correlations arise, is that of boldness. Boldness is defined as the tendency of an individual to take risks and be exploratory in novel contexts. Using the framework of behavioral syndromes, we tested for individual differences in boldness in the laboratory among field-caught juvenile bluegill sunfish (Lepomis macrochirus) within and across the contexts of exploratory behavior, activity, and risk taking (e.g., antipredator) behavior. After such testing, individuals were tagged and returned to their lake of origin as part of a mark–recapture study testing for the repeatability of individual differences in boldness. Here, we report strong and consistent individual differences in boldness within and across all 3 behavioral contexts. Additionally, we observed that at least some boldness behaviors were repeatable after a 1–3 month recapture period. This study provides novel evidence for a boldness syndrome in sunfish, as well as insight into how behavioral types (e.g., shy/bold) may evolve and be maintained in natural populations.

Nine-spined sticklebacks deploy a hill-climbing social learning strategy

Kendal, J. R., Rendell, L., Pike, T. W., Laland, K. N. — 2009-03-17

Theoretical models on the adaptive advantages of social learning lead to the conclusion that copying cannot be indiscriminate and that individuals should adopt evolved behavioral strategies that dictate the circumstances under which they copy others and from whom they learn. Strategies that exhibit hill-climbing properties, that would allow a population of individuals to converge on the fitness-maximizing behavior over repeated learning events, are of particular significance due to their potentially critical role in cumulative cultural evolution. Here, we provide experimental evidence that nine-spined sticklebacks (Pungitius pungitius) use public information adaptively and in accordance with a hill-climbing social learning strategy. Sticklebacks switch patch preferences to exploit a more profitable food patch if the returns to demonstrator fish are greater than their own but are less likely to copy when low-profitability patches are demonstrated. These findings reinforce the argument that public-information use in nine-spined sticklebacks is an adaptive specialization. More generally, the observation of this sophisticated form of learning in a species of fish supports the view that the presence of enhanced social learning may be predicted better by specific sources of selection than by relatedness to humans.

Family conflict and the evolution of sociality in reptiles

While, G. M., Uller, T., Wapstra, E. — 2009-03-17

Mating systems and parental care are predicted to coevolve because the former dictates the cost–benefit ratio of the latter by affecting genetic relatedness between adults and offspring. Reptiles show only rudimentary forms of sociality and parental care and, hence, could provide important insights into the early stages of the evolution and maintenance of social systems. The skink genus Egernia exhibits the most complex form of sociality and parental care in lizards, with the formation of stable social groups typically consisting of a monogamous pair and their offspring. Here we show that, within a social group, offspring sired by males other than the social father are restricted to the area of the parental home range that is occupied exclusively by the mother. Thus, males rarely tolerate offspring within their home range that they are not genetically related to. This may increase the cost of multiple mating for females and offspring via increased risk of infanticide, reduced parental tolerance, and increased mother–offspring competition. We outline a verbal model for how this could generate a feedback loop in which selection favors reduced multiple mating by females and increased paternal care, thereby setting the stage for the evolution of complex sociality and genetic monogamy.

Living with the dead: when the body count rises, prey stick around

Fievet, V., Le Guigo, P., Casquet, J., Poinsot, D., Outreman, Y. — 2009-03-17

Most terrestrial prey species are assumed to assess predation risk by detecting predators directly rather than using cues of previous attacks on conspecifics. However, such cues might represent valuable information, and prey can be expected to respond to the presence of congeners killed by enemies. Such cues are available in aphid colonies attacked by parasitic wasps because they do not remove parasitized hosts from the colony. Colonies are thus often a mixture of healthy, parasitized, and killed aphids, which corpses ("mummies") stay attached to the plant and can be encountered by live aphids. Aphids exhibit a dispersal polyphenism. Recent studies show that they produce more winged offspring when directly exposed to natural enemies or to alarm pheromone emitted by conspecifics. We hypothesized that aphids perceive the corpses of congeners killed by parasitoids and respond by increasing the production of winged morphs, but we surprisingly found the opposite. We determined the adaptive value of this response by analyzing the foraging behavior of parasitoids in aphid colonies with killed aphids ("mummies"). Parasitoid females responded to the presence of mummies by reducing both their time allocation and parasitism activity in the patch. The strategy of aphids to reduce emigration (i.e., they produce more wingless morphs when mummies are present) is adaptive because the presence of killed congeners reduces parasitoid pressure on colonies. This demonstrates that the remains of individuals killed by natural enemies can still have an ecological relevance in prey populations and that enemy-induced phenotypic plasticity depends on the type of predation cues.

Armament under direct sexual selection does not exhibit positive allometry in an earwig

van Lieshout, E., Elgar, M. A. — 2009-03-17

The allometric scaling relationships of armaments and ornaments have been subject to extensive debate. A large body of empirical evidence suggests that sexually selected traits typically exhibit positive static allometry, where the large individuals express proportionally larger traits. Recent theory suggests that this need not be the case. We confirm this prediction using the earwig Euborellia brunneri as a model species—unusually, the male armament in this species does not exhibit positive allometry. We experimentally assessed the strength of direct and indirect selection on armament length and morphology and on body size and weight. In a 3-stage experiment, we first permitted females to choose between 2 males and assigned 1 male as the preferred and the other the nonpreferred male. We then allowed the same pair of males to establish a dominance hierarchy in fighting trials. Last, to evaluate the implications of female choice, we conducted mating trials where half the females were mated to their preferred male and half with their nonpreferred male. We found that male armament length and body weight were under direct sexual selection through intrasexual competition. In contrast, female mate preferences did not relate to any measured male trait. Finally, mating behavior was not related to male preference status or armament length. Thus, armament size is sexually selected through intrasexual competition although it does not exhibit positive allometry. Our finding suggests that researchers should be cautious when inferring the absence of sexual selection in traits that do not exhibit positive allometry.

Long-term effects of early parasite exposure on song duration and singing strategy in great tits

Bischoff, L. L., Tschirren, B., Richner, H. — 2009-03-17

Song is a sexually selected trait in many bird species and has been suggested to function as a signal of a male's health and parasite resistance. Here we present an experimental field study on the long-term effects of parasite exposure early in life on adult bird song. We exposed nestling great tits (Parus major) to ectoparasitic hen fleas (Ceratophyllus gallinae) and on their recruitment into the local breeding population assessed the response to a playback of a challenging male in their breeding territory. We show, to our knowledge for the first time in a wild bird population, that parasite exposure early in life affects bird song: song duration of males that were exposed to parasites early in life was reduced by 32% compared with males that grew up in a flea-free environment. Early parasite exposure also significantly reduced the degree of song overlap with the playback, which has been shown to correlate with social status. There was no effect of early parasite exposure on the number of different song types sung or on the latency until the males started the vocal response to the playback. These results suggest that mates or rivals can use song duration and song overlap as proxies for an individual's exposure to parasites early in life. It thereby highlights both the importance of parasites in maintaining honesty of sexually selected traits and the costs of parasitism in terms of reduced attractiveness and competitiveness.

Intraguild predation, thermoregulation, and microhabitat selection by snakes

Webb, J. K., Pringle, R. M., Shine, R. — 2009-03-17

Intraguild (IG) predation, the killing and eating of potential competitors, can be a powerful force within faunal assemblages. If both the IG predator and its prey prefer similar microhabitats in spatially structured environments, avoidance of the predator may relegate IG prey to suboptimal habitats. In southeastern Australia, the broad-headed snake (Hoplocephalus bungaroides) is an endangered species sympatric with the small-eyed snake (Cryptophis nigrescens), an abundant and geographically widespread species known to eat other snakes. Both of these nocturnal ectotherms shelter diurnally beneath thermally distinctive "hot rocks," which are in limited supply. When selecting shelter sites, broad-headed snakes thus face a trade-off between predation risk and habitat quality. In laboratory experiments, we allowed broad-headed snakes to choose between retreat sites differing in thermal regimes, in scent cues from predators, and in the actual presence of the predator. Broad-headed snakes displayed an aversion to sites with live predators and predator scent, yet nonetheless frequently selected those sites to obtain thermal benefits. In trials with live predators, adult broad-headed snakes shared hot rocks with small-eyed snakes, but most juveniles did not; data from a 16-year field study likewise suggest that broad-headed snakes only cohabit with small-eyed snakes if the two snakes are similar in body size. Our results suggest that thermoregulatory considerations are sufficient to prompt juvenile (but not adult) broad-headed snakes to risk IG predation, emphasizing the importance of microhabitat quality and body size in mediating IG predator–prey interactions.

Prudent male mate choice under perceived sperm competition risk in the eastern mosquito fish

Wong, B. B. M., McCarthy, M. — 2009-03-17

In contrast to what is known about adaptive mate choice in females, we know far less about how fluctuating reproductive costs might affect male mate choice. In many species, sperm competition can have a direct bearing on male fertilization success, and choosy males should be expected to respond adaptively to the perceived cost of sperm competition and to adjust their mate preferences accordingly. Here, we conducted a series of experiments investigating male mate choice under sperm competition risk in the eastern mosquito fish, Gambusia holbrooki. We tested male association preferences before and after manipulating their perceptions of sperm competition risk associated with initially preferred and nonpreferred females. We found that individuals were consistent in their preferences if they did not have the opportunity to witness other males associating with the initially preferred female. By contrast, males spent significantly less time with initially preferred females if, in the interim, she had been seen in the vicinity of another male. A similar opportunity to observe the initially nonpreferred female with another male had no effect on subsequent male mate choice. Our results suggest that choosy males may be capable of adjusting their preferences in response to shifts in their perception of sperm competition risk.

Social enviroment influences aphid production of alarm pheromone

Verheggen, F. J., Haubruge, E., De Moraes, C. M., Mescher, M. C. — 2009-03-17

In most aphid species, the volatile sesquiterpene (E)-β-farnesene (Eβf) is released as an alarm pheromone in response to predation and is also emitted continuously at low levels. Some aphid predators use Eβf as a foraging cue, suggesting that the benefits to aphids of signaling via Eβf must be weighed against the cost of increasing apparency to natural enemies. To determine whether aphids vary Eβf production in response to features of their social environment, we compared the production of Eβf by Acyrthosiphon pisum (Harris) individuals reared in isolation with that of individuals reared among conspecifics or individuals of a different aphid species, Myzus persicae. Production of Eβf by A. pisum reared in isolation was significantly lower than that of aphids reared among conspecifics or among M. persicae individuals. When we reared A. pisum individuals in isolation but exposed them to odors from an aphid colony, Eβf production was similar to that of aphids reared among conspecifics, suggesting that aphids use a volatile cue to assess their social environment and regulate their production of alarm pheromone. It is likely that this cue is Eβf itself, the only volatile compound previously found in headspace collections of A. pisum colonies. Finally, we examined the attraction of a predatory hoverfly, which uses Eβf as a foraging cue, to groups of aphids reared in isolation or among conspecifics and found that groups comprising individuals reared in isolation were significantly less attractive to the predator, suggesting that the observed variation in Eβf production may be ecologically relevant.

When are vomiting males attractive? Sexual selection on condition-dependent nuptial feeding in Drosophila subobscura

Immonen, E., Hoikkala, A., Kazem, A. J.N., Ritchie, M. G. — 2009-03-17

Nuptial gifts are any nutritious items or inedible tokens transferred from the male to the female as a part of courtship or copulation. Although nuptial gift donation has been studied in a variety of taxa, this behavior has been largely overlooked in Drosophila. We studied nuptial feeding in Drosophila subobscura, where the gift is a regurgitated drop of liquid, in order to examine the importance of this behavior for male mating success and female fecundity. We varied male and female condition by dietary restriction to assess any condition dependence of male nuptial feeding ability and female feeding behavior and mate discrimination. Our results show that there was directional selection for males in good condition that produced a higher number of regurgitated gifts. Interestingly, the strength of selection was also dependent on female condition. Females in poor condition showed strongest preference for males in good condition. Such females could increase their fecundity to a level similar to that of females in good condition by feeding on regurgitated gifts from males in good condition. However, male exploitation of female gustatory response is also a plausible explanation for the courtship feeding response, as we did not find an effect of nuptial feeding on the fecundity of females in good condition. These findings suggest that, in this monandrous species, selection has favored males who invest in nuptial gifts, possibly as an example of paternal investment as well as mating effort, but that the strength of selection on nuptial feeding is strongly subject to environmental variation.

The elusive paradox: owner-intruder roles, strategies, and outcomes in parasitoid contests

Bentley, T., Hull, T. T., Hardy, I. C.W., Goubault, M. — 2009-03-17

Models of dyadic contests for indivisible resources have predicted that the owner–intruder role distinction can suffice as a cue for evolutionarily stable resolution. This outcome may be "common sense" (prior owners retain the resource) or counterintuitively "paradoxical" (the intruder takes over), but the most recent models predict paradoxes to be an infrequent result, and there are also very few candidate examples provided by empirical study. Possible paradoxical outcomes were recently reported from the parasitoid wasp Goniozus legneri in which adult females compete directly for hosts. Here we provide further investigation, taking into account influences of contest ability (body size) and the value of the host to each contestant (correlated with the developmental stage of the owner's brood). We additionally evaluate contest strategies in terms of respect for ownership as evidenced by attack behavior during contests. Goniozus legneri shows weak, and thus only partial, respect for role asymmetries: such mixed strategies are predicted by recent models that assume population-level feedback on resource value parameters. Contest outcomes are influenced by asymmetries in resource value and body size and are generally common sense. Instances of paradoxical contests remain predictably elusive.

Extrapair paternity in an insular population of house sparrows after the experimental introduction of individuals from the mainland

Ockendon, N., Griffith, S. C., Burke, T. — 2009-03-17

Several studies have found that island populations of passerines exhibit lower levels of extrapair paternity (EPP) than mainland populations. An explanation proposing that lower levels of genetic diversity in isolated populations reduce the indirect genetic benefits of EPP to females has been supported by observational study but not tested experimentally. Here, we present the results of a manipulative study on an island population of house sparrows (Passer domesticus), which previously exhibited a significantly lower frequency of EPP than mainland populations. Fifty adults from a mainland population with significantly more genetic diversity than the island population (across 17 microsatellite loci) were introduced into the island population, and the incidence of EPP was subsequently monitored over 3 breeding seasons. In the year of the introduction, the incidence of EPP rose to approximately the level seen in mainland populations of house sparrows but dropped to an intermediate frequency in the following 2 years. Unexpectedly, parentage assignment showed that in the year of the introduction, all females producing extrapair offspring were native island birds, as were all the extrapair and cuckolded males. These results suggest that EPP in this experimental population was not driven by females trying to maximize the genetic diversity of their offspring.

Differential effects of structural complexity on predator foraging behavior

Michel, M. J., Adams, M. M. — 2009-03-17

The choice of predator foraging mode has important consequences for ecological communities. Foraging mode designations are often made on the basis of predator activity, yet activity can be affected by various environmental stimuli independent of changes in foraging mode. Structural complexity can reduce predator activity by either interfering with predator vision and mobility or as part of a foraging mode shift. We examined the effects of simulated aquatic vegetation on multiple behaviors of 2 aquatic insect predators to distinguish between these 2 possible outcomes. Larvae of the diving water beetle (Dytiscus spp.) shifted from an active predator in treatments without structure to a sit-and-pursue (SAP) predator in treatments containing structure, as indicated by a decrease in activity and prey encounter rates and an increase in probability of capture. This trade-off between encounter rates and probability of capture resulted in an equal number of prey captures among the treatments. Dragonfly nymphs (Anax junius) remained SAP predators in both treatments, although interference from the simulated vegetation significantly reduced activity. Structure also slightly decreased the number of aeshnid prey captures. Physiological attributes of the predators, such as mode of respiration and method of prey detection, seemed to influence foraging behavior. This study emphasizes the benefits of measuring multiple predator behaviors when classifying predators to particular foraging modes.

When predators become prey: flight decisions in jumping spiders

Stankowich, T. — 2009-03-17

Current optimal escape theory focuses on economic distance-based models that predict that animals will flee at greater distances when risk of capture is greater. Although these models have been tested extensively on vertebrate prey animals using large approaching stimuli (e.g., humans), it has never been tested on an invertebrate generalist predator with a stimulus that is in the size range of potential prey. I presented adult jumping spiders, Phidippus princeps, with a small black model to test flight decisions when physically handicapped, under different levels of threat, and on surfaces that potentially hindered escape. Predator approach speed, running surface texture, and leg autotomy had no effect on flight decisions. I also measured running distance and speed under different levels of hunger and energy state to test how these variables affect the decision to flee or to turn and defend oneself against a predator. When prodded, larger spiders fled shorter distances before switching to a defensive posture, hungry spiders fled longer distances than sated spiders, and rested spiders ran faster than tired spiders. There is likely a trade-off between body size and energy stores when deciding to flee from a threat or turning to defend oneself. These findings 1) reflect differences in how predators and prey assess risk in their environment and the distances at which they treat an approaching object as threatening and 2) suggest that future studies should focus on how animals optimize escape decisions in ways other than traditional distance-based economic models (e.g., relying absolutely on crypsis).

Sperm precedence in monarch butterflies (Danaus plexippus)

Solensky, M. J., Oberhauser, K. S. — 2009-03-17

We characterized sperm precedence in monarch butterflies (Danaus plexippus), using a series of experiments in which we manipulated male mating histories to vary spermatophore size and the number of sperm transferred. Several factors affected the outcome of sperm competition. There was a pattern of second-male sperm precedence, but second-male precedence was rarely complete, and several other factors had significant effects on paternity patterns. Larger males outcompeted smaller males when they were not matched for size. Phosphoglucose isomerase (PGI) genotype affected the outcome of sperm competition under very hot conditions. When sperm from the same pair of males competed in different females, males fared better when they transferred more sperm. These results demonstrate that sperm precedence within a species can be affected by many factors, including the circumstances under which it is measured.

Sex allocation in response to local resource competition over breeding territories

Hjernquist, M. B., Thuman Hjernquist, K. A., Forsman, J. T., Gustafsson, L. — 2009-03-17

Sex allocation according to local resource competition suggests that investment and offspring sex ratio should be biased toward the dispersing sex to limit the competition among the natal philopatric sex. Conversely, when competition over resources is low, parents should allocate more resources toward the philopatric sex. In this study, this reciprocal scenario of sex allocation is tested. More specifically, the effect of breeding territory availability on primary sex ratio is studied in the collared flycatcher, a migratory passerine bird, where males are the natal philopatric sex. As predicted, primary sex ratios were biased toward males in areas where available territories were abundant (estimated from population growth). No relationship between sex ratio adjustment and adult phenotypes as well as date of first egg was found. We discuss potential explanation for the male-biased broods in areas with many vacant territories and low levels of competition. We suggest that sex ratio adjustment in relation to breeding territory quality and availability could be relatively common in birds.

Correlation between exploration activity and use of social information in three-spined sticklebacks

Nomakuchi, S., Park, P. J., Bell, M. A. — 2009-03-17

Individual variation in the exploration of an unfamiliar environment and joining performance probably reflects the shy–bold continuum as risk-related behavior. We experimentally examined the association between individual variation in exploration activity in an unfamiliar laboratory environment and tendency to follow other conspecifics in limnetic (primarily shoaling planktivore) and benthic (opportunistic shoaling benthic feeder) populations of three-spined stickleback from Alaska. Using a maze aquarium, we initially observed individual differences in exploration activity, that is, how far along the unfamiliar maze single immature fish could progress and how quickly they could arrive there. Then, we measured individual tendencies for individuals to follow "demonstrators" that had already been trained to solve the maze. There is a positive correlation between the 2 measurements in both populations. This may indicate that individuals active in exploration in unfamiliar environments can quickly exploit social advantages provided by demonstrators, which is consistent with the conceptual framework of behavioral syndromes in a shy–bold continuum.

Temporal variability in a multicomponent trait: nuptial coloration of female two-spotted gobies

2009-03-17

Animals that breed more than once may face different environmental and physiological conditions at each reproductive event. Costs and benefits of sexual ornaments could therefore vary both within and between breeding seasons. Despite this, the ornaments are often assumed to be fixed, and temporal changes in ornamentation have rarely been investigated. Female two-spotted gobies (Gobiusculus flavescens) have colorful orange bellies when sexually mature and nest-holding males prefer females with more colorful bellies. This nuptial coloration is caused both by the carotenoids-rich gonads being directly visible through the skin and by the chromatophore pigmentation of the abdominal skin. Toward the end of the breeding season, males become rare and females become the more competitive sex. We show that female ornamentation of G. flavescens is a complex multicomponent trait and that the separate components, as well as their interactions, are variable. As gonads matured, they became more colorful while the abdominal skin became more transparent, causing more intense belly coloration in sexually mature females. However, coloration varied greatly also among fully mature females, suggesting that it may not only be a signal of readiness to spawn. Indeed, belly coloration predicted gonad carotenoid concentration, but there were several important seasonal differences in color expression. Females sampled toward the end of the breeding season were more colorful. This was due to seasonal increases in both gonad carotenoid concentration and skin coloration. Thus, at a time when competition over males is stronger and the terminal reproductive event approaches, females appear to invest more in signaling.

Do polyandrous pygmy grasshopper females obtain fitness benefits for their offspring?

Caesar, S., Forsman, A. — 2009-03-17

Explanations for polyandry in insects invoke material and genetic benefits that enhance female fitness via the production of more viable or more variable offspring. Here we use the color polymorphic pygmy grasshopper, Tetrix subulata, to evaluate effects of male quality, mate color morph resemblance (a proxy for compatibility), and polyandry on offspring performance. We experimentally mated females with different numbers and color morph combinations of males and reared offspring under either sun-exposed or shaded conditions using a split-brood design. We find a significant male identity effect on egg hatchability, consistent with the hypothesis that males vary in paternal quality. Offspring viability posthatching varied in a complex manner with solar regime, mating treatment, and parental resemblance. The effects of parental color morph resemblance on offspring performance suggest a potential role of compatibility and offspring variability. Monandrous females produced more viable offspring than polyandrous females (under shaded conditions) and we suggest as a hypothesis that the expected positive influence of polyandry on offspring performance may have been outweighed by more intense competition and antagonistic interactions among half-siblings. That an effect of mating treatment was evident under shaded but not under sun-exposed conditions suggests that great care is called for when making inferences from studies that show negative results.

Conspicuousness-dependent antipredatory behavior may counteract coloration differences in Iberian rock lizards

Cabido, C., Galan, P., Lopez, P., Martin, J. — 2009-03-17

Sexual selection favors more conspicuous male displays, whereas natural selection (via predator pressure) favors less conspicuous displays. However, this trade-off might be altered if males with more conspicuous displays could compensate behaviorally for their increased conspicuousness by acting more cautiously toward predators. The aim of this study was to explore in 2 species of Iberian rock lizards whether or not conspicuous coloration was associated with antipredatory behavior and whether conspicuousness-dependent regulation of antipredatory behavior existed. Our results suggested that male lizards may compensate for the negative effects of conspicuous sexual coloration on predation risk by modulating their antipredatory behavior (time inside refuges, false alarms, etc). We found that male Iberolacerta monticola, but not male Iberolacerta cyreni, compensated for the negative effects of blue lateral ocelli, which increased visual conspicuousness. However, male lizards did not compensate for relatively unexposed ventral spots. We also found that male I. monticola in better condition and with more blue lateral ocelli were shier, whereas male I. cyreni in better condition and with more ventral spots were bolder. These 2 lizard species live in habitats that differ in refuge availability and in the number of potential predators, which may promote differences in the trade-off between predation risk and social behavior and may explain the observed interspecific differences in antipredatory behavior. This suggests that regulation of antipredatory behavior may also function as a condition-dependent cost promoting costly (honest) sexual signaling in some species.

Antipredator behavior in blackbirds: habituation complements risk allocation

Rodriguez-Prieto, I., Fernandez-Juricic, E., Martin, J., Regis, Y. — 2009-03-17

Several studies showed that animals allow closer approaches (measured through flight initiation distances, FIDs) by potential predators (e.g., humans) in high–predator density areas, which has been explained by habituation effects. We assessed whether this pattern could be produced by not only habituation but also risk allocation by simulating attacks on blackbirds Turdus merula by both usual (pedestrians) and novel (radio-controlled vehicle) potential predators in parks with different levels of human visitation. Individual blackbirds from parks with higher pedestrian rates showed lower FID than individuals from parks with lower pedestrian rates, in response to both usual and novel approaches. Blackbirds adjusted their antipredator behavior to the specific level of pedestrian rate encountered every morning and evening in each park, with higher FID in the period with lower pedestrian rate. Similar responses to usual and novel potential predators among parks and daily variation in antipredator behavior support the risk allocation hypothesis and could not be explained by habituation. However, the rate at which FID was reduced in individuals from low-visited parks to high-visited parks was greater for pedestrian attacks than for novel potential predator attacks, suggesting that habituation is also present in our system and complements the effects of risk allocation. Our results have applied implications: the reduction in FID with increasing human visitation in natural areas is usually attributed to habituation; however, we propose that risk allocation can also reduce antipredator behavior effort to survive in habitats with high levels of recreational activities at the expense of potential physiological costs.

Caste-specific symbiont policing by workers of Acromyrmex fungus-growing ants

Ivens, A. B.F., Nash, D. R., Poulsen, M., Boomsma, J. J. — 2009-03-17

The interaction between leaf-cutting ants and their fungus garden mutualists is ideal for studying the evolutionary stability of interspecific cooperation. Although the mutualism has a long history of diffuse coevolution, there is ample potential for conflicts between the partners over the mixing and transmission of symbionts. Symbiont transmission is vertical by default, and both the ants and resident fungus actively protect the fungal monoculture growing in their nest against secondary introductions of genetically dissimilar symbionts from other colonies. An earlier study showed that mixtures of major and minor Acromyrmex workers eliminate alien fungus fragments even in subcolonies where their resident symbiont is not present. We hypothesize that the different tasks and behaviors performed by majors and minors are likely to select for differential responses to alien fungi. Major workers forage and cut new leaves and masticate them after delivery in the upper parts of the fungus garden and so are likely to more frequently encounter alien fungus than minor workers maintaining the established fungus garden and caring for the brood. We show that major workers of Acromyrmex echinatior indeed express stronger incompatibility reactions toward alien fungus garden fragments than minor workers. This implies that only the major workers, through recognition and exclusion of foreign fungus clones at their point of entry to the nest, have a realistic possibility to eliminate alien fungal tissue before it gets incorporated in the fungus garden and starts competing with the resident fungal symbiont.

Behavioral changes associated with a population density decline in the facultatively social red fox

Iossa, G., Soulsbury, C. D., Baker, P. J., Edwards, K. J., Harris, S. — 2009-03-17

Understanding the causal mechanisms promoting group formation in carnivores has been widely investigated, particularly how fitness components affect group formation. Population density may affect the relative benefits of natal philopatry versus dispersal. Density effects on individual behavioral strategies have previously been studied through comparisons of different populations, where differences could be confounded by between-site effects. We used a single population of red foxes (Vulpes vulpes) in the city of Bristol, UK, that underwent a natural perturbation in density to compare key changes in 1) group structure, 2) within-group relatedness, 3) mating system, 4) dispersal, and 5) dominance attainment. At high densities (19.6–27.6 adults km^–2), group sex ratios were equal and included related and unrelated individuals. At low densities (4.0–5.5 adults km^–2), groups became female biased and were structured around philopatric females. However, levels of within-group relatedness were unchanged. The genetic mating patterns changed with no instances of multiple-paternity litters and a decline in the frequency of extrapair litters of cubs from ≤77% to ≤38%. However, the number of genetically monogynous groups did not differ between periods. Dispersal was male biased at both high and low densities. At high density, most dominant males in the study groups appeared to have gained dominance after dispersing, but natal philopatry was an equally successful strategy at low density; conversely, most dominant females were philopatric individuals at both high and low densities. These results illustrate how density may alter behavioral strategies such as mating patterns and how this, in turn, alters group structure in a single population.

Do hoverflies (Diptera: Syrphidae) sound like the Hymenoptera they morphologically resemble?

Rashed, A., Khan, M.I., Dawson, J.W., Yack, J.E., Sherratt, T.N. — 2009-03-17

It has long been recognized that many hoverfly species (Diptera: Syrphidae) mimic the morphological appearance of defended Hymenoptera, such as wasps and bees. However, it has also been repeatedly suggested that some mimetic hoverflies respond with sounds on attack that resemble the warning or startle sounds of their hymenopteran models. In this study, we set out to quantitatively compare the spectral characteristics of the sounds produced by a range of nonmimetic flies, wasps, bumblebees, honeybees, and their hoverfly mimics when they were artificially attacked. The sounds made by wasps and honeybees after simulated attacks were statistically distinguishable from their hoverfly mimics. Bumblebee models of their hoverfly mimics share some similarities in the sound they produce on attack, but they were no closer acoustically to their model than a range of other hoverfly species that morphologically resemble other models. All the mimetic hoverflies tested in this study tended to sound similar to one another, regardless of the model they resemble morphologically. Overall, we found little evidence that mimetic hoverflies sound like their hymenopteran models on attack, and we question whether acoustic mimicry has evolved in this complex.

Competition and brood reduction: testing alternative models of clutch-size evolution in parasitoids

2009-03-17

Competition between siblings occurs in many taxa including parasitoid wasps. Larvae of solitary species eliminate competitors by engaging in aggressive behavior, thus restricting brood size to a single individual. In gregarious species, more than one offspring can develop per host. There are 2 models by which gregariousness can arise in a population of solitary individuals: 1) through a reduction in larval mobility (with the retention of aggressive behavior) or 2) through a reduction in fighting behavior or ability. When more larvae are present than can be supported by available host resources, these 2 models make opposing predictions regarding the process of brood size reduction: Mortality occurring early in larval development under the reduced mobility hypothesis versus mortality occurring throughout larval development under the reduced aggression hypothesis. Here, we measure changes in brood size over the course of larval development of the gregarious parasitoid, Cotesia flavipes. Superparasitized hosts contained approximately twice as many C. flavipes eggs as hosts parasitized by a single parasitoid female. Brood sizes in superparasitized hosts declined gradually as C. flavipes individuals developed, whereas brood sizes remained constant during larval development in singly parasitized hosts. An absence of wounded or destroyed larvae suggested no aggressive behavior. Collectively, these results support the reduced aggression hypothesis.

Lekking satin bowerbird males aggregate with relatives to mitigate aggression

2009-03-17

Males in several lekking species aggregate with their relatives to display for females, suggesting that kin selection can affect sexual selection. Several hypotheses have been proposed to explain this behavior, but no general explanation has emerged. In most species with lek mating systems, neighboring males have intense aggressive interactions that can affect the quality of their sexual displays. Here we test the hypothesis that the presence of related neighbors mitigates the negative consequences of this aggression. Male bowerbirds build stick display structures (bowers) that are used by females in mate assessment and are commonly destroyed by males' 2 nearest neighbors. We show that kin aggregate as first or second nearest neighbors, and males direct fewer bower destructions toward kin than equidistant nonkin. Males with more relatives nearby receive fewer bower destructions. These results suggest that the restraining effect of relatedness on aggression favors the close spatial association of related males' display sites. An alternative hypothesis, that related males aggregate to gain copulations from females attracted to successful relatives, was not supported.

Conclusions beyond support: overconfident estimates in mixed models

Schielzeth, H., Forstmeier, W. — 2009-03-17

Mixed-effect models are frequently used to control for the nonindependence of data points, for example, when repeated measures from the same individuals are available. The aim of these models is often to estimate fixed effects and to test their significance. This is usually done by including random intercepts, that is, intercepts that are allowed to vary between individuals. The widespread belief is that this controls for all types of pseudoreplication within individuals. Here we show that this is not the case, if the aim is to estimate effects that vary within individuals and individuals differ in their response to these effects. In these cases, random intercept models give overconfident estimates leading to conclusions that are not supported by the data. By allowing individuals to differ in the slopes of their responses, it is possible to account for the nonindependence of data points that pseudoreplicate slope information. Such random slope models give appropriate standard errors and are easily implemented in standard statistical software. Because random slope models are not always used where they are essential, we suspect that many published findings have too narrow confidence intervals and a substantially inflated type I error rate. Besides reducing type I errors, random slope models have the potential to reduce residual variance by accounting for between-individual variation in slopes, which makes it easier to detect treatment effects that are applied between individuals, hence reducing type II errors as well.

Group size effect caused by food competition in nutmeg mannikins (Lonchura punctulata)

Rieucau, G., Giraldeau, L.-A. — 2009-03-17

When foraging group sizes increase, animals generally decrease the time devoted to antipredator detection and increase their foraging rate, the commonly reported group size effect. The increased foraging rate is thought to follow from increased safety from predators because as group size increases, more eyes are available to detect predators and the risk of being a predator's victim is diluted. This increased safety then allows higher feeding rates because individuals can reallocate time spent in vigilance to foraging. However, increased foraging rates can also be due to increased competition for resources as the number of companions increases. We tested whether increased feeding rates are the product of competition or antipredation when group size increases in nutmeg mannikins (Lonchura punctulata). We used edited video playbacks to change group size and type of competitor: vigilant only, feeding only, and controls. We found that the increased feeding rate associated with an increased group size only resulted when the companions were feeding. Video playbacks of nonforaging companions neither decreased an individual's use of vigilance while handling food nor did it release the full increase of feeding rate. Focal birds lowered their scanning time while feeding as the frequency of pecking by simulated nonvigilant companions increased. We conclude that the group size effect reported in nutmeg mannikins is not a product of safety benefits of group living but may also arise from the costs imposed by competition for resources.

Livestock grazing behavior and inter- versus intraspecific disease risk via the fecal-oral route

Smith, L. A., White, P. C.L., Marion, G., Hutchings, M. R. — 2009-03-17

Livestock herbivores are at risk of parasite/pathogen exposure from livestock and wild mammal feces during grazing. Livestock exposure to parasites/pathogens will be dependent on the behavioral contact processes between grazing livestock and host animal (both livestock and wild mammal) feces at the bite scale. Here we use 2 grazing experiments to determine the affect of feces from different species and in different defecation patterns on the grazing response of cattle. In experiment 1, there were 4 plots, each with 4 replicates of 5 patch treatments of different fecal contamination (240 g/m² of Eurasian badger feces, cattle feces, fallow deer feces, Eurasian rabbit feces, and noncontaminated control patches). In experiment 2, there were 3 treatment patterns of badger fecal contamination (one 1-m² circular patch contaminated with 960 g of badger feces; two 1-m² circular patches each contaminated with 480 g of badger feces; and four 1-m² circular patches each contaminated with 240 g of badger feces), divided into 2 plots per treatment. The cattle's grazing response was determined by measuring sward depletion at each of the treatment patches. In experiment 1, cattle-grazed control and rabbit fecal–contaminated patches the most, whereas badger-contaminated patches were grazed the least. In experiment 2, cattle grazed the treatment with the greatest number of fecal-contaminated patches the most. We conclude that cattle vary their grazing response to feces from different host species and to feces in different spatial patterns. Quantifying these behavioral responses to feces is a key step toward quantifying infection risk to herbivores via the fecal–oral route in grazing systems.

Individual differences in protandry, sexual selection, and fitness

Moller, A. P., Balbontin, J., Cuervo, J. J., Hermosell, I. G., de Lope, F. — 2009-03-17

Protandry is the difference in arrival date between males and females, with competition among males for access to preferred territories (the rank advantage hypothesis) or mating success (the mate opportunity hypothesis) supposedly driving the evolution of protandry. The fitness costs and benefits of protandry accruing to individuals differing in degree of protandry (arrival date of a male relative to the arrival date of his partner) have never been quantified. We analyzed the fitness consequences of sex differences in arrival date in the barn swallow Hirundo rustica, in which arrival date can be precisely estimated and the fitness of pairs differing in degree of individual protandry assessed. Early arriving males had greater mating success than late arriving males. The number of extrapair offspring in own nests decreased with increasing degree of individual protandry, whereas the number of offspring fathered by a focal male was unrelated to individual protandry. There was directional selection on individual protandry as shown by pairs with a larger than average degree of protandry reproducing early and, hence, supposedly producing more recruits. There was also stabilizing selection on individual protandry as shown by pairs with an intermediate degree of protandry reproducing early. Annual production of fledglings increased with early arrival of males, but not with early arrival of females, once the effect of laying date had been considered, with no additional effect of individual protandry. Neither male nor female survival was significantly related to degree of individual protandry. These findings are consistent not only with the mate opportunity hypothesis but also with a sexual conflict hypothesis, suggesting that males and females differ in their optimal timing of arrival due to sex-specific fitness costs and benefits.

Interactions between masculinity-femininity and apparent health in face preferences

Smith, F. G., Jones, B. C., DeBruine, L. M., Little, A. C. — 2009-03-17

Consistent with Getty's (2002. Signaling health versus parasites. Am Nat. 159:363–371.) proposal that cues to long-term health and cues to current condition are at least partly independent, recent research on human face preferences has found divergent effects of masculinity–femininity, a cue to long-term health, and apparent health, a cue to current condition. In light of this, we tested for interactions between these 2 cues. Participants viewed composite images of opposite-sex faces that had been manufactured in combinations of high and low apparent health and masculinity–femininity. Preferences for masculinity in men's faces and femininity in women's faces were stronger when judging the attractiveness of faces with high apparent health than when judging the attractiveness of faces with low apparent health. Similarly, preferences for high apparent health were stronger for judgments of masculine men's faces and feminine women's faces than for judgments of feminine men's faces and masculine women's faces, respectively. Interactions between apparent health and masculinity–femininity when forming face preferences may function to optimize how masculinity–femininity and apparent health are used to infer the quality of potential mates and highlight the complexity and sophistication of the perceptual mechanisms that underpin face preferences.

Negotiation between parents over care: reversible compensation during incubation

Kosztolanyi, A., Cuthill, I. C., Szekely, T. — 2009-03-17

Parental care is often beneficial for the young but costly for the caregiving parent. Because both parents benefit from care via the offspring, whereas they pay the costs individually, a conflict is expected about how much care each parent should provide. How do parents settle this conflict? We addressed this question by reducing nest temperatures during incubation in the Kentish plover Charadrius alexandrinus, a small ground-nesting shorebird in which the parents share incubation. By cooling the clutch using a remote-controlled device built under the nest, we experimentally increased the workload of either the male or the female in random order and recorded the behavioral responses of the targeted parent and its mate. Unlike most previous manipulations of parental effort, our manipulation sought to measure a parent's response to an increase, not a shortfall, in the partner's contribution. The manipulation was also short term and reversed between the members of a given pair. We found that there is a trade-off between the efforts of parents because increased (or reduced) effort by the targeted parent was associated with decreased (or increased) effort by its mate, respectively. This result is consistent with theoretical models that predict compensation as a response to changed parental effort of the mate. We also found that compensation was consistent between treatments when the male or the female of a given pair was targeted. Furthermore, our results support the notion that parents adjust their effort in response to their mate's behavior in real time, that is, they negotiate parental roles.

The social and genetic mating system in flickers linked to partially reversed sex roles

Wiebe, K. L., Kempenaers, B. — 2009-03-17

The type of social and genetic mating system observed in birds is influenced by the need of both sexes to provide parental care. In woodpeckers, unlike most birds, females are partially emancipated as males provide most of the care including nocturnal incubation. We analyzed the mating system of northern flickers Colaptes auratus and used microsatellite markers to assess parentage of 326 nestlings from 46 monogamous broods and 41 nestlings from 7 polyandrous broods. No cases of extrapair paternity were found in monogamous broods, but there was one such case in the brood of a secondary male of a polyandrous female. Intraspecific parasitism lead to 17% of broods containing at least one parasitic egg. The identity of the parasitic female was determined in 5 cases to be a close neighbor with a mate and clutch of her own. Between 0% and 5% of females annually were polyandrous with the timing of the 2 nests slightly staggered. Polyandrous females were older than average females in the population, and their primary males were older than secondary males. Polyandrous females raised nearly twice as many (10.8) nestlings compared with monogamous females (5.5). Although most female flickers are strictly socially and genetically monogamous, some can benefit from engaging in the alternate reproductive tactics of polyandry and brood parasitism. Therefore, at least in flickers, such tactics of laying eggs in multiple nests are not the result of poor-quality females "making the best of a bad situation" but are a way to increase reproductive success.

Seeing red: behavioral evidence of trichromatic color vision in strepsirrhine primates

Leonhardt, S.D., Tung, J., Camden, J.B., Leal, M., Drea, C.M. — 2008-12-29

Among primates, catarrhines (Old World monkeys and apes) and certain platyrrhines (New World monkeys) possess trichromatic color vision, which might confer important evolutionary advantages, particularly during foraging. Recently, a polymorphism has been shown to shift the spectral sensitivity of the X-linked opsin protein in certain strepsirrhines (e.g., Malagasy lemurs); however, its behavioral significance remains unknown. We assign genotypes at the X-linked variant to 45 lemurs, representing 4 species, and test if the genetic capacity for trichromacy impacts foraging performance, particularly under green camouflage conditions in which red detection can be advantageous. We confirm polymorphism at the critical site in sifakas and ruffed lemurs and fail to find this polymorphism in collared lemurs and ring-tailed lemurs. We show that this polymorphism may be linked to "behavioral trichromacy" in heterozygous ruffed lemurs but find no comparable evidence in a single heterozygous sifaka. Despite their putative dichromatic vision, female collared lemurs were surprisingly efficient at retrieving both red and green food items under camouflage conditions. Thus, species-specific feeding ecologies may be as important as trichromacy in influencing foraging behavior. Although the lemur opsin polymorphism produced measurable behavioral effects in at least one species, the ruffed lemur, these effects were modest, consistent with the modest shift in spectral sensitivity. Additionally, the magnitude of these effects varied across individuals of the same genotype, emphasizing the need for combined genetic and behavioral studies of trichromatic vision. We conclude that trichromacy may be only one of several routes toward increased foraging efficiency in visually complex environments.

Dragonfly larvae and tadpole frog space use games in varied light conditions

Luttbeg, B., Hammond, J. I., Sih, A. — 2008-12-29

Predators and prey often engage in a game where predators attempt to be in areas with higher prey densities and prey attempt to be in areas with lower predator densities. A few models have predicted the resulting distributions of predators and prey, but little empirical data exist to test these predictions and to examine how abiotic and biotic factors shape the distributions. Thus, we observed how Anax dragonfly nymphs and Pacific tree frog tadpoles (Pseudacris regilla) either together or separately distributed themselves in an arena with a high- and a low-prey resource patch. Trials were conducted in high- and low-light conditions to manipulate predation risk and to view the effects of this abiotic factor. Counter to the model predictions, we found that predators were not more abundant in high-resource (HR) patches, and they thus did not force prey toward being uniformly distributed. Using a model selection approach to assess what factors affected predator and prey patch-switching movement, we found that prey more often left patches that had more predators present, but predators surprisingly more often left patches with more prey present. Light levels did not affect predation risk; however, in the dark with the associated reduction in visual information predators preferred HR patches. This caused a lower coincidence of prey and predators in patches. Predators also switched patches less often when they occupied the same patch as the other predator. This suggests that predator distributions, and indirectly prey distributions, are affected by the risk of intraguild predation.

Vigilance and its complex synchrony in the red-necked pademelon, Thylogale thetis

Pays, O., Dubot, A.-L., Jarman, P. J., Loisel, P., Goldizen, A. W. — 2008-12-29

Several adaptive functions, including gaining information from other group members and detecting predators, are generally ascribed to vigilance in groups of animals subject to predation. Most studies of the effects of neighbors on vigilance have focused on individual vigilance. We investigated the effects of neighbors on vigilance in wild red-necked pademelons Thylogale thetis foraging at night in nonpersistent aggregations in a clearing in rain forest. Neither the total number of pademelons in the clearing nor the numbers at various distances around focal individuals affected the individual vigilance of focal animals. However, focal animals’ individual vigilance did change with the distance to their nearest neighbor and also with distance to cover. Pairs of individuals closer than 10 m apart tended to synchronize their bouts of individual vigilance and foraging. The degree of synchrony within pairs increased with both distance to cover and the total number of pademelons foraging in the area and decreased with increasing distance to the pair's nearest neighbor but did not vary with the distance separating the members of the pair. Thus, despite their individual vigilance being unaffected by the number of other pademelons in the feeding aggregation, pademelons were nonetheless sensitive to the presence of conspecifics and adjusted their behavior in relation to their separation from neighbors. Thus, some vigilance benefits may be obtained from the presence of conspecifics even in species that aggregate only temporarily on food patches without forming more permanent social groups.

Behavioral syndrome over the boundaries of life--carryovers from larvae to adult damselfly

Brodin, T. — 2008-12-29

Activity is an important behavioral trait that mediates a trade-off between obtaining food for growth and avoiding predation. Active individuals usually experience a higher encounter rate with food items and suffer higher predation pressure than less active individuals. I investigated how activity of the damselfly Lestes congener is affected by larval state and predator presence and if larval behavioral type (BT) can be used to predict larval boldness, foraging success, and adult BT. Activity level of individual larvae was studied without predator at 2 different physiological states (hungry and fed) and in 2 predator treatments: familiar predator cues and unfamiliar predator cues. Larvae did not adjust their activity depending on state or when subjected to unfamiliar predator cues, but a general reduction in activity was seen in the familiar predator treatment. Hence, active individuals remained active compared with their conspecifics, independent of state or predator treatment. Active individuals were also bolder and more efficient foragers than their less active conspecifics. Furthermore, both adult activity and boldness were correlated with larval BT. The results illustrate that BT of a larvae is carried over many different situations keeping active larvae active even in maladaptive situations, demonstrating how a behavioral syndrome may constrain behavioral plasticity. Furthermore, results showed that behavioral syndromes can carry over from larvae through metamorphosis and dictate the BT of the adult.

Sperm length evolution in the fungus-growing ants

Baer, B., Dijkstra, M. B., Mueller, U. G., Nash, D. R., Boomsma, J. J. — 2008-12-29

Eusocial insects offer special opportunities for the comparative study of sperm traits because sperm competition is absent (in species with obligatory monandry) or constrained (in lineages where queens mate multiply but never remate later in life). We measured sperm length in 19 species of fungus-growing ants, representing 9 of the 12 recognized genera, and mapped these onto the ant phylogeny. We show that average sperm length across species is highly variable and decreases with mature colony size in basal genera with singly mated queens, suggesting that sperm production or storage constraints affect the evolution of sperm length. Sperm length does not decrease further in multiply mating leaf-cutting ants, despite substantial further increases in colony size. In a combined analysis, sexual dimorphism explained 63.1% of the variance in sperm length between species. As colony size was not a significant predictor in this analysis, we conclude that sperm production trade-offs in males have been the major selective force affecting sperm length across the fungus-growing ants, rather than storage constraints in females. The relationship between sperm length and sexual dimorphism remained robust in phylogenetically independent contrasts. Some of the remaining variation was explained by the relative size of the sperm-storage organ, but only in the multiply mating leaf-cutting ants, suggesting that sperm-storage constraints become important for the evolution of sperm length in this derived group. Mate number affected sperm length to a minor extent, and only in interaction with other predictor variables, suggesting that sperm competition has not been a major selective force for sperm length evolution in these ants.

Kin recognition via cuticular hydrocarbons shapes cockroach social life

Lihoreau, M., Rivault, C. — 2008-12-29

Genetic relatedness plays a key role in the organization and the functioning of societies. A large diversity of species has developed kin recognition abilities, allowing individuals to discriminate conspecifics in relation to relatedness. In social insects, many studies showed that discrimination generally acts at the level of nestmateship and only few studies report kin recognition abilities. Our results highlight the importance of kin recognition in shaping social life in the urban cockroach Blattella germanica (L.) and present a complete description of the recognition system from expression to action components. Cockroaches of all developmental stages (nymphs and adults) discriminate siblings from nonsiblings independently of any prior social experience. Preference is context dependent so that siblings are preferred as social partners, whereas nonsiblings are preferred as mating partners. Discrimination is based on quantitative differences of cuticular hydrocarbons that are perceived through antennal contacts. As individual cuticular profiles remain stable over time, they constitute reliable discrimination cues correlated with relatedness. Our results offer interesting perspectives for the study of kin recognition and for the understanding of evolution toward sociality in insects.

An intimidating ornament in a female pipefish

Berglund, A., Rosenqvist, G. — 2008-12-29

A sexually selected signal may serve a dual function being both attractive to mates and deterring rivals. Presently, there are few unambiguous demonstrations of an ornament functioning in both a mate choice and mate competition context and none regarding female ornaments. We have shown earlier that a temporary ornament, a striped pattern, in a sex-role reversed female pipefish, Syngnathus typhle, attracts males. Here we show that this ornament also intimidates rival females: in one experiment a male could interact with either 1 or 2 females. Latency until copulation was longer when 2, rather than 1, females were present. Moreover, when 2 females were present, competition lasted longer and time until mating took place increased when females displayed their ornaments more equally. In another experiment, a focal female could see 1 stimulus female and 1 stimulus male, the latter 2 being unaware of each other. The ornament of the stimulus female was manipulated, either strengthened by being painted black or left unaltered by being sham-painted. As a result, focal females experiencing black-painted stimulus females decreased courtship as well as competitive activities compared with focal females seeing sham-painted females. Moreover, focal females seeing black-painted females displayed less of their own ornament compared with controls. This decrease was due to a decrease in display toward males rather than to stimulus females. Thus, this female ornament indeed has a dual function, attracting mates and deterring rivals. In addition, the social costs invoked by this intimidating effect on rivals may help to maintain signal honesty.

Female-biased natal dispersal in the Siberian flying squirrel

Hanski, I. K., Selonen, V. — 2008-12-29

Natal dispersal is usually sex biased in birds and mammals. Female-biased natal dispersal is the prevailing pattern in birds but is rare among mammals. Hypotheses explaining sex bias in dispersal include the mate-defense mating hypothesis, which predicts male-biased dispersal, the resource-defense hypothesis predicting female-biased dispersal, and the competition hypothesis, which predicts that if dispersal is caused by competition for resources between sexes, then the subdominant sex will disperse. We studied natal dispersal of Siberian flying squirrels Pteromys volans using radio telemetry in Southern Finland in 1996–2004. Of 86 juveniles that survived over the dispersal period, almost all young females dispersed from the natal site, whereas almost 40% of males were philopatric. Dispersal was farther for females than males. Females began dispersal on average 2 weeks earlier than males and were lighter in mass at the onset of dispersal than later dispersing males. No mate- or resource-defense mating system could be found among males, but females seemed to defend nest and apparently food resources, in contrast to the expectation of dispersal bias in resource-defense systems. Competition for resources between sexes does not explain female bias either: in the flying squirrel, the female seems to be the dominant sex. We propose that young females are subordinate to their mothers and have to disperse to find a vacant, suitable site for reproduction.

Previous experiences shape adaptive mate preferences

Fawcett, T. W., Bleay, C. — 2008-12-29

Existing models of mate choice assume that individuals have perfect knowledge of their own ability to attract a mate and can adjust their preferences accordingly. However, real animals will typically be uncertain of their own attractiveness. A potentially useful source of information on this is the feedback from previous encounters with potential mates. Here, we develop a dynamic model of mutual mate choice in which both males and females are initially ignorant of their own attractiveness. Individuals sequentially sample potential mates and retain some information about the outcome of these encounters (e.g., the number of times they are accepted or rejected). We use a simplified process of mutation and selection to evolve an adaptive strategy for mate choice under these conditions. The stable strategy we find is the one in which individuals are sensitive to this previous experience, adjusting their mate preferences according to the interest received from members of the opposite sex. In general, experiences of rejection tend to reduce choosiness, whereas experiences of acceptance tend to increase it. Sensitivity to previous experiences allows individuals to exercise a prudent mate-choice strategy in which their preferences are gradually tuned to their prospects on the mating market. This flexibility is based on simple rules and does not require sophisticated cognitive abilities. Our basic predictions can be tested in systems where both males and females are choosy, and it is possible to manipulate the level of interest shown by potential mates.

Female grooming markets in a population of gray-cheeked mangabeys (Lophocebus albigena)

Chancellor, R. L., Isbell, L. A. — 2008-12-29

Primate female allogrooming models based on biological markets theory predict that grooming is "time matched" within bouts, that is, the amount of time the first female grooms predicts the amount of time the second one grooms. The models also predict that when female–female contest competition is weak, grooming is traded for grooming, but when female–female contest competition is strong, grooming may be traded for other commodities such as feeding tolerance, and grooming discrepancy between members of dyads is rank related. We tested these predictions using data collected from adult and subadult female gray-cheeked mangabeys (Lophocebus albigena) (N = 26) in 5 groups in Kibale National Park, Uganda. We found that, overall, females reciprocated in 33% of grooming bouts. Among reciprocated bouts, females in all 5 groups showed time matching. In 2 groups, we also found rank-related grooming discrepancies but showing opposite patterns to each other. Consistent with predictions based on biological markets theory, these groups may have been under greater feeding competition, revealed more by adjustments in ranging behavior than increased agonistic rates. Although these results support current allogrooming models, they also suggest that the models may become more robust if the influence of scramble competition is incorporated. In addition, they emphasize the flexibility and dynamic nature of female competitive relationships within the same population of primates.

Postsettlement movement patterns and homing in a coral-associated fish

Wall, M., Herler, J. — 2008-12-29

Coral-associated gobies are highly specialized reef fishes with high host-coral fidelity. Flexibility in habitat choice, however, is important to compensate for potential habitat alteration or loss, but detailed information about the postsettlement movement behavior of such gobies is lacking. We examined movement patterns in Gobiodon histrio, both under natural conditions and during subsequent field experiments, involving breeding pair or partner removal from 3 of the 4 investigation plots. Additionally, we investigated homing behavior, and 2 aquaria experiments were designed to assess home coral and partner recognition of adult fish taken from breeding pairs. Under natural conditions, the movement rate was high for single adults, whereas breeding pairs showed high home-coral fidelity. Manipulations revealed little change of natural patterns except in single adults, which slightly decreased their movement rate in the breeding pair removal plot. In the homing experiment, 17% of tested fish returned to their home coral even after displacement of 4 m, and homing success was much higher at shorter distances (100% at 0.5 m, 53% at 2.25 m). In the aquarium, G. histrio exhibited higher recognition of its home-coral colony (75%) than of its breeding pair partner (60%). Our study shows that G. histrio frequently moves between corals, although this depends on the social status (juvenile, single adult, breeding pair) of the individuals. The high proportion and movement rate of single adults indicate low sensitivity to habitat alteration but also limited high-quality habitats in which breeding pairs could be established. Hence, vulnerability to habitat loss increases when individuals breed.

The protective value of conspicuous signals is not impaired by shape, size, or position asymmetry

Stevens, M., Castor-Perry, S. A., Price, J. R.F. — 2008-12-29

Various conspicuous signals in nature promote initial and learned avoidance by predators. It is widely thought that such signals are most effective when highly symmetrical in features such as size and shape, supported by recent laboratory experiments with domestic chicks and artificial prey. However, no study has investigated the effect of asymmetry on conspicuous signals in a natural setting, where viewing distances, angles, predator species, and light conditions vary and where predators encounter prey sequentially rather than simultaneously. We undertook 2 field experiments with artificial gray-scale prey, marked with a pair of white markings presented to wild avian predators, to test the effect of asymmetry on the survival value of conspicuous signals in the field. Experiment 1 had treatments with symmetrical spots or with spots asymmetrical in area between 5 and 50%. All marked treatments survived better than unmarked controls, but there was no benefit of being symmetrical. Experiment 2 tested the effect of possessing markings asymmetrical for shape or position and any additive effect of these 2 features. Again, symmetry conferred no benefit and targets with markings asymmetrical for position and/or shape survived equally well as those with symmetrical arrangements. These findings indicate that asymmetry in warning signals may not be costly to prey in nature or be of less importance compared with other features of the signal, such as color and overall size.

The development of foraging microhabitat preferences in meerkats

Thornton, A., Hodge, S. J. — 2008-12-29

Animals of many species tend to target their foraging attempts toward particular microhabitats within their habitat. Although these preferences are critical determinants of the foraging niche and have important ecological and evolutionary implications, we know little about how they develop. Here, we use detailed longitudinal data from meerkats (Suricata suricatta) to examine how individual learning and the use of social information affect the development of foraging microhabitat preferences. Despite living in an open, arid environment, adult meerkats frequently foraged at the base of vegetation. Young pups seldom did so, but their foraging microhabitat choices became increasingly adult-like as they grew older. Learning about profitable microhabitats may have been promoted in part by positive reinforcement from prey capture. Foraging may also have become increasingly targeted toward suitable locations as pups grew older because they spent more time searching before embarking on foraging bouts. The development of microhabitat preferences might also have been influenced by social cues. Foraging in close proximity to adults may increase the probability that pups would dig in similar microhabitats. Also, pups often dug in holes created by older individuals, whereas adults never dug in existing holes. Foraging in existing holes was no more profitable to pups than creating their own foraging hole but could provide pups with important informational benefits. The integration of personal and social information is likely to be a common feature in the development of the foraging niche in generalist species.

Chick-a-dee call variation in Carolina chickadees and recruiting flockmates to food

Mahurin, E. J., Freeberg, T. M. — 2008-12-29

The "chick-a-dee" call of many Paridae species (titmice, tits, and chickadees) is structurally complex and functions in social cohesion. Studies with different Parid species suggest that variation in the note composition of calls relates to a wide variety of contexts. An earlier study with Carolina chickadees (Poecile carolinensis), the focal species of the present study, found that receivers responded differently to playback calls differing in note composition in feeding contexts. Here, we addressed whether signalers actually produce calls differing in note composition in feeding contexts and whether those calls might serve a recruitment function. In a first study, we found that the first chickadee to take seed from a feeding station produced calls with a greater number of D notes before the second chickadee arrived to take seed, compared with after the second chickadee arrived to take seed. This suggests that calls with a large number of D notes might serve a general recruitment function. We tested this idea in a second study, using playbacks of calls containing a large number of D notes or a small number of D notes at different sites. We found that the latency for a first chickadee to come into a site and take seed was shorter for playback variants containing a large number of D notes. Thus, in Carolina chickadees, chick-a-dee calls containing a large number of D notes may function to recruit other flock members to a discovered food source.

Fecundity compromises attractiveness when pigments are scarce

Morales, J., Velando, A., Torres, R. — 2008-12-29

Theory predicts that the trade-off between ornamentation and fecundity limits female attractiveness. However, there is little evidence on this theoretical trade-off and its proximate background. Our aim was to study whether pigment availability modulates this potential relationship in blue-footed booby females. We supplemented females with dietary carotenoids after laying the first egg and assessed the change in foot color, a carotenoid-based sexually selected trait in both sexes. We measured the change in body mass and in the levels of plasma antioxidants and carotenoids. Also, we registered the mass and volume of eggs. Surprisingly, experimental females reduced zeaxanthin concentration in plasma, but not other carotenoids or total antioxidant levels. Conversely, they increased foot color intensity and laid heavier second eggs and larger second and third eggs than controls. Furthermore, under natural conditions (controls), ornamentation was negatively associated with the mass and volume of second eggs, but the association was reversed under conditions of high carotenoid availability (experimental females). Results suggest that carotenoid availability may mediate the theoretical trade-off between ornamentation and fecundity. We highlight that pigment limitation for females could represent an evolutionary pathway to male choosiness in the blue-footed booby.

Caribbean damselfish with varying territory quality: correlated behaviors but not a syndrome

Snekser, J. L., Leese, J., Ganim, A., Itzkowitz, M. — 2008-12-29

The behavioral syndrome hypothesis suggests that individual animals within a population behave differently due to specific behavioral types, and these should be consistent across behaviors or in different contexts. In contrast, for animals that live within an environment in which territory quality can change over time, natural selection should have favored behavioral flexibility and modulation of the cost of defense in relation to territory quality. This would require assessment of the territory followed by displays of appropriate types and intensities of behavior. We examined the territorial behavior of male beaugregory damselfish (Stegastes leucostictus) by enhancing territory quality using artificial breeding sites and comparing their behavior to males on lower quality natural sites. When male fish were defending high-quality artificial territories, they had higher levels of aggression toward male conspecifics and courtship toward females than when on low-quality natural territories. We also found that aggression and courtship behaviors were correlated on natural sites but not on artificial sites. Behaviors were not correlated within individuals when males switched from natural to artificial territories or from artificial to natural territories. These results indicate that males assess their current territories and adjust behaviors accordingly and that courtship and aggressive behaviors are not linked within a permanent behavioral syndrome.

Food availability and parasite infection influence mating tactics in guppies (Poecilia reticulata)

Kolluru, G. R., Grether, G. F., Dunlop, E., South, S. H. — 2008-12-29

Despite the important effects of diet and parasite infection on male reproductive behavior, few studies have simultaneously addressed their influence on intrasexual selection (male–male competition). We examined the synergistic effects of 2 naturally varying environmental factors, lifetime food intake and infection, with the monogenean parasite Gyrodactylus turnbulli on the mating tactics and foraging behavior of male guppies (Poecilia reticulata). We allowed fish to interact directly with each other during observations and found that unparasitized males won more intermale contests, courted females more frequently, and received positive responses to courtship displays more frequently than males that had been infected. Infected males devoted more time to foraging and less time to courtship and competition than uninfected males, suggesting that they were energetically limited and could not increase reproductive effort despite their reduced expected lifespan. This interpretation was supported by the observation that greater food intake ameliorated the negative effects of parasite infection on courtship effort. Our results have bearing on how natural variation in food availability and parasite prevalence influence geographic variation in reproductive behavior.

Color polymorphism and intrasexual competition in assemblages of cichlid fish

Dijkstra, P. D., Hemelrijk, C., Seehausen, O., Groothuis, T. G.G. — 2008-12-29

The origin and maintenance of phenotypic polymorphisms is a classical problem in evolutionary ecology. Aggressive male–male competition can be a source of negative frequency-dependent selection stabilizing phenotypic polymorphisms when aggression is biased toward the own morph. We studied experimental assemblages of red and blue color morphs of the Lake Victoria cichlid fish Pundamilia. Aggression was investigated in mixed-color and single-color assemblages. We found that aggression was indeed biased toward males of the same color, which could in theory reduce aggression levels in mixed-color assemblages and promote coexistence. However, previous studies showed high aggression levels in red and dominance of red over blue males in dyadic interactions, which could hinder coexistence. We found that coexistence in mixed-color assemblages reduced the level of aggression in red males but not in blue males. Red and blue males were equally dominant in mixed-color assemblages, suggesting that predictions derived from dyadic interactions may not be valid for an assemblage situation. The results are consistent with field data: the geographic range of red is nested within that of blue, suggesting that red cannot displace blue. Our study suggests that male–male competition may be a significant force for maintaining phenotypic diversity.

Effects of group size and personality on social foraging: the distribution of sheep across patches

Michelena, P., Sibbald, A. M., Erhard, H. W., McLeod, J. E. — 2008-12-29

Group cohesion in social animals foraging in patchy environments depends on 2 opposite forces, intraspecific competition, and attraction. The decision to leave or stay in a group may vary according to the individual personality. We investigated the role of personality when feeding competition increases as a result of increasing group size. Individuals referred as "bold" and "shy" were identified from an indoor exploration test according to their propensity to leave the group to explore a novel environment, using 12 novel objects placed at increasing distances from the group. Groups of 2, 4, 6, or 8 shy or bold sheep were tested in 45 x 5–m grass arenas, with one 5 x 5–m patch of preferred vegetation at each end. Sheep grazed on or close to these patches, but the number grazing the patches seldom reached more than 4–5 individuals, suggesting that crowding might affect foraging at the highest densities. The smallest groups grazed together on the same patch, but there was an increasing likelihood of splitting into subgroups as group size increased, with equal-sized subgroups most commonly grazing the 2 patches simultaneously. This effect was greatest in bold sheep, which tended to split into subgroups at smaller group sizes than shy sheep. This study provides new insight into the mechanisms by which group-living herbivores distribute themselves across patchy resources in a way that minimizes interference competition and demonstrates the importance of individual variability for spatial organization at the level of the group.

Experience does not alter alternative mating tactics in the burying beetle Nicrophorus vespilloides

Walling, C. A., Stamper, C. E., Salisbury, C. L., Moore, A. J. — 2008-12-29

Alternative male mating tactics are widespread, but the cues that determine which tactic is adopted remain unclear. Size is commonly associated with alternative mating tactics, but it is not known how individuals gauge their size effectively, especially given that size is relative and frequency dependent. One possibility is that interactions with conspecifics are used to assess size, relative to potential competitors, and thus fine-tune tactics. Success in mating might also influence mating tactics given that this should indicate the potential availability of mates in the population. We tested these ideas in the burying beetle Nicrophorus vespilloides, examining whether individuals use the outcome of larval or adult interactions as cues to adjust the tactics used to acquire mates. Male N. vespilloides employ 2 tactics; search for a carcass, a resource required for reproduction, or release a pheromone (call) to attract a mate. Males are plastic in the amount of time they invest in each tactic, and in a related species (Nicrophorus orbicollis), male size influences the tactic adopted. We examine the potential effects of parental care, sibling competition, relative size within a brood, and adult experience of agonistic interactions and mating on tactic adoption. Absolute size was consistently the best predictor of calling rate, with smaller males calling more often than larger males. We suggest that the lack of a response to adult cues may reflect unpredictability in the occurrence of social interactions or stable size distributions in this population.

Mate choice, operational sex ratio, and social promiscuity in a wild population of the long-snouted seahorse Hippocampus guttulatus

Naud, M.-J., Curtis, J. M.R., Woodall, L. C., Gaspar, M. B. — 2008-12-29

Mate competition and mate choice are not mutually exclusive behaviors. Both behaviors may drive sexual selection in one or both sexes of a population. One of several factors affecting which behavior is exhibited by which sex is the operational sex ratio (OSR) in the study population. The present study combines behavioral observations in the field with controlled experiments in aquaria to investigate social interactions and mate choice in both male and female long-snouted seahorses Hippocampus guttulatus in the context of the population OSR. Compared with the more readily studied pipefishes, data on OSR and mate choice in seahorses are scarce in the published literature. Our field data provide novel evidence of social promiscuity, size-assortative mating, and an OSR that varies from being unbiased early and midseason to male biased at the end of the breeding season. Our mate choice experiments revealed intersexual differences in mate preference with males significantly preferring larger females to familiar ones. Taken together, our field and experimental results suggest that mate choice rather than intrasexual competition could drive sexual selection in seahorses.

Shoal composition determines foraging success in the guppy

Dyer, J. R.G., Croft, D. P., Morrell, L. J., Krause, J. — 2008-12-29

The composition of an animal group can impact greatly on the survival and success of its individual members. Much recent work has concentrated on behavioral variation within animal populations along the bold/shy continuum. Here, we screened individual guppies, Poecilia reticulata, for boldness using an overhead fright stimulus. We created groups consisting of 4 bold individuals (bold shoals), 4 shy individuals (shy shoals), or 2 bold and 2 shy individuals (mixed shoals). The performance of these different shoal types was then tested in a novel foraging scenario. We found that both bold and mixed shoals approached a novel feeder in less time than shy shoals. Interestingly, we found that more fish from mixed shoals fed than in either bold or shy shoals. We suggest that this can be explained by the fact that nearly all the cases where one fish was followed into the feeder by another occurred within mixed shoals and that it was almost always a shy fish following a bold one. These results suggest clear foraging benefits to shy individuals through associating with bold ones. Surprisingly, our results also suggest potential foraging benefits to bold individuals through associating with shy individuals. This study highlights a possible mechanism by which interindividual variation in behavioral types is maintained in a population.

Manipulation of male attractiveness induces rapid changes in avian maternal yolk androgen deposition

2008-12-29

Avian eggs contain maternal androgens that may adjust offspring development to environmental conditions. We review evidence and functional explanations for the relationship between androgen concentrations in avian eggs and male attractiveness. Experimental studies in captive birds show generally positive relationships, but results from correlational and experimental field studies are less consistent, perhaps because they lack a within-female design to control for confounding between-female variation. We analyzed the effect of male attractiveness on yolk levels of maternal androgens in a wild bird, using a correlational and experimental approach with a within-female design. We manipulated the sexually selected UV coloration of the crown feathers of male blue tits (Cyanistes caeruleus) after their female had laid the second egg and measured the subsequent effect on androgen concentrations (testosterone and androstenedione) in the fifth, seventh, and ninth eggs relative to that in the second egg. Levels of testosterone, but not androstenedione, in eggs 5 and 7 were higher for control (attractive) than for UV-reduced (unattractive) males. This effect disappeared in the ninth egg, coinciding with the recovery of UV coloration after manipulation. This suggests that females are capable of rapid adjustments of testosterone deposition in response to changes in their mate's ornamental plumage. However, androgen concentrations in the second egg and pretreatment male crown coloration were not correlated. Possibly, the combination of relatively small variation in UV coloration before treatment and the influence of unknown confounding variables in the correlative approach resulted in insufficient statistical power to detect such a correlation.

Aggression and fitness differences between plumage morphs in the common buzzard (Buteo buteo)

Boerner, M., Kruger, O. — 2008-12-29

Genetic plumage polymorphism in birds is increasingly recognized as a potentially important trait influencing birds’ life histories. In the common buzzard Buteo buteo, the 3 color morphs vary in lifetime reproductive success (LRS), but the proximate mechanisms leading to these differences are unknown. We first confirmed the stability of the fitness differences found previously, using a greatly extended LRS data set. To find potential causes for these differences, we experimentally studied variation in aggressive behavior of the morphs, both against an interspecific predator and intraspecific competitors. The morphs showed substantial variation in aggressive behavior. Light-colored males were most aggressive toward an interspecific predator, followed by intermediate and dark males. In females, this pattern was reversed, resulting in sex-related differences of aggression in 2 morphs. When defending their territory against intraspecific competitors, no absolute difference in aggression was found, but the morphs reacted strongest toward intruders of a morph similar to their own. This suggests that aggression differs both between and within morphs, leading to a complex pattern on the population level. Coupled with the strong fitness differences, our results suggest that the genetic basis of the polymorphism has far-reaching behavioral consequences.

Effect of predation risk, body size, and habitat characteristics on emigration decisions in mallards

Legagneux, P., Inchausti, P., Bourguemestre, F., Latraube, F., Bretagnolle, V. — 2008-12-29

Understanding the processes underlying emigrating behavior is fundamental to better understand animal dispersal. Because of the difficulties involved in carrying out controlled manipulation of the proximate drivers of emigration over large spatial scales, results from laboratory or small field enclosures suitable for small-bodied species remains to be validated in natural habitats. We investigate whether emigration is driven either by intraspecific competition or resulted from hunting-risk avoidance and assessed the effect of phenotypic variation on individual decisions. We made use of a quasi-experimental situation by using hunting recoveries of mallards (Anas platyrhynchos) released as ducklings in a fragmented landscape managed for duck hunting. Our results suggest an indirect effect of hunting on emigration. Body size plays a major role in modulating individual emigrating decisions, with small-bodied individuals emigrating more to escape from high levels of predation pressure while larger bodied individuals being more vulnerable to predation.

Females decide whether size matters: plastic mate preferences tuned to the intensity of male-male competition

Lehtonen, T. K., Lindstrom, K. — 2008-12-29

Recent studies have indicated that mating success of large males may improve under increasing levels of mating competition. This outcome is explained 1) if male mating competition is overriding female preferences for male traits that are unrelated to, or negatively correlated with, male size and dominance and, in so doing, dictates the distribution of matings or 2) if females alter their preferences with respect to large males when male–male competition is intense. Under both hypotheses, one could expect large, dominant males to be more successful under intense competition than under weak competition. However, only the first explanation predicts that male mating success under intense competition should be determined by dominance; traits that are unrelated to male dominance should be uncorrelated to mating success. In contrast, if females change their preferences (explanation 2), males with traits beneficial to females independent of the competitive environment can maintain a high mating success under all levels of male–male competition. We tested these alternatives using a small marine fish, the sand goby, Pomatoschistus minutus. The mating success of large males increased under conditions allowing intense male competition, whereas females showed a preference for good nest building independent of the level of competition. These findings suggest that females are in control of their choice by altering their preference for male size in response to the intensity of male–male competition rather than female preference being overshadowed by male dominance. This plasticity of preferences implies that the strength of sexual selection is not constant at the population level.

The deterrent effect of bird song in territory defense

de Kort, S. R., Eldermire, E. R. B., Cramer, E. R. A., Vehrencamp, S. L. — 2008-12-29

Using the responses of territory owners to playback to infer the territorial function of acoustic signals is common practice. However, difficulties with interpreting the results of such experiments have obscured our understanding of territorial signalling. For instance, a stronger response to playback is often interpreted as more aggressive, but there is no consensus as to whether this should be in response to the least or most threatening simulated intruder. Rather than following a gradual increase or decrease, the relationship between signal intensity and response strength may instead describe a peaked curve. We manipulated banded wren (Thryophilus pleurostictus) songs to simulate low-, median-, and high-performance singers and used these songs as stimuli in playback experiments. Banded wrens were less likely to approach the high-performance stimulus compared with the low- and median-performance stimuli. However, the birds that did approach the high-performance stimulus sang more than those that approached the low-performance stimulus. In addition, birds were more likely to match the songs when exposed to the median- and high-performance stimuli compared with the low-performance stimuli, and song matching predicted approach behavior. These results are in accordance with theoretical models of aggressive encounters in which low-performance opponents are challenged without further assessment. Median- and high-performance opponents, however, may require further assessment, and the latter may be perceived as too intimidating for approach.

Experimental evaluation of sex differences in territory acquisition in a cooperatively breeding bird

Eikenaar, C., Richardson, D. S., Brouwer, L., Bristol, R., Komdeur, J. — 2008-12-29

In many species, territory ownership is a prerequisite for reproduction; consequently, factors that affect success in territory acquisition can have a large impact on fitness. When competing for territories, some individuals may have an advantage if, for example, they are phenotypically superior or more familiar with the site than others. The relative importance of the many factors involved in territory acquisition is, at present, unclear. We studied patterns of natural territory acquisition in a closed and saturated population of Seychelles warblers. Furthermore, by removing breeders, we experimentally investigated the relative importance, to territory acquisition, of a range of factors and assessed whether this differed between the sexes. In both sexes, the main route to natural territory acquisition was to disperse from the natal territory to immediately claim a vacant dominant position. Males were older than females when acquiring a territory for the first time. In the removal experiment, for both sexes, the proximity of an individual's natal territory to a vacant dominant position was positively related to the individual's chance of claiming the vacancy. Older males were more likely to gain an experimental vacant dominant position than were young males, whereas age did not affect territory acquisition in females. In the Seychelles warbler, the degree of intrasexual competition for territory ownership may be stronger for males than for females because territory ownership is a prerequisite for male reproduction, whereas females can reproduce on their natal territory. In such competition, young males subsequently lose out to older ones.

Begging and cowbirds: brood parasites make hosts scream louder

Boncoraglio, G., Saino, N., Garamszegi, L. Z. — 2008-12-29

Avian brood parasites have evolved striking begging ability that often allows them to prevail over the host progeny in competition for parental resources. Host young are therefore selected by brood parasites to evolve behavioral strategies that reduce the cost of parasitism. We tested the prediction that the intensity of nestling begging displays functioning to attract parental care increases across species with the frequency of parasitism by the brown-headed cowbird (Molothrus ater). This was expected because host young should try to prevail over highly competitive parasitic broodmates in scramble interactions, act more selfishly when frequency of parasitism is high because brood parasites often affect more severely host condition than conspecific broodmates, and discount the kin selection costs of subtracting resources to unrelated parasites. Across 31 North American host species, begging loudness positively covaried with parasitism rate in Passerines, and such effect was stronger in species with small compared with large clutches. Begging loudness increased with brood parasitism and nest predation among the most suitable host species. These results held after controlling for concomitant ecological factors and for common ancestry effects. Our results support the hypothesis that avian brood parasitism has played a role in the evolution of begging behavior of host young.

Do male paternity guards ensure female fidelity in a duetting fairy-wren?

Hall, M. L., Peters, A. — 2008-12-29

Most socially monogamous bird species engage in extra-pair mating, and consequently males may adopt various behavioral strategies to guard paternity. However, the relationship between mate guarding and extra-pair paternity is unclear: low levels of extra-pair paternity can be associated either with no mate guarding or with intense mate guarding. We investigate paternity guards in the purple-crowned fairy-wren (Malurus coronatus), a duetting species where extra-pair paternity is rare. This species is unusual in a genus known for extremely high levels of extra-pair mating. We examine the behavioral interactions between the sexes underlying these low rates of extra-pair paternity and show that male purple-crowned fairy-wrens do not use frequent copulation or courtship feeding to assure paternity or guard females acoustically by duetting. Males maintain close proximity to females both when they are fertile and when they are not breeding and do not invest in courtship displays to extra-pair females. Consistent with predictions of theoretical models, low extra-pair paternity in this species may be related to female fidelity rather than male paternity assurance strategies, but short-term removal of males would be necessary to test this experimentally.

Erratum

2008-12-29