Female competition for access to males affects birth rate in baboons

Cycle length flexibility: is the duration of sexual receptivity associated with changes in social pressures?

Research in social mammals has revealed the complexity of strategies females use in response to female-female reproductive competition and sexual conflict. One point at which competition and conflict manifests acutely is during sexual receptivity, indicated by swellings in some primates. Whether females can adjust their sexual receptivity from cycle to cycle to decrease reproductive competition and sexual conflict in response to social pressures has not been tested. As a first step, this study explores whether sexual receptivity duration is predicted by social pressures in wild female chacma baboons (Papio ursinus). Given that female baboons face intense reproductive competition and sexual coercion, we predicted that: females could shorten the duration of their sexual receptive period to reduce female-female aggression and male coercion or increase it to access multiple or their preferred male(s). We quantified 157 ovulatory cycles from 46 wild females living in central Namibia recorded over 15 years. We found no support for our hypothesis; however, our analyses revealed a negative correlation between maximal-swelling duration and group size, a proxy of within-group competition. This study provides further evidence that swelling is costly as well as a testable framework for future investigations of 'cycle length manipulation'.

The Infertility Trap: The Fertility Costs of Group-Living in Mammalian Social Evolution

Mammal social groups vary considerably in size from single individuals to very large herds. In some taxa, these groups are extremely stable, with at least some individuals being members of the same group throughout their lives; in other taxa, groups are unstable, with membership changing by the day. We argue that this variability in grouping patterns reflects a tradeoff between group size as a solution to environmental demands and the costs created by stress-induced infertility (creating an infertility trap). These costs are so steep that, all else equal, they will limit group size in mammals to ∼15 individuals. A species will only be able to live in larger groups if it evolves strategies that mitigate these costs. We suggest that mammals have opted for one of two solutions. One option (fission-fusion herding) is low cost but high risk; the other (bonded social groups) is risk-averse, but costly in terms of cognitive requirements.

Sexual selection and sperm diversity in primates

Many aspects of primate sperm physiology and reproductive behavior have been influenced by sexual selection, especially in taxa exposed to sperm competition where females mate with multiple partners. Primate sperm diversity reflects therefore the evolutionary divergences of the different primate species and the impact of a combination of variables exerting selection pressures on sperm form, function, and competition. Thereby, mating systems, life cycle or ecological variables are some of the important factors driving sperm diversity and explaining variation in terms of sperm morphology, parameters or male sexual characters. Here, we address primate sperm diversity through a compilation of all data available in the literature concerning primate sperm parameters and relationships between them. We also review the factors that can influence primate sperm diversity (e.g. mating systems, trade-off between investments in precopulatory and postcopulatory sexual traits, male and female sexual behaviors, seasonality, social constraints, testosterone levels), and discuss also their relevance to our understanding of human reproduction.

Fertility as a constraint on group size in African great Apes

Gorillas and chimpanzees live in social groups of very different size and structure. Here I test the hypothesis that this difference might reflect the way fertility maps onto group demography as it does in other Catarrhines. For both genera, birth rates and the number of surviving offspring per female are quadratic (or ∩-shaped) functions of the number of adult females in the group, and this is independent of environmental effects. The rate at which fertility declines ultimately imposes a constraint on the size of social groups that can be maintained in both taxa. The differences in group size between the two genera seem to reflect a contrast in the way females buffer themselves against this cost. Gorillas do this by using males as bodyguards, whereas chimpanzees exploit fission–fusion sociality to do so. The latter allows chimpanzees to live in much larger groups without paying a fertility cost (albeit at a cognitive cost).

Social structure as a strategy to mitigate the costs of group living: a comparison of gelada and guereza monkeys

In mammals, and especially primates, group size and social complexity are typically correlated. However, we have no general explanation why this is so. I suggest that the answer may lie in one of the costs of group living: mammalian reproductive endocrinology is extremely sensitive to stress, and forms one of the hidden costs of living in groups. Fertility declines with group size widely across the social mammals, including primates, and will ultimately place a constraint on group size. However, some species seem to have been able to mitigate this cost by forming bonded relationships that reduce the impact of experienced aggression, even if rates of aggression remain high. The downside is that they reduce network connectivity and hence risk fragmenting the group by providing fracture lines for group fission. To explore this, I compare network indices and fertility patterns across the same range of group sizes for two species of Old World monkeys, Colobus guereza and Theropithecus gelada: the former relatively unsocial, the latter intensely social with frequent use of grooming-based alliances. Compared to those of the guereza, gelada social networks lose density more slowly, maintain connectedness more effectively and are less likely to fragment as they increase in size. Although fertility declines with group size in both species, in gelada the impact of this effect is deferred to larger group sizes. The differences in fertility and network structure both predict the very different maximum group sizes typical of these two species, as well as the typical sizes at which their groups undergo fission. This finding may explain aspects of wider mammalian sociality.

Sexual Selection and the differences between the sexes in Mandrills (Mandrillus sphinx)

Sexual selection has become a major focus in evolutionary and behavioral ecology. It is also a popular research topic in primatology. I use studies of mandrills (Mandrillus sphinx), a classic example of extravagant armaments and ornaments in animals, to exemplify how a long-term, multidisciplinary approach that integrates field observations with laboratory methods can contribute to on-going theoretical debates in the field of sexual selection. I begin with a brief summary of the main concepts of sexual selection theory and the differences between the sexes. I then introduce mandrills and the study population and review mandrill life history, the ontogeny of sex differences, and maternal effects. Next, I focus on male-male competition and female choice, followed by the less well-studied questions of female-female competition and male choice. This review shows how different reproductive priorities lead to very different life histories and divergent adaptations in males and females. It demonstrates how broadening traditional perspectives on sexual selection beyond the ostentatious results of intense sexual selection on males leads to an understanding of more subtle and cryptic forms of competition and choice in both sexes and opens many productive avenues in the study of primate reproductive strategies. These include the potential for studies of postcopulatory selection, female intrasexual competition, and male choice. These studies of mandrills provide comparison and, I hope, inspiration for studies of both other polygynandrous species and species with mating systems less traditionally associated with sexual selection.

Infertility, abortion, and biotechnology : When it's not nice to fool mother nature

Patterns of reproductive failure described in humans and other mammals suggest that reproductive failure may in many instances be the result of adaptations evolved to suppress reproduction under temporarily harsh conditions. By suppressing reproduction under such conditions, females are able to conserve their time and energy for reproductive opportunities in which reproduction is most likely to succeed. Such adaptations have been particularly important for female mammals, given (a) the amount of time and energy that reproduction requires, and (b) the degree to which reproductive conditions can vary.The existence of conscious and unconscious mechanisms to suppress reproduction under poor conditions has several implications for obstetric/gynecologic practices. Two implications are discussed with reference to biotechnological advancements in our ability to facilitate conceptions and manage problem pregnancies: (a) potential dangers of sophisticated technologies overriding natural fertility controls; and (b) the need for greater appreciation of the association between psychosocial stress and reproductive failure in the treatment of reproductive problems. Implications for elective abortion practices are discussed as well.

Female competition and aggression: interdisciplinary perspectives

This paper introduces a Theme Issue combining interdisciplinary perspectives in the study of female competition and aggression. Despite a history of being largely overlooked, evidence is now accumulating for the widespread evolutionary significance of female competition. Here, we provide a synthesis of contributions to this Theme Issue on humans and other vertebrates, and highlight directions for future research. Females compete for resources needed to survive and reproduce, and for preferred mates. Although female aggression takes diverse forms, under most circumstances relatively low-risk competitive strategies are favoured, most probably due to constraints of offspring production and care. In social species, dominance relationships and threats of punishment can resolve social conflict without resort to direct aggression, and coalitions or alliances may reduce risk of retaliation. Consistent with these trends, indirect aggression is a low cost but effective form of competition among young women. Costs are also minimized by flexibility in expression of competitive traits, with aggressive behaviour and competitive signalling tailored to social and ecological conditions. Future research on female competition and the proximate mediators of female aggression will be greatly enhanced by opportunities for interdisciplinary exchange, as evidenced by contributions to this Theme Issue.

Evidence for intra-sexual selection in wild female baboons

Because female reproductive success in social mammals is determined largely by parental rather than mating effort, intra-sexual competition among females has typically been assumed to occur primarily over food. Recently, however, renewed attention has been paid to the importance of other sources of variation in female fitness, with a concomitant focus on a broader definition of intra-sexual selection that encompasses both competition for resources and competition for mates and social partners. We present behavioural and demographic data gathered over 15 years on a group of wild chacma baboons (Papio hamadryas ursinus) which show that females incur several costs when living in groups with many females and comparatively few males: increased intra-sexual aggression, less stable bonds with female partners, and increased mortality. Female-female aggression was higher in years when the adult sex ratio was more highly skewed toward females, with low-ranking female exhibiting particularly high rates of aggression toward other females. Females' social bonds with other females also became less stable in years when the group contained many females. Finally, female mortality rates were highest in years when the group contained comparatively more females and fewer males. The negative correlation between the number of males and female mortality suggests that the increase in mortality was due to increased predation rather than food competition. Results indicate that intra-sexual competition for social partners, mates, paternal investment, and perhaps also male protection occurs even in species where females exhibit low reproductive skew.

Sexual Selection and Condition-Dependent Mate Preferences

The last decade has witnessed considerable theoretical and empirical investigation of how male sexual ornaments evolve. This strong male-biased perspective has resulted in the relative neglect of variation in female mate preferences and its consequences for ornament evolution. As sexual selection is a co-evolutionary process between males and females, ignoring variation in females overlooks a key aspect of this process. Here, we review the empirical evidence that female mate preferences, like male ornaments, are condition dependent. We show accumulating support for the hypothesis that high quality females show the strongest mate preference. Nonetheless, this is still an infant field, and we highlight areas in need of more research, both theoretical and empirical. We also examine some of the wider implications of condition-dependent mating decisions and their effect on the strength of sexual selection.

The function of Barbary macaque copulation calls

In a wide variety of animal species, females produce vocalizations specific to mating contexts. It has been proposed that these copulation calls function to incite males to compete for access to the calling female. Two separate advantages of inciting male-male competition in this way have been put forward. The first suggests that as a result of calling, females are only mated by the highest ranking male in the vicinity (indirect mate choice hypothesis). The second proposes that copulation calling results in a female being mated by many males, thus promoting competition at the level of sperm (sperm competition hypothesis). In this paper, I give results from the first experimental study to test these hypotheses. Playback was used to examine the function of copulation calls of female Barbary macaques (Macaca sylvanus) in Gibraltar. Although rank did not affect lone males' likelihood of approaching copulation calls, when playbacks were given to pairs of males only the higher ranking individual approached. Moreover, females were mated significantly sooner after playback of their copulation call than after playback of a control stimulus. These results suggest that the copulation calls of female Barbary macaques play a key role in affecting patterns of male reproductive behaviour, not only providing an indirect mechanism of female choice, but also promoting sperm competition by reducing the interval between copulations. Potential fitness benefits of inciting male-male competition at these two levels are discussed.

The evolution of nonhuman primate social behavior

A review of the recent literature concerning evolutionary mechanisms and possible genetic contributions to social behavior reveals a concentration on function rather than mechanism. Although functional consequences may influence future genetic changes in a population, they do not necessarily reflect evolutionary history. More important, genes cannot code for functions. Only when the anatomical structures and behavioral patterns of individuals are described can we study genetic contributions to social organization. Discussions of function in the abstract, without specification of mechanism, do not fall within the realm of scientific testing.