The efficacy of female choice in chimpanzees of the Taï Forest, Côte d’Ivoire

Strategies, costs and counter-strategies to sexual coercion

Sexual conflict, the conflict between the evolutionary interests of females and males over mating, occasionally results in the evolution of traits favourable for one sex and adverse for the other. In this context, males can use sexual coercion to increase their mating success, at the expense of their female targets' mate choice. An increasing number of studies highlight a great diversity of male and female behaviours that serve as strategies and counter-strategies, respectively, to sexual coercion. Previous studies have reviewed the literature on infanticide but not the literature on forced copulation, sexual harassment, intimidation or punishment. This qualitative review synthesises the empirical evidence and draws a unified framework of the ecology of sexual coercion across animals, presenting male sexually coercive strategies and co-evolved female counter-strategies that can reduce coercion and its fitness costs. Using examples from insects to humans, it shows that different strategies of sexual coercion can lead to the evolution of similar counter-strategies. These counter-strategies include female promiscuity, deception of males (e.g. concealed ovulation or pseudo-oestrus), avoidance of certain males and association with others for protection, female aggregation to dilute coercion and ultimately physical resistance by single or allied females. Extending previous work, this review provides compelling evidence of sexually antagonistic coevolution amid sexual coercion. It also calls for future work to clarify, first, which individual traits are linked to greater coercion rates in males and a higher likelihood of receiving coercion in females and, second, any causal relationships between different strategies of sexual coercion and the evolution of different social and mating systems.

Drivers of female power in bonobos

In mammals, female dominance over males is a rare phenomenon. However, recent findings indicate that even in species with sexual dimorphism biased towards males, females sometimes occupy high status. Here we test three main hypotheses explaining intersexual power relationships, namely the self-reinforcing effects of winning and losing conflicts, the strength of mate competition, and female coalition formation. We test these for bonobos (Pan paniscus), one of our closest living relatives, where females have high status relative to males despite male-biased size dimorphism. We compiled demographic and behavioral data of 30 years and 6 wild living communities. Our results only support predictions of the female coalition hypothesis. We found that females target males in 85% of their coalitions and that females occupy higher ranks compared to males when they form more frequent coalitions. This result indicates that female coalition formation is a behavioral tool for females to gain power over males.

Focal Coordination and Language in Human Evolution

We study game-theoretic models of human evolution to analyze fundamentals of human nature. Rival-claimants games represent common situations in which animals can avoid conflict over valuable resources by mutually recognizing asymmetric claiming rights. Unlike social-dilemma games, rival-claimants games have multiple equilibria which create a rational role for communication, and so they may be good models for the role of language in human evolution. Many social animals avoid conflict by dominance rankings, but intelligence and language allow mutual recognition of more complex norms for determining political rank or economic ownership. Sophisticated forms of economic ownership could become more advantageous when bipedalism allowed adaptation of hands for manufacturing useful objects. Cultural norms for claiming rights could develop and persist across generations in communities where the young have an innate interest in learning from their elders about when one can appropriately claim desirable objects. Then competition across communities would favor cultures where claiming rights are earned by prosocial behavior, such as contributions to public goods. With the development of larger societies in which many local communities share a common culture, individuals would prefer to interact with strangers who identifiably share this culture, because shared cultural principles reduce risks of conflict in rival-claimants games.

Female Mate Choice in Wild Kenyan Blue Monkeys (Cercopithecus mitis)

Female mate choice may drive sexual selection, but discerning whether female behaviors reflect free expression of choice or responses to constraints can be difficult. We investigated the efficacy of female choice in wild blue monkeys using 10 years of behavior and paternity data (N = 178 male-female dyads). Although blue monkeys live modally in one-male polygynous groups, where male-biased intersexual power is expected, females can access multiple potential mates during seasonal male influxes and occasional intergroup encounters. Additionally, extra-group males sire offspring. We examined female resistance rates to male-initiated sexual interactions, and unsolicited proceptive behavior that females directed to males (corrected for male availability). Females seldom resisted male solicitation, but initiated sexual interactions more than males. Females generally preferred residents. Those who preferred non-residents tended to have residents with longer tenures, but neither female parity nor rank influenced the tendency to prefer non-residents vs. residents. The male most solicited by a particular female fathered that female's infant 82% of the time; odds of siring were 26 times higher for most vs. nonpreferred males. Female preference predicted paternity even more strongly among non-resident males, with odds of siring 33 times higher for most vs. nonpreferred non-residents. Neither female rank nor parity influenced her likelihood of having her preferred partner as sire. Paternity by preferred males did not affect infant survival. While we cannot fully discount the effect of male-male competition on paternity, these results suggest that blue monkey females can exercise choice successfully, even in a polygynous mating system.

Male-philopatric nonhuman primates and their potential role in understanding the evolution of human sociality

In most primate species, males transfer out of their natal groups, resulting in groups of unrelated males. However, in a few species, including humans, males remain in their groups and form life-long associations with each other. This pattern of male philopatry is linked with cooperative male behaviors, including border patrols and predator defense. Because females in male-philopatric species form weaker kin networks with each other than in female-philopatric species, they are expected to evolve counter-strategies to male sexual coercion that are relatively independent of support from other females. Studies of male-philopatric nonhuman primates can provide insight into the evolutionary basis of prosocial behaviors, cooperation, and group action in humans and offer comparative models for understanding the sociality of other hominin species. This review will discuss patterns of dispersal and philopatry across primates, explore the resulting male and female behaviors, and argue that male-philopatric nonhuman primate species offer insight into the social and sexual dynamics of hominins throughout evolution.

Reproductive inequality among males in the genus Pan

Reproductive inequality, or reproductive skew, drives natural selection, but has been difficult to assess, particularly for males in species with promiscuous mating and slow life histories, such as bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). Although bonobos are often portrayed as more egalitarian than chimpanzees, genetic studies have found high male reproductive skew in bonobos. Here, we discuss mechanisms likely to affect male reproductive skew in Pan, then re-examine skew patterns using paternity data from published work and new data from the Kokolopori Bonobo Reserve, Democratic Republic of Congo and Gombe National Park, Tanzania. Using the multinomial index (M), we found considerable overlap in skew between the species, but the highest skew occurred among bonobos. Additionally, for two of three bonobo communities, but no chimpanzee communities, the highest ranking male had greater siring success than predicted by priority-of-access. Thus, an expanded dataset covering a broader demographic range confirms that bonobos have high male reproductive skew. Detailed comparison of data from Pan highlights that reproductive skew models should consider male-male dynamics including the effect of between-group competition on incentives for reproductive concessions, but also female grouping patterns and factors related to male-female dynamics including the expression of female choice. This article is part of the theme issue 'Evolutionary ecology of inequality'.

Why bonobos show a high reproductive skew towards high-ranking males: analyses for association and mating patterns concerning female sexual states

Among non-human primates, male dominance rank is not necessarily a good indicator of mating success, and relationships between male dominance rank and mating or reproductive success are affected by female behavior and sexual states implying their probability of conception. Although comparisons of the behavior of male chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) suggest that the effect of male dominance rank on mating success is expected to be less prominent in bonobos, recent genetic studies have shown that high-ranking male reproductive skew is higher in bonobos than in chimpanzees. One possible reason for the higher reproductive skew in bonobos could be that female behavior and their sexual states may have much more influence on male mating and reproductive success in bonobos than in chimpanzees. In the current study on bonobos, we conducted focal animal observation of females and analyzed the influence of female sexual swelling, the number of days after parturition, and dominance rank of males on female associations, and copulation with adult males. Our results showed that females with maximum swelling (MS) had more proximity with high-ranking males and copulated more frequently with higher-ranking males than with lower-ranking males. Females for whom longer time had elapsed since parturition, and therefore had higher probabilities of conception, had 5-m proximity with adult males more frequently than females whom shorter time had elapsed since parturition, but did not have more copulation with adult males. Females with MS had proximity and copulated with high-ranking males frequently, which partly explains why the reproductive skew is so high in bonobos. These results are discussed in relation to previous hypotheses on the influence of long-lasting mother-son relationships on mating success of males and on the contribution of female receptivity during non-conceptive nursing periods to moderate intermale aggression in bonobos.

Olfactory Inspection of Female Reproductive States in Chimpanzees

In mammalian species, olfactory cues are important for within and between species communication. These cues can be part of multimodal signals indicating, for example, female fertility potentially perceived by male conspecifics. However, a large gap exists in our understanding of multimodal signaling in non-human catarrhines. Chimpanzees (Pan troglodytes) live in fission-fusion societies, mate promiscuously and express high levels of male-male competition. Females show a striking visual signal, an exaggerated sexual swelling, known to be a proxy of ovulation, while the maximum swelling is not matching the exact time of ovulation. The question remains if males use additional olfactory cues when being able to approach females closely. This would allow males to pinpoint the exact timing of ovulation and could be one reason why high-ranking males sire offspring more successfully than other males. Here, we present the first systematic test of such multimodal signaling by investigating male sniffing behavior directed to females in relation to their fertility in a group of 13 captive chimpanzees. Our results show that male sniffing behavior significantly increased with female swelling size, with female age as well as when fewer male competitors were present. Hence, odors might be part of a multimodal fertility cue, supporting the idea that males monitor both visual and olfactory cues to gain comprehensive information on female fertility.

Towards an understanding of multimodal traits of female reproduction in chimpanzees

Although primates have long been regarded as microsmatic, recent studies indicate that olfaction is an important sensory mode of primate communication, e.g., in the context of reproduction. However, large gaps remain in our understanding of primate olfactory traits, especially in the great apes. Female chimpanzees (Pan troglodytes) possess an exaggerated sexual swelling, which is an imprecise signal of fertility that is thought to serve to confuse paternity. However, some high-ranking males that copulate most frequently on the days when females are most fertile seem to have more precise information on the timing of ovulation, which suggests the existence of an olfactory fertility trait. In order to examine, and provide evidence for, fertility-related chemical information in female chimpanzees, we used gas chromatography-mass spectrometry to analyze the chemical composition of female body odor collected across the menstrual cycle during various stages of sexual swelling (97 samples of six females). The chemical composition was significantly affected by swelling stage, and eight substances were detected that were strongly related to the latter. The existence of an additional, olfactory, fertility trait may help males to fine-tune their sexual behavior or allow females to strengthen concealment of the exact timing of ovulation, and needs to be further investigated in follow-up studies. The results of our study provide much-needed evidence for the existence of an olfactory cue related to reproduction in chimpanzees, and form a basis for future studies on the interplay between visual and olfactory information on female fertility.

How Language and Human Altruism Evolved Hand in Hand - The Backchannel Hypothesis

This paper contributes to two debates: the debate about language evolution and the debate about the foundations of human collaboration. While both cooperation and language may give the impression of being adaptations that evolved for the "good of the group," it is well established that the evolution of complex traits cannot be a direct result of group selection. In this paper I suggest how this tension can be solved: both language and cooperation evolved in a unique two-level evolutionary system which was triggered by a well-documented geological event-the drying out of the climate-in East Africa, which subsequently reduced the intermating between groups and thus made it possible that the mechanism that produced differences between groups (including social forms of selection such as female choice) could be the target of natural selection on the group level. If a social form of selection (e.g., sexual selection) produced differences in fitness between groups, the displacement process between groups would indirectly select those forms of social selection that produce groups that would displace all others. The main hypothesis presented in this paper is that, in this situation, a backchannel between the two levels of selection naturally evolves. A backchannel between the two levels would, for example, emerge when sexual selection (or any other form of social selection) was sensitive to the individual's contribution to the group. Examples of systems utilizing a backchannel are nerve cells being better nourished when used more frequently, enabling them to be conducive to the survival of the whole organism, or a law firm in which all employees get paid to the extent that they contribute to the survival and success of the firm. In both cases, the selection on the higher level informs the selection on the lower level. The aim of the paper is to illuminate these rather opaque claims, to which the reader probably has many objections in this abridged form.

Male chimpanzee sexual coercion and mating success at Ngogo

Chimpanzees (Pan troglodytes) have a complex mating system in which both sexes use multiple tactics. Most copulations occur in group contexts, and high-ranking males can gain high mating and reproductive success, but females typically mate with multiple males and the potential for sperm competition is high. Also, male-female dyads sometimes form temporary exclusive mating associations (consortships). Male aggression to receptive females is common. Several studies have supported the hypothesis that this is sexual coercion, but debate exists regarding the importance of coercion relative to that of female choice. The number of adult males in a community can influence the balance between these processes. In the large Ngogo community, male dominance ranks and rates of aggression to fully-swollen females were positively related to mating success as estimated by copulation rates and by proportions of copulations achieved. Aggression rates were higher than at other sites, overall and per male, especially during periovulatory periods, and increased with the number of males associating with a female. Aggression impaired female foraging efficiency. Males initiated most copulations and females rarely refused mating attempts. Male-to-female grooming was positively associated with male mating success and with the proportion of copulations that females initiated, but the amount of grooming was typically small and whether grooming-for-mating trading occurs is uncertain. These results are consistent with the hypothesis that sexual coercion is an important component of male chimpanzee mating strategies in many sociodemographic circumstances, but also show that male tactics vary both in response to and independently of those circumstances.

Countering infanticide: Chimpanzee mothers are sensitive to the relative risks posed by males on differing rank trajectories

OBJECTIVES

Infanticide by males is common in mammals. According to the sexual selection hypothesis, the risk is inversely related to infant age because the older the infant, the less infanticide can shorten lactational amenorrhea; risk is also predicted to increase when an infanticidal male's chance of siring the replacement infant is high. Infanticide occurs in chimpanzees (Pan troglodytes), a species in which male dominance rank predicts paternity skew. Infanticidal male chimpanzees (if low-ranking) are unlikely to kill their own offspring, whereas those who are currently rising in rank, particularly when this rise is dramatic, have an increased likelihood of fathering potential future infants relative to any existing ones. Given that mothers should behave in ways that reduce infanticide risk, we predicted that female chimpanzees, and specifically those with younger, more vulnerable infants, would attempt to adjust the exposure of their infants to potentially infanticidal males. Specifically, mothers of young infants should reduce their association with adult males in general, and to a greater extent, with both low-ranking males and those rising in rank from a position where paternity of current infants was unlikely, to a rank where the probability of siring the next infant is significantly higher. We also investigated the alternative possibility that rather than avoiding all adult males, mothers would increase association with males of stable high rank on the basis that such males could offer protection against infanticide.

MATERIALS AND METHODS

We examined data on female association patterns collected from the Budongo Forest, Uganda, during a period encompassing both relative stability in the male hierarchy and a period of instability with a mid-ranking male rising rapidly in rank.

RESULTS

Using linear mixed models, we found that mothers reduced their association with the rank-rising male, contingent on infant age, during the period of instability. We also found evidence that females preferentially associated with a potential protector male during the high-risk period.

DISCUSSION

Our results support the sexually selected hypothesis for infanticide and demonstrate that female chimpanzees are sensitive to the relative risks posed by adult males.

Incomplete control and concessions explain mating skew in male chimpanzees

Sexual selection theory predicts that because male reproductive success in mammals is limited by access to females, males will attempt to defend access to mates and exclude rivals from mating. In mammals, dominance rank is correlated with male reproductive success; however, the highest-ranking (alpha) male rarely monopolizes reproduction completely. To explain why, incomplete control models propose that alpha males simply cannot control other males' access to mates. If true, then dominance rank should be a key factor influencing subordinate (non-alpha) male mating success. Alternatively, the concession model states that alpha males can prevent other males from gaining access to mates but posits that they concede matings to subordinates in exchange for social favours. This predicts that a male's grooming interactions with the alpha should mediate his access to females. We test these predictions using 36 years of data, encompassing the tenures of eight alpha male chimpanzees at Gombe National Park, Tanzania. Incomplete control models were most strongly supported. At a given copulation event, the probability that the alpha was the male that mated was negatively associated with the number of males and sexually receptive females in the party. Additionally, as the number of males increased, high dominance rank was associated with an increased likelihood that a particular non-alpha male mated. The concession model, however, was also supported. The amount of time a male spent grooming with the alpha was positively associated with his likelihood of mating when the alpha was present in the party. As grooming is a major affiliative component of male social relationships, our results suggest that social bonds with dominant individuals are leveraged for mating access, particularly in species in which males form coalitions.

Do females use their sexual status to gain resource access? Investigating food-for-sex in wolves and dogs

While food sharing among related individuals can be explained by kin selection, food sharing between unrelated individuals has been more of an evolutionary puzzle. The food-for-sex hypothesis provides an explanation for the occurrence of food sharing among nonkin. However, little is known about the socio-ecological factors that can promote such a commodity exchange. A species mating system is a factor potentially influencing food-for-sex patterns of behavior. Here, we compared wolves, which form pair-bonds, with dogs, which are typically promiscuous in free-ranging contexts, to investigate the effect of reproductive stages on the behavior around a food source in 2 different contexts. Furthermore, we considered the roles of both the males and the females in the potential food-for-sex exchange. Results indicate that in both species and for both sexes the breeding period promotes decreased aggression. Additionally, females were more persistent in their attempts to access the food and were able to monopolize the resource more when in heat as compared to outside the breeding period. Finally, in dogs, but not wolves, females spent more time in proximity to the male's bone and had a shorter latency to start eating it when in heat. Overall, this study demonstrates that the food-for-sex hypothesis plays a part in intersexual food sharing in canids, and highlights the role of females in the interaction. These effects were especially the case in dogs, suggesting a potential effect of mating system on food-for-sex responses.

Non-invasive monitoring of physiological markers in primates

The monitoring of endocrine markers that inform about an animal's physiological state has become an invaluable tool for studying the behavioral ecology of primates. While the collection of blood samples usually requires the animal to be caught and immobilized, non-invasively collected samples of saliva, urine, feces or hair can be obtained without any major disturbance of the subject of interest. Such samples enable repeated collection which is required for matching behavioral information over long time periods with detailed information on endocrine markers. We start our review by giving an overview of endocrine and immune markers that have been successfully monitored in relation to topics of interest in primate behavioral ecology. These topics include reproductive, nutritional and health status, changes during ontogeny, social behavior such as rank relationships, aggression and cooperation as well as welfare and conservation issues. We continue by explaining which hormones can be measured in which matrices, and potential problems with measurements. We then describe different methods of hormone measurements and address their advantages and disadvantages. We finally emphasize the importance of thorough validation procedures when measuring a specific hormone in a new species or matrix.

Hormonal predictors of women's extra-pair vs. in-pair sexual attraction in natural cycles: Implications for extended sexuality

In naturally cycling women, Roney and Simmons (2013) examined hormonal correlates of their desire for sexual contact. Estradiol was positively associated, and progesterone negatively associated, with self-reported desire. The current study extended these findings by examining, within a sample of 33 naturally cycling women involved in romantic relationships, hormonal correlates of sexual attraction to or interests in specific targets: women's own primary partner or men other than women's primary partner. Women's sexual interests and hormone (estradiol, progesterone, and testosterone) levels were assessed at two different time points. Whereas estradiol levels were associated with relatively greater extra-pair sexual interests than in-pair sexual interests, progesterone levels were associated with relatively greater in-pair sexual interests. Both hormones specifically predicted in-pair sexual desire, estradiol negatively and progesterone positively. These findings have implications for understanding the function of women's extended sexuality - their sexual proceptivity and receptivity outside the fertile phase, especially during the luteal phase.

Obstacles and catalysts of cooperation in humans, bonobos, and chimpanzees: behavioural reaction norms can help explain variation in sex roles, inequality, war and peace

Our closest living relatives, bonobos and chimpanzees, along with small-scale human societies figure prominently in debates about human nature. Here we emphasize and explain behavioural variation within and among these three species. In the logic of behavioural ecology, individuals have been selected to adjust their behaviour along evolved reaction norms that maximize fitness given current socio-ecological conditions. We discuss variation in three behavioural contexts: relationships between the sexes, hierarchy and inequality, and intergroup interactions. In each context, behavioural variation can be related to two broad socio-ecological conditions: (i) the defensibility of contested resources, and (ii) differences in bargaining power. When defensibility of resources and differences in bargaining power are great, interactions are rife with conflict; when they are minimal, interactions are more harmonious. These socio-ecological conditions therefore constitute key catalysts and obstacles of cooperation. We conclude that human nature should be seen as consisting of evolved reaction norms.

Trading or coercion? Variation in male mating strategies between two communities of East African chimpanzees

Across taxa, males employ a variety of mating strategies, including sexual coercion and the provision, or trading, of resources. Biological market theory (BMT) predicts that trading of commodities for mating opportunities should exist only when males cannot monopolize access to females and/or obtain mating by force, in situations where power differentials between males are low; both coercion and trading have been reported for chimpanzees (Pan troglodytes). Here, we investigate whether the choice of strategy depends on the variation in male power differentials, using data from two wild communities of East African chimpanzees (Pan troglodytes schweinfurthii): the structurally despotic Sonso community (Budongo, Uganda) and the structurally egalitarian M-group (Mahale, Tanzania). We found evidence of sexual coercion by male Sonso chimpanzees, and of trading-of grooming for mating-by M-group males; females traded sex for neither meat nor protection from male aggression. Our results suggest that the despotism-egalitarian axis influences strategy choice: male chimpanzees appear to pursue sexual coercion when power differentials are large and trading when power differentials are small and coercion consequently ineffective. Our findings demonstrate that trading and coercive strategies are not restricted to particular chimpanzee subspecies; instead, their occurrence is consistent with BMT predictions. Our study raises interesting, and as yet unanswered, questions regarding female chimpanzees' willingness to trade sex for grooming, if doing so represents a compromise to their fundamentally promiscuous mating strategy. It highlights the importance of within-species cross-group comparisons and the need for further study of the relationship between mating strategy and dominance steepness.

Sexually coercive male chimpanzees sire more offspring

In sexually reproducing animals, male and female reproductive strategies often conflict. In some species, males use aggression to overcome female choice, but debate persists over the extent to which this strategy is successful. Previous studies of male aggression toward females among wild chimpanzees have yielded contradictory results about the relationship between aggression and mating behavior. Critically, however, copulation frequency in primates is not always predictive of reproductive success. We analyzed a 17-year sample of behavioral and genetic data from the Kasekela chimpanzee (Pan troglodytes schweinfurthii) community in Gombe National Park, Tanzania, to test the hypothesis that male aggression toward females increases male reproductive success. We examined the effect of male aggression toward females during ovarian cycling, including periods when the females were sexually receptive (swollen) and periods when they were not. We found that, after controlling for confounding factors, male aggression during a female's swollen periods was positively correlated with copulation frequency. However, aggression toward swollen females was not predictive of paternity. Instead, aggression by high-ranking males toward females during their nonswollen periods was positively associated with likelihood of paternity. This indicates that long-term patterns of intimidation allow high-ranking males to increase their reproductive success, supporting the sexual coercion hypothesis. To our knowledge, this is the first study to present genetic evidence of sexual coercion as an adaptive strategy in a social mammal.

Mating behavior of adolescent male chimpanzees (Pan troglodytes) at Ngogo, Kibale National Park, Uganda

Male mating tactics vary extensively in many primates. Some variation occurs because adolescent males often are sexually active but cannot invest heavily in mating effort because of their limited ability to compete directly with adults and because they are still investing in growth; consequently, most of their mating attempts may be surreptitious and/or with females whose fecundity is low. Chimpanzees (Pan troglodytes) have a complex mating system: most copulations occur between estrous females with full sexual swelling and multiple males in group settings where the potential for sperm competition is high, but males sometimes mate-guard females, and sometimes male-female pairs mate exclusively with each other while avoiding other males during "consortships." Among other factors, dominance ranks, coalition formation, and variation in male-female association influence male mating and reproductive success. Mating effort increases from adolescence into prime adulthood. At Gombe and Mahale, adolescent males copulated more with nulliparous than with parous females, and mostly when females were unlikely to be ovulating, partly because of low adult male interest in nulliparous females and partly because of aggression from or avoidance of adult males. Adolescents thus had low probabilities of siring infants. However, adolescents are known to have gained some paternity at Gombe and in other populations, and their mating behavior deserves more study. I present data on mating by adolescent males in an unusually large chimpanzee community at Ngogo, Kibale National Park, Uganda. Adolescents at Ngogo also copulated more with nulliparous than parous females and mostly copulated outside of periovulatory periods. Also, they directed less aggression at estrous females than did adult males. However, they gained lower shares of copulations than reported for Gombe and Mahale, regardless of female parity, and received more aggression from adult males. These differences might partly reflect the influence of variation in the number of males per community on male mating tactics.

Social variables affecting mate preferences, copulation and reproductive outcome in a pack of free-ranging dogs

Mating and reproductive outcome is often determined by the simultaneous operation of different mechanisms like intra-sexual competition, mating preferences and sexual coercion. The present study investigated how social variables affected mating outcome in a pack of free-ranging dogs, a species supposed to have lost most features of the social system of wolves during domestication. We found that, although the pack comprised multiple breeding individuals, both male copulation success and female reproductive success were positively influenced by a linear combination of dominance rank, age and leadership. Our results also suggest that mate preferences affect mating outcome by reinforcing the success of most dominant individuals. In particular, during their oestrous period bitches clearly searched for the proximity of high-ranking males who displayed affiliative behaviour towards them, while they were more likely to reject the males who intimidated them. At the same time, male courting effort and male-male competition for receptive females appeared to be stronger in the presence of higher-ranking females, suggesting a male preference for dominant females. To our knowledge, these results provide the first clear evidence of social regulation of reproductive activities in domestic dogs, and suggest that some common organizing mechanisms may contribute to shape the social organization of both dogs and wolves.

Sexual conflict in primates

Sexual conflict is increasingly recognized as a major force for evolutionary change and holds great potential for delineating variation in primate behavior and morphology. The goals of this review are to highlight the rapidly rising field of sexual conflict and the ongoing shift in our understanding of interactions between the sexes. We discuss the evidence for sexual conflict within the Order Primates, and assess how studies of primates have illuminated and can continue to increase our understanding of sexual conflict and sexual selection. Finally, we introduce a framework for understanding the behavioral, anatomical, and genetic expression of sexual conflict across primate mating systems and suggest directions for future research.

Mating skew in Barbary macaque males: the role of female mating synchrony, female behavior, and male-male coalitions

A fundamental question of sexual selection theory concerns the causes and consequences of reproductive skew among males. The priority of access (PoA) model (Altmann, Ann NY Acad Sci 102:338-435, 1962) has been the most influential framework in primates living in permanent, mixed-sex groups, but to date it has only been tested with the appropriate data on female synchrony in a handful of species. In this paper, we used mating data from one large semi-free ranging group of Barbary macaques: (1) to provide the first test of the priority-of-access model in this species, using mating data from 11 sexually active females (including six females that were implanted with a hormonal contraceptive but who showed levels of sexual activity comparable to those of naturally cycling females) and (2) to determine the proximate mechanism(s) underlying male mating skew. Our results show that the fit of the observed distribution of matings with sexually attractive females to predictions of the PoA model was poor, with lower-ranking males mating more than expected. While our work confirms that female mating synchrony sets an upper limit to monopolization by high-ranking individuals, other factors are also important. Coalitionary activity was the main tactic used by males to lower mating skew in the study group. Coalitions were expressed in a strongly age-related fashion and allowed subordinate, post-prime males to increase their mating success by targeting more dominant, prime males. Conversely, females, while mating promiscuously with several males during a given mating cycle, were more likely to initiate their consortships with prime males, thus reducing the overall effectiveness of coalitions. We conclude that high-ranking Barbary macaque males have a limited ability to monopolize mating access, leading to a modest mating skew among them.

Mating promiscuity and reproductive tactics in female black and gold howler monkeys (Alouatta caraya) inhabiting an island on the Parana river, Argentina

In several primate species, females mate promiscuously and several adult males peacefully co-reside in the same social group. We investigated female mating behavior in two neighboring multimale-multifemale groups of Alouatta caraya in northern Argentina (27 degrees 20'S-58 degrees 40'W). All adult individuals in each group were marked with identification anklets and ear tags, and followed for five consecutive full days per month during 20 consecutive months. We recorded 219 copulations for eight resident females in these two groups. Thirty-two percent of matings involved extra-group copulations and 68% were with resident males. During periods when females were likely to conceive and during periods when females were nonfertile (pregnancy and lactation), there were no significant differences in the average number of resident and nonresident males with which they copulated (G-test: G(adj)=0.1, df=3, P>0.05). In both of our study groups, adult males were tolerant of the mating activities between resident males and resident females, but acted aggressively and collectively (howling, border vigilance, and fighting) when extragroup males attempted to enter the group and mate with resident females. Given the frequency of extragroup matings, we examined the distance females traveled to engage in these copulations, time engaged in pre- and postcopulatory behavior, and the risk of injury during extragroup copulations. These costs were found to be relatively small. We suggest that female promiscuity is the prime driver or constraint on male reproductive opportunities in this species. Female promiscuity in A. caraya appears to represent a mixed mating strategy that may serve to increase opportunities for genetic diversity between a female's successive offspring as well as minimize the risk of infanticide by spreading paternity estimates across a larger number of adult males.

Paternity and social rank in wild chimpanzees (Pan troglodytes) from the Budongo Forest, Uganda

We analyzed patterns of paternity and male dominance rank in the Sonso community of wild East African chimpanzees (Pan troglodytes schweinfurthii) in the Budongo Forest, Uganda. Our major objective was to determine whether and how social rank influenced paternity success. We successfully genotyped 52 individuals at up to nine microsatellite loci, using DNA extracted from fecal samples. Of 24 offspring analyzed, we identified sires for 21. Paternity success was significantly correlated with social rank, with alpha males siring a disproportionate number of offspring. However, both middle- and low-ranking males also fathered offspring, and the priority-of-access model provided a relatively poor prediction of which males would be successful and under what circumstances. The concentration of paternities among only seven males and the tendency for high-ranking males to sire offspring of multiparous females suggest that both individual variation in male quality and the resource value of particular females may be mediating factors. In comparison with other chimpanzee studies, our results support the hypothesis that larger male cohort size reduces the ability of the alpha male to monopolize females, though within our study, male number did not affect the success of the alpha. Successful sires were not necessarily those who achieved the highest mating success with the females whose offspring they sired, but were those who demonstrated higher investment by spending significantly more time in association with these females. Finally, we estimate extra-group paternity at 0-5%, supporting other evidence that the community serves as the primary reproductive unit in chimpanzees.

Mothers matter! Maternal support, dominance status and mating success in male bonobos (Pan paniscus)

Variation in male mating success is often related to rank differences. Males who are unable to monopolize oestrous females alone may engage in coalitions, thus enhancing their mating success. While studies on chimpanzees and dolphins suggest that coalitions are independent of kinship, information from female philopatric species shows the importance of kin support, especially from mothers, on the reproductive success of females. Therefore, one might expect a similar effect on sons in male philopatric species. We evaluate mating success determinants in male bonobos using data from nine male individuals from a wild population. Results reveal a steep, linear male dominance hierarchy and a positive correlation between dominance status and mating success. In addition to rank, the presence of mothers enhances the mating success of sons and reduces the proportion of matings by the highest ranking male. Mothers and sons have high association rates and mothers provide agonistic aid to sons in conflicts with other males. As bonobos are male-philopatric and adult females occupy high dominance status, maternal support extends into adulthood and females have the leverage to intervene in male conflicts. The absence of female support to unrelated males suggests that mothers gain indirect fitness benefits by supporting their sons.

No evidence of short-term exchange of meat for sex among chimpanzees

The meat-for-sex hypothesis posits that male chimpanzees (Pan troglodytes) trade meat with estrous females in exchange for short-term mating access. This notion is widely cited in the anthropological literature and has been used to construct scenarios about human evolution. Here we review the theoretical and empirical basis for the meat-for-sex hypothesis. We argue that chimpanzee behavioral ecology does not favor the evolution of such exchanges because 1) female chimpanzees show low mate selectivity and require little or no material incentive to mate, violating existing models of commodity exchange; and 2) meat-for-sex exchanges are unlikely to provide reproductive benefits to either partner. We also present new analyses of 28 years of data from two East African chimpanzee study sites (Gombe National Park, Tanzania; Kanyawara, Kibale National Park, Uganda) and discuss the results of previously published studies. In at least three chimpanzee communities, 1) the presence of sexually receptive females did not increase hunting probability, 2) males did not share preferentially with sexually receptive females, and 3) sharing with females did not increase a male's short-term mating success. We acknowledge that systematic meat sharing by male chimpanzees in expectation of, or in return for, immediate copulations might be discovered in future studies. However, current data indicate that such exchanges are so rare, and so different in nature from exchanges among humans, that with respect to chimpanzees, sexual bartering in humans should be regarded as a derived trait with no known antecedents in the behavior of wild chimpanzees.

Wild chimpanzees exchange meat for sex on a long-term basis

Humans and chimpanzees are unusual among primates in that they frequently perform group hunts of mammalian prey and share meat with conspecifics. Especially interesting are cases in which males give meat to unrelated females. The meat-for-sex hypothesis aims at explaining these cases by proposing that males and females exchange meat for sex, which would result in males increasing their mating success and females increasing their caloric intake without suffering the energetic costs and potential risk of injury related to hunting. Although chimpanzees have been shown to share meat extensively with females, there has not been much direct evidence in this species to support the meat-for-sex hypothesis. Here we show that female wild chimpanzees copulate more frequently with those males who, over a period of 22 months, share meat with them. We excluded other alternative hypotheses to exchanging meat for sex, by statistically controlling for rank of the male, age, rank and gregariousness of the female, association patterns of each male-female dyad and meat begging frequency of each female. Although males were more likely to share meat with estrous than anestrous females given their proportional representation in hunting parties, the relationship between mating success and sharing meat remained significant after excluding from the analysis sharing episodes with estrous females. These results strongly suggest that wild chimpanzees exchange meat for sex, and do so on a long-term basis. Similar studies on humans will determine if the direct nutritional benefits that women receive from hunters in foraging societies could also be driving the relationship between reproductive success and good hunting skills.

Male dominance rank and reproductive success in chimpanzees, Pan troglodytes schweinfurthii

Competition for fertile females determines male reproductive success in many species. The priority of access model predicts that male dominance rank determines access to females, but this model has been difficult to test in wild populations, particularly in promiscuous mating systems. Tests of the model have produced variable results, probably because of the differing socioecological circumstances of individual species and populations. We tested the predictions of the priority of access model in the chimpanzees of Gombe National Park, Tanzania. Chimpanzees are an interesting species in which to test the model because of their fission-fusion grouping patterns, promiscuous mating system and alternative male mating strategies. We determined paternity for 34 offspring over a 22-year period and found that the priority of access model was generally predictive of male reproductive success. However, we found that younger males had higher success per male than older males, and low-ranking males sired more offspring than predicted. Low-ranking males sired offspring with younger, less desirable females and by engaging in consortships more often than high-ranking fathers. Although alpha males never sired offspring with related females, inbreeding avoidance of high-ranking male relatives did not completely explain the success of low-ranking males. While our work confirms that male rank typically predicts male chimpanzee reproductive success, other factors are also important; mate choice and alternative male strategies can give low-ranking males access to females more often than would be predicted by the model. Furthermore, the success of younger males suggests that they are more successful in sperm competition.