Harder,better, faster,stronger: Weapon size is more sexually dimorphic than weapon biomechanical components in two freshwater anomuran species

Sexual selection influences the evolution of morphological traits that increase the likelihood of monopolizing scarce resources. When such traits are used during contests, they are termed weapons. Given that resources are typically linked to monopolizing mating partners, theory expects only males to bear weapons. In some species, however, females also bear weapons, although typically smaller than male weapons. Understanding why females bear smaller weapons can thus help us understand the selective pressures behind weapon evolution. However, most of our knowledge comes from studies on weapon size, while the biomechanics of weapons, such as the size of the muscles, efficiency, and shape are seldom studied. Our goal was to test if the theoretical expectations for weapon size sexual dimorphism also occur for weapon biomechanics using two aeglid crab species. Males of both species had larger claws which were also stronger than female claws. Male claws were also more efficient than females' claws (although we used only one species in this analysis). For weapon shape, though, only one species differed in the mean claw shape. Regarding scaling differences, in both species, male claws had higher size scaling than females, while only one species had a higher shape scaling. However, male weapons did not have higher scaling regarding strength and efficiency than females. Thus, males apparently allocate more resources in weapons than females, but once allocated, muscle and efficiency follow a similar developmental pathway in both sexes. Taken together, our results show that sexual dimorphism in weapons involves more than differences in size. Shape differences are especially intriguing because we cannot fully understand its causes. Yet, we highlight that such subtle differences can only be detected by measuring and analysing weapon shape and biomechanical components. Only then we might better understand how weapons are forged.     

Ecological and social determinants of association and proximity patterns in the fission–fusion society of spider monkeys (Ateles geoffroyi)

Some social species exhibit high levels of fission–fusion dynamics (FFD) that improve foraging efficiency. In this study, we shed light on the way that FFD allows animal groups to cope with fluctuations in fruit availability. We explore the relative contribution of fruit availability and social factors like sex in determining association and proximity patterns in spider monkeys. We tested the influence of fruit availability and social factors on the association and proximity patterns using three-year data from a group of spider monkeys in the Yucatan Peninsula of Mexico. We identified subgroup members and estimated their Interindividual distances through instantaneous scan sampling. We evaluated fruit availability by monitoring the phenology of the 10 most important food tree species for spider monkeys in the study site. Social network analyses allowed us to evaluate association and proximity patterns in subgroups. We showed that association patterns vary between seasons, respond to changes in fruit availability, and are influenced by the sex of individuals, likely reflecting biological and behavioral differences between sexes and the interplay between ecological and social factors. In contrast, proximity patterns were minimally affected by changes in fruit availability, suggesting that social factors are more important than food availability in determining cohesion within subgroups.     

Hunger‐dependent and sex‐specific antipredator behaviour of larvae of a size‐dimorphic mosquito

1. Modification of behaviours in the presence of predators or predation cues is widespread among animals. The costs of a behavioural change in the presence of predators or predation cues depend on fitness effects of lost feeding opportunities and, especially when organisms are sexually dimorphic in size or timing of maturation, these costs are expected to differ between the sexes. 2. Larval Aedes triseriatus (Say) (Diptera: Culicidae) were used to test the hypothesis that behavioural responses of the sexes to predation cues have been selected differently due to different energy demands. 3. Even in the absence of water‐borne predation cues, hungry females (the larger sex) spent more time browsing than did males, indicating a difference in energy needs. 4. In the presence of predation cues, well‐fed larvae of both sexes reduced their activity more than did hungry larvae, and males shifted away from high‐risk behaviours to a greater degree than did females, providing the first evidence of sex‐specific antipredator behaviour in foraging mosquito larvae. 5. Because sexual size dimorphism is common across taxa, and energetic demands are probably correlated with size dimorphism, this research demonstrates the importance of investigating sex‐specific behaviour and behavioural responses to enemies, and cautions against generalising results between sexes.     

Losses of female song with changes from tropical to temperate breeding in the New World blackbirds

Birds in which both sexes produce complex songs are thought to be more common in the tropics than in temperate areas, where typically only males sing. Yet the role of phylogeny in this apparent relationship between female song and latitude has never been examined. Here, we reconstruct evolutionary changes in female song and breeding latitude in the New World blackbirds (Icteridae), a family with both temperate and tropical representatives. We provide strong evidence that members of this group have moved repeatedly from tropical to temperate breeding ranges and, furthermore, that these range shifts were associated with losses of female song more often than expected by chance. This historical perspective suggests that male-biased song production in many temperate species is the result not of sexual selection for complex song in males but of selection against such songs in females. Our results provide new insights into the differences we see today between tropical and temperate songbirds, and suggest that the role of sexual selection in the evolution of bird song might not be as simple as we think.     

Sperm number trumps sperm size in mammalian ejaculate evolution

Postcopulatory sexual selection is widely accepted to underlie the extraordinary diversification of sperm morphology. However, why does it favour longer sperm in some taxa but shorter in others? Two recent hypotheses addressing this discrepancy offered contradictory explanations. Under the sperm dilution hypothesis, selection via sperm density in the female reproductive tract favours more but smaller sperm in large, but the reverse in small, species. Conversely, the metabolic constraint hypothesis maintains that ejaculates respond positively to selection in small endothermic animals with high metabolic rates, whereas low metabolic rates constrain their evolution in large species. Here, we resolve this debate by capitalizing on the substantial variation in mammalian body size and reproductive physiology. Evolutionary responses shifted from sperm length to number with increasing mammalian body size, thus supporting the sperm dilution hypothesis. Our findings demonstrate that body-size-mediated trade-offs between sperm size and number can explain the extreme diversification in sperm phenotypes.     

Sexual size dimorphism and selection in the wild in the waterstrider Aquarius remigis: Body size,components of body size and male mating success

Sexual size dimorphism is assumed to be adaptive and is expected to evolve in response to a difference in the net selection pressures on the sexes. Although a demonstration of sexual selection is neither necessary nor sufficient to explain the evolution of sexual size dimorphism, sexual selection is generally assumed to be a major evolutionary force. If contemporary sexual selection is important in the evolution and maintenance of sexual size dimorphism then we expect to see concordance between patterns of sexual selection and patterns of sexual dimorphism. We examined sexual selection in the wild, acting on male body size, and components of body size, in the waterstrider Aquarius remigis, as part of a long term study examining net selection pressures on the two sexes in this species. Selection was estimated on both a daily and annual basis. Since our measure of fitness (mating success) was behavioral, we estimated reliabilities to determine if males perform consistently. Reliabilities were measured as ? statistics and range from fair to perfect agreement with substantial agreement overall. We found significant univariate sexual selection favoring larger total length in the first year of our study but not in the second. Multivariate analysis of components of body size revealed that sexual selection for larger males was not acting directly on total length but on genital length. Sexual selection for larger male body size was opposed by direct selection favoring smaller midfemoral lengths. While males of this species are smaller than females, they have longer genital segments and wider forefemora. Patterns of contemporary sexual selection and sexual size dimorphism agree only for genital length. For total length, and all other components of body size examined, contemporary sexual selection was either nonsignificant or opposed the pattern of size dimporhism. Thus, while the net pressures of contemporary selection for the species may still act to maintain sexual size dimorphism, sexual selection alone does not.