Sharing mating opportunities by coalition males in stump-tailed macaques: elucidation of ecological mechanism by diversion of PvS model

ABSTRACTCooperation, or the act of benefiting others at the cost of the benefactor’s fitness, has been a central issue in evolutionary theory. Non-human animals sometimes show coalitions or male-male “cooperation” to confront a male rival and challenge the rank hierarchy. Here we observed novel types of coalitions in wild stump-tailed macaques (Macaca arctoides); multiple males actively shared the mating opportunities, i.e., a male copulated with a female, while his ally waited his turn and guarded them. Our mathematical simulations revealed that lack of estrous signs, as well as large numbers of males in a group, possibly enhance facultative sharing of females. This is the first demonstration of the sharing of females in non-human primates., and shed light on the evolutionary theory of cooperation. Overall, our novel observations have revised the existing socioecological models in primate social systems. The characteristics of stump-tailed macaque societies now range from despotism to egalitarianism, and from monopolization of females by a dominant male to male-male coalition coupled with active sharing of mating opportunities, which increases the chances of reproductive success.

Developmental constraints on behavioural flexibility

We suggest that variation in mammalian behavioural flexibility not accounted for by current socioecological models may be explained in part by developmental constraints. From our own work, we provide examples of constraints affecting variation in behavioural flexibility, not only among individuals, but also among species and higher taxonomic units. We first implicate organizational maternal effects of androgens in shaping individual differences in aggressive behaviour emitted by female spotted hyaenas throughout the lifespan. We then compare carnivores and primates with respect to their locomotor and craniofacial adaptations. We inquire whether antagonistic selection pressures on the skull might impose differential functional constraints on evolvability of skulls and brains in these two orders, thus ultimately affecting behavioural flexibility in each group. We suggest that, even when carnivores and primates would theoretically benefit from the same adaptations with respect to behavioural flexibility, carnivores may nevertheless exhibit less behavioural flexibility than primates because of constraints imposed by past adaptations in the morphology of the limbs and skull. Phylogenetic analysis consistent with this idea suggests greater evolutionary lability in relative brain size within families of primates than carnivores. Thus, consideration of developmental constraints may help elucidate variation in mammalian behavioural flexibility.

Socioecology of wild bearded capuchin monkeys (Sapajus libidinosus): an analysis of social relationships among female primates that use tools in feeding

Socioecology considers that the features of food sources affect female social relationships in group-living species. Among primates, the tests of socioecological models are largely focused on Old World species and do not evaluate if the use of feeding tools affects the competitive regime over food and females’ relations in wild populations. We studied female social relationships among a wild population of bearded capuchins monkeys (Sapajus libidinosus) that use percussive tools (stones) to crack encased foods, in a semi-arid habitat in Brazil. Females fed mainly on clumped, high quality resources, indicating that the habitat provides a high quality diet year-round. Females experienced contest competition within and between-groups. As predicted by socioecological models, females’ social relationships were characterized by philopatry, linear dominance hierarchies, coalitions, and tolerance in feeding bouts. Females spent a small proportion of their feeding time using tools. Nevertheless, tool sites generated high rates of contest competition and lower indices of tolerance among females. Although the social structure of our study population did not differ significantly from the pattern observed in wild populations of Sapajus that do not use tools, tool use increased within-group contest competition and apparently contributed to the linearity of the dominance hierarchies established among females. We predict that when tool use results in usurpable food resources, it will increase contest competition within group-living species.

Testing mate, resource and infant defence functions of intergroup aggression in non-human primates: issues and methodology

Intergroup aggression is relatively common in many primate species and its functions have long been debated and incorporated as important parts of primate socioecological models. Recent studies have improved upon previous research by empirically testing the potential mate, food/resource and infant defence functions of intergroup aggression. I raise three main issues regarding the predictions and methods that have been used and make suggestions that should improve future studies. First, I address what constitutes evidence of defence and suggest that encounter rates and contexts should generally not be used to help test mate, resource and infant defence hypotheses. Secondly, I address the problem of disentangling mate, resource and infant defence hypotheses and suggest that because they are not mutually exclusive and potentially confound one another, tests of one hypothesis should adequately control for other possible functions, preferably using multiple logistic regression. Lastly, I discuss the importance of examining intergroup aggression using a cost-benefit approach and suggest that measures should be taken to control for species-specific measures of fighting ability, and/or other potential costs or influential factors that might influence participation in intergroup aggression, including groups' identities. For the latter two issues, I provide simulations that illustrate the methodologies I suggest and the potential effects of employing them, relative to previously employed tests.