Behavioral Ecology - Advance Access

Male songbirds provide indirect parental care by guarding females during incubation

Fedy, B. C., Martin, T. E. — 2009-07-02

Across many taxa, guarding of fertile mates is a widespread tactic that enhances paternity assurance. However, guarding of mates can also occur during the nonfertile period, and the fitness benefits of this behavior are unclear. Male songbirds, for example, sometimes guard nonfertile females during foraging recesses from incubation. We hypothesized that guarding postreproductive mates may have important, but unrecognized, benefits by enhancing female foraging efficiency, thereby increasing time spent incubating eggs. We tested the hypothesis in 2 songbird species by examining female behavior during natural and experimentally induced absences of males. Male absence caused increased vigilance in foraging females that decreased their efficiency and resulted in less time spent incubating eggs. Male guarding of nonfertile females can thus provide a previously unrecognized form of indirect parental care.

Breeding systems, climate, and the evolution of migration in shorebirds

Garcia-Pena, G. E., Thomas, G. H., Reynolds, J. D., Szekely, T. — 2009-07-02

Migratory behavior incurs energetic costs that may influence the time and energy available for reproduction including territory establishment, courtship, pair formation, incubation, and brood care. Conversely pair formation and parental care may leave less time and energy available for migration and other nonbreeding behaviors. Therefore, natural selection favoring migratory behavior may influence breeding system evolution and vice versa. We used phylogenetic comparative methods to investigate relationships between migration distance and the wide diversity of breeding systems in shorebirds (sandpipers, plovers and allies). Consistent with previous studies, we show that long-distance migration is associated with reduced male care across shorebird species. We then use directional phylogenetic analyses to test whether migration distances have tended to increase or decrease over time and whether such evolutionary changes have preceded or followed changes in parental care. We show that evolutionary transitions from short-distance migration to long-distance migration have coevolved with changes from full biparental care to reduced male care. Furthermore, our directional analyses suggest that increments in migration distance are more likely to have preceded reductions in male care than vice versa. We also show that male polygamy is associated with northern breeding latitudes when the nonbreeding latitude is controlled statistically. Although this suggests that mating systems, parental care, and migration have more complex relationships than previously thought, our results are consistent with the hypothesis that migration influences breeding system evolution.

Queen reproductive state modulates pheromone production and queen-worker interactions in honeybees

Kocher, S. D., Richard, F.-J., Tarpy, D. R., Grozinger, C. M. — 2009-07-02

The mandibular glands of queen honeybees produce a pheromone that modulates many aspects of worker honeybee physiology and behavior and is critical for colony social organization. The exact chemical blend produced by the queen differs between virgin and mated, laying queens. Here, we investigate the role of mating and reproductive state on queen pheromone production and worker responses. Virgin queens, naturally mated queens, and queens instrumentally inseminated with either semen or saline were collected 2 days after mating or insemination. Naturally mated queens had the most activated ovaries and the most distinct chemical profile in their mandibular glands. Instrumentally inseminated queens were intermediate between virgins and naturally mated queens for both ovary activation and chemical profiles. There were no significant differences between semen- and saline-inseminated queens. Workers were preferentially attracted to the mandibular gland extracts from queens with significantly more activated ovaries. These studies suggest that the queen pheromone blend is modulated by the reproductive status of the queens, and workers can detect these subtle differences and are more responsive to queens with higher reproductive potential. Furthermore, it appears as if insemination substance does not strongly affect physiological characteristics of honeybee queens 2 days after insemination, suggesting that the insemination process or volume is responsible for stimulating these early postmating changes in honeybee queens.

Should females prefer males with elaborate nests?

Lehtonen, T. K., Wong, B. B.M. — 2009-07-01

It is widely assumed that when males alone are responsible for nest building, nest characteristics should reflect the quality of its owner and that the nest itself should be an important cue in female choice. This, however, does not always have to be the case if, for example, nest attributes are an unreliable reflection of male quality. Here, we investigate whether females should prefer nest characteristics in a small marine fish, the sand goby, Pomatoschistus minutus. Previous findings suggest that female sand gobies prefer males that possess well-built nests (based on the amount of sand piled on top). It was unclear, however, whether females chose males based on the quality of the nest per se or some other, correlated quality of the builder. In the current study, we found conflicting evidence of whether males in good condition are able to bring a greater percentage of eggs to the hatching stage. In a field investigation, we also found that the relationship between body condition and the degree of nest construction was temporally unstable. Furthermore, when we experimentally disentangled nest quality from other male traits, we showed that females did not prefer to spawn with males that had the most elaborate nests. Together, these results suggest that females do not necessarily prefer males based on nest characteristics and may, instead, rely on multiple cues when choosing mates, the relative importance of which should vary depending on context.

Remotely sensed productivity, regional home range selection, and local range use by an omnivorous primate

Willems, E. P., Barton, R. A., Hill, R. A. — 2009-06-30

Remote sensing of the environment has proved an invaluable tool to the study of animal ecology at continental to regional scales. Here, we investigated the utility of a remotely sensed index of plant productivity (the normalized difference vegetation index [NDVI]) at a much finer spatial scale to account for the range use of an omnivorous primate (the vervet monkey: Cercopithecus aethiops) foraging in a multipredator environment. Vervet monkey home range location suggested that the animals prefer areas with elevated productivity and reduced seasonality as indexed by simple NDVI metrics. Within the annual home range area, monthly NDVI values were linearly related to field measurements of leaf cover and quadratically associated with vervet monkey food availability. Temporal variation in parameters of local range use could subsequently be expressed in terms of local NDVI: Monthly averaged day journey length showed a second-order polynomial response, and the amount of time the monkeys spent on the ground increased with group size whereas linearly decreasing with monthly NDVI. The first finding signifies a behavioral response to food availability, whereas the latter is interpreted as an antipredatory response to changes in habitat visibility, associated with leaf cover. As a spatially explicit and temporally varying measure of habitat structure and productivity, the NDVI thus offers considerable scope for studies of animal behavioral ecology not only at broad spatiotemporal scales but also at a much finer grained level of analysis.

Alternative foraging tactics and risk taking in brook charr (Salvelinus fontinalis)

Farwell, M., McLaughlin, R. L. — 2009-06-29

Recently emerged brook charr (Salvelinus fontinalis) foraging in still-water pools along the sides of streams tend to be sedentary, feeding from the lower portion of the water column (sitting and waiting), or active, feeding from the upper portion of the water column (active search). Individuals exhibiting intermediate behavior are observed less frequently. We assessed the perceptual, energetic, and locomotor bases of the individual differences in foraging tactics by testing whether an individual's activity while searching for prey in the field was linked to its willingness to take risks, resting metabolic rate (RMR), and swimming capacity. Proportion of time an individual spent moving during prey search was quantified in the field, the individual was captured, and willingness to take risks (field), resting oxygen consumption (lab), and locomotor ability (lab) were measured. Individuals that spent a lesser proportion of time moving in the field took longer to exit from a dark tube into an unfamiliar field environment, and delayed their exit times more in response to a novel object, than did individuals that spent a greater proportion of time moving in the field. Proportion of time spent moving in the field was unrelated to resting oxygen consumption and swimming capacity measured in the laboratory. Dispositions in foraging behavior and risk taking early in life could influence encounter rates with novel prey and habitats, which are important steps in the initial stages of resource polymorphisms.

Vigilance and predator detection vary between avian species with different visual acuity and coverage

Tisdale, V., Fernandez-Juricic, E. — 2009-06-23

Interspecific variations in avian visual systems have been suggested to influence antipredator strategies, yet little empirical evidence exists on how morphological and ecological factors associated with visual properties can constraint predator detection. We investigated antipredator responses (predator detection probabilities and vigilance behavior) in 2 species with different visual properties (European starlings, Sturus vulgaris, have higher acuity and wider blind areas at the rear of their heads than house sparrows, Passer domesticus) in relation to distance to the predator, body posture, and head orientation. Visual acuity may affect the distance at which a predator is detected; while the size of the blind area may influence the body postures and head orientation with the highest predator detection probabilities. Distance to the decreased predator detection probabilities of house sparrows (lower acuity) but not those of European starlings. Certain body postures and head orientations, influenced both species despite the interspecific differences in visual field configuration. Times allocated to scanning were similar, but European starlings had longer head-up scan bouts likely to enhance scanning coverage, whereas house sparrows had higher head-up scan rates probably to reduce the length of interscan intervals. We discuss alternative interpretations; however, our findings suggest that sensory configurations may limit the effectiveness of some antipredator strategies under certain ecological conditions, which has implications for understanding the evolution of different behavioral mechanisms that reduce predation risk.

Flexible cuckoo chick-rejection rules in the superb fairy-wren

Langmore, N. E., Cockburn, A., Russell, A. F., Kilner, R. M. — 2009-06-22

Recognition of brood parasitic cuckoo nestlings poses a challenge to hosts because cues expressed by cuckoos and host young may be very similar. In theory, hosts should use flexible recognition rules that maximize the likelihood of rejecting cuckoo nestlings while minimizing the risk of rejecting their own young. Our previous work revealed that female superb fairy-wrens Malurus cyaneus often abandoned nestling cuckoos and that the presence of a single chick in the nest was 1 trigger for abandonment because fairy-wrens also sometimes abandoned a single fairy-wren chick. Here we use a combination of 20 years of observational data, a cross-fostering experiment, and a brood size reduction experiment to determine the basis for individual variability in the chick-rejection rules of superb fairy-wrens in response to parasitism by Horsfield's bronze-cuckoos Chalcites basalis. We show that the decision to abandon a single chick is based on integration of learned recognition cues and external cues. Experienced females were relatively more likely to abandon a single cuckoo chick and accept a single fairy-wren chick than naive females. Breeding experience therefore facilitates the ability to make an accurate rejection decision, perhaps through learned refinement of the recognition template. In addition, fairy-wrens modified their rejection threshold in relation to the presence of adult cuckoos in the population, becoming more likely to abandon single nestlings with increasing risk of parasitism. By using these flexible rejection rules, female superb fairy-wrens are more likely to defend themselves successfully against exploitation by the cuckoo and are less prone to mistakenly reject their own offspring.

Condition-dependent alternative mating tactics in a sexually cannibalistic wolf spider

Wilgers, D. J., Nicholas, A. C., Reed, D. H., Stratton, G. E., Hebets, E. A. — 2009-06-22

Condition-dependent male mating tactics often involve high-quality males capitalizing on the outcome of intersexual selection, whereas low-quality males use behaviors that circumvent female choice to achieve copulation. In the wolf spider Rabidosa punctulata, males display 2 distinct mating tactics: 1) "courtship"—consisting of visual and seismic components or 2) "direct mount"—consisting of males grappling/holding females until they assume a copulatory posture. We tested for condition-dependent tactic expression using both field-collected individuals (representing natural variation in body mass and condition) and individuals whose diet we manipulated in the laboratory (representing extreme divergences in body size and condition). Across both natural and diet manipulated individuals, mating tactic was found to be condition dependent; however, the pattern of tactic expression was initially unexpected. Larger males with better body condition primarily adopted a direct mount tactic, whereas smaller males with poorer body condition primarily utilized courtship. Across all males, copulation success tended to be greatest for the direct mount tactic. We suggest that small, poor condition males predominantly utilize the less-successful mating tactic (courtship) in part due to their increased susceptibility to female cannibalism when attempting a direct mount.

Variation in dominance hierarchies among group-living animals: modeling stability and the likelihood of coalitions

Broom, M., Koenig, A., Borries, C. — 2009-06-22

Dominance hierarchies based on resource holding potential (RHP) or age are central to the social structure of many group-living animals. Nonhuman primate females and some other mammals are unusual because ranks can depend on kin support or follow an inverse age-graded pattern independent of kin. Whereas the former hierarchies are mediated by frequent coalitions leading to long-term stability, the latter are unstable due to individualistic contests and occasional revolutionary coalitions. We use mathematical modeling to investigate this variation, based on the interaction of triads, subgroups of the group consisting of 3 individuals. This produces 4 key situations, namely that relationships can be either stable or unstable, and involving coalitionary support or not. Varying the parameter values of our model and applying it to real-world examples, we conclude that the emergence of different dominance hierarchies depends on 6 major factors. In addition to RHP, reproductive skew and demography often suggested previously, we stress the importance of 1) the variance in relatedness; 2) the costs and likelihood of interventions; and 3) the value of resources and rank in different life stages, which might be tightly linked to the life history of a species.

Evolution and life-history correlates of female song in the New World blackbirds

Price, J. J. — 2009-06-19

Female song is much more prevalent in tropical than in temperate songbirds but, we know surprisingly little about the evolutionary origins of this striking latitudinal difference. Here I reconstruct the evolution of female song in the New World blackbird family (Icteridae) and compare historical changes in this trait to changes in several other life-history characters: social mating system, nesting pattern, and migratory behavior. Reconstructions using both parsimony and maximum likelihood methods show that female song has been lost repeatedly in this clade and that tropical ancestors with frequent female song almost invariably were monogamous, had dispersed nest sites, and were nonmigratory. Losses of female song were not consistently associated with changes in any single life-history characteristic across the family, but rather appear to have occurred for different reasons in different lineages, including the evolution of migration in the oriole genus (Icterus), the evolution of brood parasitism in the cowbirds (Molothrus), and the evolution of polygynous, colonial breeding in the oropendolas (Psarocolius, Gymnostinops) and caciques (Cacicus). These results support previous suggestions that the prevalence of female song in the tropics is largely explained by the life-history traits associated with tropical habitats.

Age-related reproductive performance in the parental burying beetle, Nicrophorus orbicollis

Trumbo, S. T. — 2009-06-19

Age-related improvements in reproductive performance have been explained by the selection, constraint, and restraint hypotheses (increased performance related to greater effort and a declining residual reproductive value). Using a laboratory population of the burying beetle, Nicrophorus orbicollis, 6 measures of performance were assessed. Consideration of the selection and constraint hypotheses was minimized by reducing mortality to near zero and comparing reproductive outcomes of equal-sized females with equivalent mating and breeding experience. Young and old females were provided a resource for breeding (mouse carcass) and were confronted by a potentially infanticidal intruder (young or old) after offspring were present in the nest. Younger females were more likely to experience a brood failure and were slower to oviposit eggs. Younger intruding females also were less likely to usurp a carcass from a resident female. Age, however, did not affect any measure of reproductive performance once a female had a brood. There were no differences between young and old females in the number and mass of offspring in original or replacement broods, or in the probability of successfully defending offspring against an intruder. In a follow-up experiment with smaller carcasses, there was, again, no significant difference in the ability of young and old females to defend their brood. In N. orbicollis, the reproductive restraint observed in younger females appears to inhibit the initiation but not the continuance of reproduction. The usefulness of the residual reproductive, targeted reproductive effort and the maturational limits hypotheses to explain age-related variation in reproductive performance are discussed.

Adaptive learning in the foraging behavior of the predatory mite Phytoseiulus persimilis

Rahmani, H., Hoffmann, D., Walzer, A., Schausberger, P. — 2009-06-19

Arthropod learning in the context of food acceptance and choice is commonly assumed to be adaptive but documentation of the adaptive value is scarce and lacking for true predators. We examined learning in juveniles of the predatory mite Phytoseiulus persimilis, which has a feeding preference for herbivorous spider mites but may use conspecific larvae as alternative prey. Adult predator females that had experienced conspecific larvae as prey during their juvenile phase attacked conspecific prey earlier than naïve predator females did. Shorter latency to attack of adult females did not depend on the length of exposure to alternative prey in the juvenile phase (24 h or whole juvenile phase). Experience decreased the predation rates of adult females but enhanced their survival chances when feeding on the alternative prey. The suggested proximate cause for enhanced survival of experienced females was greater energetic efficiency in foraging as compared with naïve females. Experience resulted in faster prey recognition and acceptance (indicated by shorter latency to attack) increasing prey profitability, and optimized daily predation rates. Ultimately, juvenile learning allows P. persimilis to better cope with shortage of the innately preferred spider mite prey.

Individual quality and reproductive effort mirrored in white wing plumage in both sexes of south polar skuas

2009-06-17

It is well established that female choice may lead to sexual selection on quality-revealing ornaments in males. However, in many species, both sexes display conspicuous characters, which may reflect individual quality and condition. We examined the correlations between measures of individual condition, reproductive performance and variation in size and whiteness of white wing patches in both sexes of the south polar skua (Catharacta maccormicki). Females with a whiter patch had a lower mean clutch size and higher survival, and males with whiter patches had higher immune responses against the injected immunogen tetanus. Birds with a larger white patch, on the other hand, had a higher mass loss, and females with large white patches laid larger eggs and had reduced survival probability. Thus, variation in 2 measured aspects of the wing patches in the south polar skua seems to mirror a gradient of reproductive investment strategies; from 1) reduced reproductive investment, better immune defense, and higher survival in birds with more intensely white wing patches to 2) high reproductive investment but survival costs in birds with larger patches.

Cryptic female choice by female control of oviposition timing in a soldier fly

Barbosa, F. — 2009-06-17

There is substantial evidence that cryptic female choice (CFC) is present in numerous taxa. Several mechanisms have been proposed for CFC; however, we only have experimental evidence for a few of them. Female control of oviposition timing is a potentially widespread mechanism of CFC, but it has never been experimentally demonstrated. The aims of this study are to test 2 critical predictions of the hypothesis that CFC through control of oviposition timing occurs in the soldier fly Merosargus cingulatus: 1) to determine if M. cingulatus females are less likely to oviposit immediately after mating when the male does not perform copulatory courtship than when he does and 2) to determine if failure to immediately oviposit by the female results in lower reproductive success for the male she just mated with. To answer the first question, I compared the oviposition behavior of females that mated with control males versus females that mated with manipulated males that could not perform copulatory courtship. I showed that M. cingulatus females fail to oviposit immediately after copulation when males do not perform copulatory courtship. To answer the second question, I showed that there is last male sperm precedence in M. cingulatus. Because the last male to mate fertilizes most of the female's eggs, a male will benefit when females oviposit immediately after mating with him and before remating with another male.

Song types, song performance, and the use of repertoires in dark-eyed juncos (Junco hyemalis)

Cardoso, G. C., Atwell, J. W., Ketterson, E. D., Price, T. D. — 2009-06-15

Song performance encompasses the idea of how physiologically demanding different songs are to sing, and this is thought to reflect the singing ability of individual birds. In the dark-eyed junco (Junco hyemalis), each male has a repertoire of song types, some of which are shared with other males in the population. We used 4 measures of performance, based on trade-offs between song traits, to test if song performance is consistent among the song types making up the repertoire of individual males. We also tested if song types differ consistently in performance regardless of which males sing them. We found low but significant correlations of performance measures among the song types of individual males. This contrasts with highly consistent differences in performance among song types, regardless of which males sing them. We conclude that performance of single song types, as evaluated by trade-off–based measures of performance, gives little information about male singing ability. As song types differ in performance, we asked if males use the song types in their repertoires differently. We found that juncos use higher performance song types during bouts of more motivated singing, as evaluated by the length of songs, suggesting that song types may be preferentially used in different contexts depending on their performance.

Foraging theory and partial consumption in a tardigrade-nematode system

Hohberg, K., Traunspurger, W. — 2009-06-15

We investigated foraging behavior of a carnivorous eutardigrade, Macrobiotus richtersi, living in soil or fresh water sediments, where the distribution of nematode prey is heterogeneous and where, consequently, the value of information about prey availability should be high. We directly observed diet choice in various mixtures of 2 size classes of nematodes Acrobeloides nanus, A1 (small prey) and A2 (large prey), differing in profitability (biomass divided by handling time) but not in digestive quality. At various prey densities of A2, we measured how much of each prey item was consumed as a function of search time. Additionally, we derived cumulative biomass uptake rates per single A2 prey from 154 complete feeding acts, and we used the marginal value theorem (MVT) to predict optimal residence time and prey exploitation as a function of environmental quality and search time. Macrobiotus richtersi did not preferentially select the more profitable size class A2 but increased its biomass uptake rate by modifying the amount consumed per prey item (partial consumption): when encounter rates were high and there were more high-quality prey, the tardigrade abandoned food more quickly and consumed less biomass per captured prey. We conclude that 1) hungry tardigrades under food-rich or food-limited conditions maximize biomass gain according to MVT and 2) tardigrades differentiate at least 2 prey situations, high supply (at high encounter rates) and low supply (at moderate and low prey densities). 3) Partial consumption, performed under food-rich conditions or by a satiated predator, leads to a reduction of prey numbers up to 3 times as much as under food-poor conditions.

Sex-specific parental strategies according to the sex of offspring in the Adelie penguin

2009-06-08

In sexually dimorphic species, the sex of the offspring may induce different constraints for parents. At the same time, within pairs, males and females may have conflicting optimal reproductive strategies. As a result, they may adjust their level of parental investment differently according to the sex of the young. In this study, we examined whether Adélie penguin (Pygoscelis adeliae) chicks were sexually dimorphic and whether parents adjusted their parental investment accordingly. Male chicks were on average approximately 10% heavier than female chicks but not larger. Despite the presumed additional cost associated with male chick growth, no fitness cost differences were observed between parents rearing 1 chick whatever its sex: Adult body mass changes and resight rates during the subsequent breeding season were similar. However, the sex of offspring affected the duration of foraging trips during the early guard stage: At this stage, female adults rearing a female chick performed longer foraging trips than female adults rearing a male chick and males rearing either a male or a female chick. We propose that, because female adults present a lower survival rate after a breeding attempt, they are more prone to modify their level of parental investment than male adults. Moreover, the modulation of the foraging behavior by female adults according to the sex of the chick is likely to reduce intraspecific competition at a time when resource availability at sea is not optimal and when food requirement for female chicks may be lower than for male chicks.

Behavior of kestrels feeding on frugivorous lizards: implications for secondary seed dispersal

Padilla, D. P., Nogales, M. — 2009-06-08

Secondary seed dispersal is a multistep system that includes 2 or more dispersal processes that can increase the distance from which seeds arrive. This phenomenon is relatively common in some habitats of subtropical oceanic islands due to the frequent frugivore–predator interactions found in them. In this study, we describe how the Eurasian Kestrel is an effective disperser of plants in the secondary seed dispersal process, through interaction with frugivorous lizards. Experiments using captive wild kestrels, along with field data, showed that predation of kestrels on lizards leads to a secondary seed dispersal with 2 possible outcomes: 1) most seeds (89%) are not consumed by kestrels because they reject the lizards’ digestive tracts and so receive only the gut treatment of lizards and 2) a small fraction of seeds (11%) appeared inside the kestrel pellets as a result of indirect ingestion by this raptor, thus undergoing double gut treatment. So, 2 different seed dispersal distances may result from this interaction: 1) when the kestrels capture the lizard and transport it to a perch where the seed-containing guts are discarded and 2) when they indirectly ingest a few seeds from lizards, consequently increasing the dispersal distance. Seeds from the Macaronesian plant species Rubia fruticosa were tested, finding that those passed through kestrels had a lower germinability than those that remained inside the rejected lizards’ digestive tracts, which had similar germination rates to those from control plants (uningested seeds). The kestrel can be considered an important and effective long-distance seed disperser due to the high abundance of frugivorous lizards in their diet, their stereotyped consumption behavior, and the effectiveness of their seed dispersal.

The influence of the acoustic community on songs of birds in a neotropical rain forest

Luther, D. — 2009-06-08

The efficacy of communication depends on the detection of species-specific signals in background noise that includes other species’ signals. To avoid confusion with each others’ signals, species should partition communication space. I investigated this possibility for the dawn chorus of birds in an Amazonian rain forest. Acoustic censuses at a location in Matto Grosso, Brazil, detected 82 sedentary species of birds that sang frequently during dawn choruses. Eleven features of these species’ songs were analyzed to characterize the acoustic space of this community. The Euclidean distances between species’ songs in this acoustic space were then used to investigate spatial, temporal, and phylogenetic influences on the divergence of songs. Songs of species in the same stratum of the forest and during the same 30-min interval had the most dispersed songs. Songs of congeners and family members were more dispersed than songs of random species. These results indicate that in this complex acoustic environment, species singing at the same place and time partition signal space. These species either choose times and places for singing to minimize acoustic interference from other species or they have evolved different songs to reduce this interference.

Condition-dependent mate choice and its implications for population differentiation in the wolf spider Pirata piraticus

Eraly, D., Hendrickx, F., Lens, L. — 2009-06-08

When populations face different environmental conditions, both local adaptation and phenotypic plasticity may cause interpopulation divergence of behavioral or phenotypic properties on which mate choice is based. If sustained, this may result in genetic differentiation even in the presence of extant gene flow. Condition dependence of mate choice is one of the main mechanisms explaining these environmental effects. We tested whether experimental food stress affects mate choice in male and female Pirata piraticus spiders from one heavily polluted and one unpolluted reference population. Compared with control females, food-stressed females from the reference population showed a decreased probability of copulation and preferred smaller mates. Females from the polluted population, in contrast, did not show a significant response to food stress and showed size-assortative mating, most strongly under food stress. We explain these results in 2 complementary ways. First, spiders from populations that are not adapted to cope with stress may be less willing to mate when eggs are not fully matured. Second, food-deprived females may show a larger responsiveness toward smaller males because the latter resemble prey more and hungry females tend to attack moving objects more often. Results from this study support the prediction that variation in body condition, driven by local ecological factors, may affect mating behavior and may ultimately lead to population divergence in important life-history traits such as body size.

Mate choice copying and mate quality bias: different processes, different species

Vakirtzis, A., Roberts, S. C. — 2009-06-04

Mate choice copying is the most studied type of nonindependent mate choice, in which the probability of a male being chosen by a female increases if he has previously been chosen by other females and decreases if he has been rejected. Recent studies suggest that what can sometimes influence females is not so much a male's success at securing mates but the quality of the females that choose him. Here, we show that, though hitherto described as mate choice copying, this type of nonindependent mate choice is characterized by distinct evolutionary dynamics and ecological requirements, will have usually evolved in different species, and must therefore be urgently distinguished from mate choice copying. The term mate quality bias is suggested as an appropriate description of this phenomenon.

An avian eavesdropping network: alarm signal reliability and heterospecific response

Magrath, R. D., Pitcher, B. J., Gardner, J. L. — 2009-06-01

Alarm calls potentially provide information about predators to heterospecifics, but little is known about patterns of eavesdropping among species. Many cases of eavesdropping in birds and mammals involve social species in mixed-species groups, but this is not always true and the reliability of information may also be critical. We used a playback experiment and observations of natural alarm calling to test for understanding of aerial "hawk" alarms among 3 species of passerine and assess call reliability. Superb fairy-wrens and white-browed scrubwrens are ecologically similar and can share mixed-species flocks, whereas New Holland honeyeaters are ecologically distinct and do not flock with the other species. Fairy-wrens and scrubwrens fled to cover to each other's alarm calls, but they also both fled to honeyeater alarms. Honeyeaters fled to scrubwren but usually not fairy-wren alarms. The pattern of heterospecific responses appears related to call reliability from each species’ perspective. Honeyeaters called only to predators of all 3 species and so provided reliable information to all. From a honeyeater's perspective, fairy-wrens were least reliable, as they gave 52% of their calls to nonpredators, whereas scrubwrens gave only 18% to nonpredators. However, from a scrubwren's perspective, fairy-wrens were largely reliable because most calls to nonpredators were to red wattlebirds, which pose a physical threat to fairy-wrens and scrubwrens but not honeyeaters. We conclude that there can be mutual responses to alarm calls between ecologically distinct species, that responses can be symmetrical or asymmetrical between species, and that call reliability appears to affect response.

Nutritional enrichment increases courtship intensity and improves mating success in male spiders

Lomborg, J. P., Toft, S. — 2009-06-01

The development of male sexual ornaments and the intensity of male courtship behavior are often used by females as criteria for mate choice and by other males to evaluate the strength of a rival. We tested the hypotheses that courtship intensity and mating success depend on the males' nutritional status (enriched or deficient) and that courtship intensity predicts mating success in males of the same nutritional status. We used wolf spiders, Pardosa prativaga, which have an elaborate display of courtship behaviors, including encircling, palp vibrations, abdomen vibrations, hopping, etc. Viability parameters indicated enhanced condition of enriched males. Mating success was higher for nutrient-enriched males in direct competition with deficient males. Enriched males had higher courtship intensity and were also larger (carapace width) but not heavier than deficient males. The statistical analysis indicated that diet effects on courtship intensity were indirect, through its effect on size. In competition tests between males of equal mass and the same diet treatment, the previously most active male (high levels of palp vibrating, abdomen vibrating, and hopping) had the highest mating success, though this result depended on male nutrient status. The widely used residual condition index (RCI) did not distinguish the treatments. It is suggested that the index is unsuitable in a situation of nutritional stress caused by nutrient imbalance. The results underscore the importance of nutrient balancing to all aspects of performance also in predatory animals.

Beyond waist-hip ratio: experimental multivariate evidence that average women's torsos are most attractive

Donohoe, M. L., von Hippel, W., Brooks, R. C. — 2009-05-21

One of the most iconic findings in human behavioral ecology is the fact that women with waist-hip ratios (WHRs) of approximately 0.7 are most attractive and that this ratio indicates maximum fecundity and reproductive value. However, the effects of WHR and of other indices of body shape and size on attractiveness are far from fully resolved. We adopt a recently developed method that combines multivariate manipulation of experimental stimuli with evolutionary selection analysis to test the linear and nonlinear effects of waist, hip, and shoulder width and the interactions between these traits on the attractiveness of 200 line-drawn models to 100 men. There was no general support that WHR or body mass (expressed as perimeter–area ratio) significantly influences attractiveness. There was, however, strong preference for average values of all 3 traits indicating that attractiveness is due to the tight integration of these 3 traits. We plot the mean waist and hip sizes of 8 samples of women on our response surface, including Playboy centerfolds, models from the 1920s and 1990s, Australian escorts, and Australian women in 4 different age categories (collectively we refer to this latter group as the "regular women"). The regular women in the 25- to 44-year age-group were closest to the peak attractiveness value on our response surface. Our results highlight the strong integration of and interrelationships among different parts of the body as determinants of attractiveness.

Larger colonies do not have more specialized workers in the ant Temnothorax albipennis

Dornhaus, A., Holley, J.-A., Franks, N. R. — 2009-05-19

Social insects are distinguished by their extraordinary degree of cooperation and the complexity of their group organization. However, a high proportion of individuals (often >50% at any one time) in a social insect colony tend to be inactive. It has been hypothesized that larger colonies can afford such inactivity because of efficiencies gained through stronger division of labor. We quantify the degree to which colonies of different sizes exhibit division of labor, and what proportion tends to be inactive, in the ant Temnothorax albipennis. Colony size neither influenced individual specialization nor overall division of labor in this species and larger colonies did not show a higher proportion of inactive workers. Interestingly, small colonies seemed to rely more on a small number of high-performance workers: the proportion of work performed by the single most active worker is significantly higher in smaller colonies for several tasks. More research is needed to resolve when and how colony size affects collective organization and division of labor in insect colonies.

Who Cares? Between-group variation in alloparental caregiving in sperm whales

Gero, S., Engelhaupt, D., Rendell, L., Whitehead, H. — 2009-05-19

Although the details of the various systems of allocare in primates, rodents, and carnivores have been well described, little is known about the existence of alloparental care in cetaceans. It is believed that the matrilineal social organization of the sperm whale functions to provide vigilant allomothers for calves at the surface while mothers make deep dives for food. Sperm whale females do have a system of allocare, but details are unknown. This study aimed to elucidate sperm whale allocare, in particular: who escorts whose calf and whether or not calves suckle from nonparent females. Using photo identification and behavioral calf follows, we examined patterns of adult–infant interactions for 23 sperm whale calves in the Sargasso and Caribbean Seas. Although multiple individuals of both sexes escorted the calves, the system of escorting differed between the 2 sites. For all calves studied in the Caribbean, we found that 1 female provided most of the allocare but did not nurse the calf, whereas in the Sargasso, multiple females provided care for, and nursed, the young. We discuss differences between populations that may have resulted in the observed differences in these 2 systems of allocare and how these findings fit with current hypotheses on the roles of kin selection and reciprocal altruism in cooperative care in mammals.

Reproductive effort in biparental care: an experimental study in long-lived Cape gannets

Bijleveld, A. I., Mullers, R. H.E. — 2009-05-19

Life-history theory predicts a trade-off between current and future reproduction, such that long-lived species should not increase their reproductive effort (RE) at a cost to their own survival. In species with long-term pair bonds and biparental care, each parent must balance its reproductive investment against that of its partner. Although the effects of "handicapping" studies on the focal individual are sometimes difficult to interpret, they are a powerful approach for investigating compensatory responses of the partner. In the present experiment, we manipulated flying ability of one parent in long-lived Cape gannets (Morus capensis), thereby indirectly increasing the demands on RE of the unmanipulated partner. Handicapped birds doubled their foraging trip duration and reduced nest attendance. Their partners showed behavioral compensation via increased nest attendance for chicks younger than 30 days and increased foraging trip frequency for older chicks. The behavioral responses of partners did not fully compensate for the reduced care of handicapped adults. For manipulated nests, overall foraging trip frequency was 21% lower, chicks were left unattended at 5 days younger, and their growth and survival was reduced compared with control nests. Handicapped adults lost 10% of their body mass during the experiment, but their partners showed no decrease in body mass. Our results show that long-lived Cape gannets can increase current RE when needed, without negative effects on body condition or survival. The reduced care of one parent was partly compensated for by its partner, and remaining costs were borne by the chick.

Protein content of diets dictates the daily energy intake of a free-ranging primate

2009-05-19

An important goal in nutritional ecology is to understand what governs the diet selection of free-living animals. Relevant information is however scarce because of the considerable challenges of collecting and interpreting such data. Here we use recent advances in nutritional theory to analyze data on food selection and nutrient intake by wild spider monkeys (Ateles chamek). We show that hypotheses traditionally used to explain vertebrate diet selection, such as energy or protein maximization, or avoidance of plant secondary metabolites, cannot explain the observed pattern of nutrient intake. Instead, spider monkeys maintained a stable daily protein intake but allowed total energy intake to vary as a function of the composition of available food items. A similar "protein-leverage effect" has been reported in humans for whom it appears to play a role in the development of obesity.

Personality traits in wild starlings: exploration behavior and environmental sensitivity

Minderman, J., Reid, J. M., Evans, P. G.H., Whittingham, M. J. — 2009-05-14

Animal personalities, defined as consistent and correlated individual differences in behavioral traits, are suggested to be common in the animal kingdom and can have important fitness consequences. Individual differences in sensitivity to environmental cues are predicted to be part of animal personalities and are important because they will affect an individual's ability to respond to environmental change. Such environmental sensitivity as a personality trait needs further study because existing studies have rarely directly related environmental sensitivity to well-established personality traits such as exploration behavior and have focused on captive animals of specific model species. Using standardized assays of exploration behavior, we show that individual variation in 1) the speed of exploration behavior and 2) the parts of the environment that are explored are repeatable in juvenile wild starlings (Sturnus vulgaris). Environmental sensitivity was measured in separate assays and was not correlated with the speed of exploration behavior. Instead, environmental sensitivity was strongly predicted by what part of the environment was used during the preceding exploration behavior assays. Thus, in juvenile wild starlings, behavioral traits other than the speed of exploration behavior better predicted environmental sensitivity. These results suggest that the relevance of exploration behavior as a personality trait may not be easily generalized across species. Furthermore, although unrelated to exploration speed, this study illustrates how environmental sensitivity correlates with well-known personality traits and thus further highlights how animal personalities can limit behavioral phenotypic plasticity in wild populations.

Nestling begging increases predation risk, regardless of spectral characteristics or avian mobbing

McDonald, P. G., Wilson, D. R., Evans, C. S. — 2009-05-14

Models of parent–offspring conflict and nestling begging honesty often assume that signaling is associated with increased predation risk. However, little evidence exists that begging actually increases predation in the context in which it evolved, especially when the potentially modulating effects of parental defense are taken into account. We measured the cost of begging in cooperatively breeding bell miners (Manorina melanophrys) by baiting 168 inactive nests with a wax egg and broadcasting sounds from nearby speakers. Nests were randomly assigned to 1 of 3 treatments: silence, unmanipulated begging calls, or shaped white noise pulses that matched the amplitude envelope of each corresponding begging call. Moreover, half of the nests were placed outside and half inside bell miner colonies, where miners vigorously mob potential nest predators. Predation was not influenced by vegetation cover, distance of the nest from the speaker, or placement inside the colony. Sounds were costly, however, as nests broadcasting begging signals or white noise were predated more often and more quickly than silent controls. Contrary to theoretical predictions regarding "stealthy" design, we found that predators were just as likely to locate nests with broadband white noise playback as nests broadcasting begging signals. Further, there was an interaction between playback amplitude and predator type (avian vs. rodent): Louder playback led to decreased nest survival for those taken by avian predators. As increased begging drives provisioning rates in many species, including bell miners, this reveals an inescapable trade-off between nestling begging intensity, parental provisioning effort, and predation risk.

Hunting efficiency and predation risk shapes the color-associated foraging traits of a predator

Fan, C.-M., Yang, E.-C., Tso, I-M. — 2009-05-14

When animals forage or court their behaviors are often constrained by factors such as predation risk. Predator–prey interactions govern the evolution of many behavioral and morphological traits. However, animals with foraging or courtship tightly linked to morphology cannot make quick behavioral adjustments when encountering a dilemma. In this study, we investigate how opposing pressures of maximizing prey intake and minimizing predation risk shape the morphology-associated foraging traits of a sit-and-wait predator. Recently, the conspicuous body colorations of certain orb-weaving spiders have been demonstrated to be attractive to both insect prey and predators. In this study, we performed field manipulations to assess how visual luring signals of such predators trade off opposing pressures of feeding and surviving. We created dummies made of cardboard to test how changing size of conspicuous signal affected attractiveness to prey and predators. Dummies mimicked the coloration pattern and chromatic properties of giant wood spider Nephila pilipes. We found that dummies were similarly attractive to prey and predators as real spiders were. Uniformly yellow colored dummies attracted significantly more prey than those dummies that mimicked the color pattern of N. pilipes. However, such dummies also attracted far more hymenopteran predators. Our findings indicate that current morphology-associated foraging traits of certain animals do not necessarily provide the best feeding performance but reflect a trade-off between opposing pressures of foraging intake and predation risk.

A possible case of contemporary selection leading to a decrease in sexual plumage dimorphism in a grassland-breeding shorebird

Schroeder, J., Lourenco, P. M., Hooijmeijer, J. C.E.W., Both, C., Piersma, T. — 2009-05-14

In sexually dimorphic species, males with more exaggerated plumage ornamentation generally have higher body condition, are preferred by females, and have higher reproductive output. In contrast to the majority of studies, we describe that less ornamented males of the monogamous and sexually dimorphic black-tailed godwit Limosa limosa were larger and heavier during late incubation than more ornamented males and were mated with females that laid larger eggs. In females, we detected no such correlations. If paler and larger males indeed have higher fitness, this suggests ongoing selection for a reduction in male ornamentation, leading to a decrease in plumage ornamentation. We found evidence for such a change since 1840: Male ornamentation in museum specimens became progressively less in all our measured plumage traits. One explanation for this could be that formerly sexually selected plumage traits are now selected against and that the sexual plumage dimorphism of West-European breeding black-tailed godwits might be waning. Concomitant with this change in plumage coloration, this species has experienced enormous human-mediated changes in both breeding and nonbreeding habitat, and we suggest that these might have led to increased costs or reduced fitness benefits for highly ornamented male godwits.

Maternal weight, offspring competitive ability, and the evolution of communal breeding

2009-05-11

Despite the widespread occurrence of communal breeding in animal societies, the fitness consequences for mothers are poorly understood. One factor that may have an important influence on the net benefits mothers gain from breeding communally is the competitive ability of their offspring, as mothers are likely to gain substantial advantages from producing young who can outcompete the offspring of other females for access to resources. Here, we investigate the factors that influence offspring competition in the communally breeding banded mongoose (Mungos mungo). We show that heavy offspring are more likely to win competitive interactions with their littermates. Heavy offspring also receive more care and are more likely to survive to independence in large communal litters where competition is most intense. Our results also indicate that offspring weight at emergence is positively correlated with the weight of the mother at conception. As a consequence, the offspring of heavy mothers are likely to enjoy marked competitive advantages during early life. Together, our findings strongly suggest that the competitive ability of offspring will influence the costs and benefits that females experience while breeding communally and highlights the need for closer examination of the factors that influence offspring competitive ability and the influence this may have on the evolution of communal breeding.

Aggressive Ural owl mothers recruit more offspring

2009-05-06

Animals are thought to adjust their behavior optimally to any given environment. So-called behavioral syndromes, or consistent patterns of behavior across environments, contradict this assumption of unlimited plasticity. We studied nest defense aggressiveness of female Ural owls (244 females with 482 breeding attempts) breeding in a highly variable environment created by fluctuations in the abundance of their main prey (field and bank voles) across years. Ural owls were more aggressive when voles were increasing in density as well as when the Ural owls had large brood sizes and laid early in the season. Aggressive nest defense was highly repeatable between breeding attempts (r = 0.52 ± 0.05 standard error), but individuals also differed in their plasticity (the extent to which they adjusted the level of their aggression to the varying food conditions). Fierce nest defenders produced more recruits to the local breeding population, but a female's survival was not affected by her intensity of nest defense. A path analysis revealed that nest defense aggressiveness, rather than its correlates vole abundance, brood size, or laying date, best explained offspring recruitment. Our findings provide an ultimate explanation for the Ural owl's extremely aggressive nest defense.

A behavioral syndrome linking courtship behavior toward males and females predicts reproductive success from a single mating in the hissing cockroach, Gromphadorhina portentosa

Logue, D. M., Mishra, S., McCaffrey, D., Ball, D., Cade, W. H. — 2009-05-06

Suites of correlated behaviors, or "behavioral syndromes," have been shown to occur throughout the animal kingdom. Behavioral syndromes involving sexual selection are expected to have significant evolutionary ramifications, but few studies have linked behavioral syndromes to sexual selection. We measured the behavior of male hissing cockroaches (Gromphadorhina portentosa) during male–male competition, female choice, and 3 other ecologically relevant contexts and quantified between-context correlations in behavior. We found that aggression directed toward an opponent and retreat and courtship elicited from an opponent were repeatable among males, suggesting that individuals exhibit stable behavioral types in the context of male–male interaction. Our analyses also revealed a "fast–slow" syndrome, linking behavior in a self-righting context to behavior in a foraging context. In contrast to data from several other species, fast–slow scores in hissing cockroaches were not correlated with aggression in a male–male context. Finally, we identified a new type of behavioral syndrome, which we call "libido." Libido was defined by a positive relationship between courtship directed toward opponents in a male–male context and courtship directed toward potential mating partners in a male–female context. Among males that copulated, libido scores predicted reproductive success. We conclude that the libido syndrome, coupled with sexual selection favoring high courtship intensity in a male–female context, may be responsible for the persistence of male–male courtship behavior in this population.

Cues, concessions, and inheritance: dominance hierarchies in the paper wasp Polistes dominulus

Zanette, L., Field, J. — 2009-05-06

Hierarchies constitute the base of many social groups. Hence, understanding how they are established is critical. Here we examine how hierarchies are formed in foundresses associations of the common paper wasp Polistes dominulus. By comparing field data with computer simulations, we evaluate order of arrival at the nest, body size, facial color patterns, and within-group kinship structure as determinants of inheritance rank. Hierarchies (ranks 1–5) were experimentally inferred for 53 nests. Overall, the order in which foundresses arrived at the nest and their body size were not significantly correlated with rank. A foundress's rank was negatively correlated with the number of full sisters it had in its group. Highly ranked wasps (ranks 1 and 2) were less likely to share a nest with their full sisters than wasps of lower rank. A wasp's rank was not determined by the relative rank of its nest-mate sisters. A foundress's rank was significantly correlated with the size of its black clypeal marks, but the number of foundresses with clypeal marks in each nest was small. On 15 of 20 nests where wasps with marks were present, only 1 wasp had such marks. Overall, our results suggest that within-group relatedness structure is important in the establishment of dominance hierarchies in P. dominulus foundress associations.

Female reproductive tactics in a sex-role reversed pipefish: scanning for male quality and number

Silva, K., Almada, V.C., Vieira, M.N., Monteiro, N.M. — 2009-05-06

Theoretical studies predict that females should invest in current reproduction according to both the expected payoffs from mating with different-quality males and their future mating prospects. The Syngnathidae family, with its male pregnancy together with the occurrence of varying degrees of sex-role reversal, constitutes an exceptional model to study female allocation strategies. The present work tests for the influence of male availability and quality (translated into body size) on the egg allocation pattern of different-sized females of the sex role–reversed pipefish Syngnathus abaster. Besides revealing a multiple mating strategy and showing that females do not produce enough eggs to fully occupy a male's brood pouch during the extent of a pregnancy, results reveal a complex pattern with different-sized females adopting different investment tactics. In contrast to small, less attractive females, who show a much more constant reproductive effort through the tested mating contexts, large females seem able to monitor the number and quality of available males responding accordingly by 1) laying more eggs in the presence of several large males or saving efforts for future breeding and 2) laying larger eggs in larger males while depositing smaller ones in lower quality individuals as a consequence of a serial mating process (large males first, small males later).

Vigilance and predation of a forest-living bird species depend on large-scale habitat structure

Griesser, M., Nystrand, M. — 2009-05-06

Prey often use visual cues to detect predators, and consequently, many studies have assessed the effect of small-scale habitat structure on prey antipredator vigilance. This scale may be inappropriate to assess the link between habitat structure and vigilance, however, because visually hunting predators often detect prey from several hundred meters away. As a result, large-scale habitat structure could affect both the hunting decisions of predators and antipredator behaviors of prey. Here we investigated the effect of small- and large-scale habitat structure, as well as group composition (kinship) on vigilance allocation of breeders in the Siberian jay Perisoreus infaustus. Vigilance had an antipredator function and was increased after exposure to a predator model. Small-scale habitat structure did not affect vigilance rates, however, habitat structure of the whole territory, measured as the proportion of visual cover, affected vigilance depending on group composition. Breeders with retained offspring (kin) in their group were more vigilant in managed open territories than on pristine dense territories, whereas breeders without kin in their groups did not adjust vigilance rates in relation to large-scaled habitat structure. Earlier studies have revealed that hawks, the main predators of jays, primarily kill non-kin group members living in groups inhabiting open territories. Therefore, we suggest that breeders adjusted their vigilance depending on the habitat-specific predation risk to protect their retained offspring. This demonstrates that large-scale habitat structure affects predator–prey interactions and is crucial to understanding spatial variation in antipredator allocation and mortality.

Parasitic cowbirds may defeat host defense by causing rejecters to misimprint on cowbird eggs

Strausberger, B. M., Rothstein, S. I. — 2009-05-06

Experiments suggest that some hosts that reject parasitic brown-headed cowbird (Molothrus ater) eggs develop egg recognition by imprinting on their own eggs, probably at the first nest naïve birds attend. This raises the possibility that parasitism of naïve individuals might cause them to mistakenly misimprint on, and accept, cowbird eggs. In an egg-learning experiment designed to cause misimprinting, hosts accepted cowbird eggs when they had laid only 1 egg even though most rejected cowbird eggs when nests had ≥2 host eggs. These findings show that cowbirds can cause misimprinting by parasitizing naïve hosts. If done opportunistically, misimprinting should become more likely as cowbird abundance increases. In accord with this expectation, simulated brood parasitism showed that 3 host species that usually reject cowbird eggs were more tolerant of cowbird eggs in the Great Plains, where cowbirds reach maximal historic abundance, than in eastern North America where cowbirds are less common. This relation between parasite abundance and host responses is opposite to that found in cuckoo hosts. In addition, these 3 rejecter species had high rates (10.8–30.4%) of natural cowbird parasitism at an Illinois site where cowbirds are extremely, and historically, abundant. These rejecters accepted most cases of researcher-detected parasitism, even though they rarely accept where cowbirds are less common. Despite the potentially high cost resulting from egg rejection, we demonstrate that parasitism of "rejecters" can be adaptive for cowbirds particularly when host egg recognition involves learning.

Queenless colonies of the Asian red dwarf honey bee (Apis florea) are infiltrated by workers from other queenless colonies

2009-05-04

In all honey bee species studied thus far, 2–4% of the workers were not born in the sampled colony. These unrelated (nonnatal) workers are thought to arise via orientation errors while returning from foraging trips. Interestingly, in colonies of the red dwarf honey bee, Apis florea, the proportion of nonnatal workers increases significantly when the colonies become queenless, and these workers are more likely to have active ovaries and lay eggs than natal workers. As a result, queenless colonies are heavily parasitized with the eggs of nonnatal workers, but the origin of the parasitizing workers is currently unknown. Here we show that workers from queenless A. florea colonies are far more likely to leave their colony and join another colony compared with workers from queenright colonies. Choice experiments showed that these drifted workers are much more likely to join a queenless colony than a colony with a queen. Perhaps surprisingly, not many workers from queenright colonies joined queenless colonies despite the opportunity for direct reproduction in queenless colonies. We suggest that the inclusive fitness benefits of remaining in the natal colony in the presence of the queen exceed the benefits of direct reproduction in an unrelated queenless colony.

Brood size in a polyembryonic parasitoid wasp is affected by relatedness among competing larvae

Segoli, M., Harari, A. R., Bouskila, A., Keasar, T. — 2009-04-22

Brood size has important implications for the fitness of both parents and offspring. In polyembryonic parasitoid wasps, each egg develops into many genetically identical embryos through clonal division inside the host. Thus, offspring may have the potential to affect brood size by adjusting the degree of embryonic division. In some species, a proportion of embryos develop into soldier larvae, which attack competitors inside the host. This may be another mechanism for offspring to affect final brood size. We investigated the effect of relatedness between competing clones on brood size in the polyembryonic wasp Copidosoma koehleri. We predicted that final brood size would be affected by the number and relatedness between competing clones inside the host. Additionally, we predicted that due to a competitive asymmetry between male and female clones (apparently only female clones produce a soldier larva), this effect would depend on the sex composition of wasps inside the host. We allowed 2 wasp eggs (laid either by 1 female or by different females) to develop in a host and counted the emerging adults. Relatedness between male clones did not affect brood size. However, female-containing broods of related clones were larger than broods of nonrelated clones, suggesting higher aggression of the soldier toward less related individuals. Dissections of hosts parasitized by 2 clones indicate that normally only 1 soldier survives and that it often eliminates unrelated clones. Thus, offspring control over brood size in response to relatedness is probably mediated by soldier aggression and not by clonal division.

Secondary sexual trait size reveals competitive fertilization success in Drosophila bipectinata Duda

Polak, M., Simmons, L. W. — 2009-04-13

The evolution of male secondary sexual traits traditionally has been ascribed to precopulatory sexual selection. In contrast, the importance of postcopulatory sexual selection for the evolution of secondary sexual traits is uncertain, and what little evidence exists for this process to contribute to the evolution of such traits is mixed. Here we test the hypothesis in Drosophila bipectinata Duda that the male sex comb, a rapidly evolving secondary sexual trait, is under positive postcopulatory sexual selection. We extracted replicate genetic lines exhibiting relatively large and small sex comb size from a natural population. Males from these lines were subjected to an assay of competitive fertilization ability, measured as P₂, the proportion of a female's clutch of eggs fertilized by the second male to mate. Males with the largest sex combs sired more offspring than less ornamented individuals, demonstrating for the first time in any Drosophila species that postcopulatory sexual selection favors increasing sex comb size. This study identifies a postcopulatory selective mechanism that may be contributing to the evolutionary diversification of a secondary sexual trait.

Preexisting behavior renders a mutation adaptive: flexibility in male phonotaxis behavior and the loss of singing ability in the field cricket Teleogryllus oceanicus

Tinghitella, R. M., Wang, J. M., Zuk, M. — 2009-04-09

Flexibility in behavior and other traits can pave the way for rapid evolutionary change. A wing mutation, "flatwing," eliminates the ability of >90% of male field crickets (Teleogryllus oceanicus) from one Hawaiian population to produce song to attract females. The morphological change was favored because calling is risky in Hawaii, attracting deadly parasitoid flies. An earlier study suggested that instead of calling, silent flatwing males use satellite behavior, approaching one of the few remaining callers and intercepting females attracted to them. Satellite-like behavior may have existed as a behavioral option prior to the wing mutation, accommodating the loss of singing ability, or behavior may have changed simultaneously with the spread of the mutation. In phonotaxis trials, males from different populations across the crickets' range varied in the distance at which they settled from the source of broadcast song, behaving more or less like satellites. Anecdotally, we noted satellite behavior in all populations, and importantly, males from the mutated population and its direct ancestor behaved similarly. This suggests that the alternative behavior is not strictly associated with the mutation and instead predated the change in morphology. We hypothesized further that satellite behavior may have been preexisting because it is beneficial under other circumstances, such as poor mating success. Virgin males did not show enhanced phonotaxis relative to multiply mated males, however. We conclude that satellite behavior predated the mutation in wing morphology, providing obligately silent males with an alternative method of locating mates and effectively rendering the wing mutation adaptive. Whereas mating history does not appear to influence phonotaxis behavior in males, other factors such as the acoustic environment and demographic conditions may be important contributors to behavioral decisions during male mate location.