Behavioral Ecology - current issue

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.