Male Barbary Macaques Research Articles

Male Barbary macaques choose loyal coalition partners which may increase their coalition network betweenness

AbstractReciprocity in the form of contingent exchanges of goods and services is widespread across animals. While there is ample evidence for helping to be contingent upon the help received from a partner, less attention has been paid to partner avoidance based on harm inflicted by a partner. Here, we investigated whether partner choice for agonistic support against powerful targets is guided by loyalty received, i.e., the tendency to refrain from attacking the subject in a coalition with any third partner. We further assessed whether loyalty received by all cooperation partners may generate increased levels of betweenness in the coalition network of a group, a measure of indirect connectedness that has previously been associated with fitness benefits. Based on observational data from male coalitions against male group mates in Barbary macaques (Macaca sylvanus), loyalty received was found to predict the frequency of cooperation in coalitions and the loyalty given to a partner. We propose that loyalty‐guided reciprocity will be favored in species with rank‐changing coalitions where defection is particularly risky. The more loyal a male's cooperation partners were, the more central he was in the coalition network in terms of higher in betweenness, suggesting a cognitively simple strategy underlying complex network positioning. Analyses of simulated data suggest strong correlations of loyalty and betweenness to be more prevalent in the relatively small groups characteristic of many primate species.

Simultaneous Determination of Cortisol and Testosterone from Rat Serum by Liquid Chromatography-Tandem Mass Spectrometry

A liquid chromatography-electrospray ionization tandem mass spectrometry in a positive mode (LC-ESI-MS/MS) was developed to determine simultaneously the levels of cortisol and testosterone in rat serum. The MRM (Multiple Reaction Monitoring) was used to optimize ion precursor, fragmentor voltage, ion product, and collision energy. Analytical samples were treated by solid-phase extraction with Bond Elut C18 cartridge. The method was validated according to FDA guidance for bioanalytical method validation. The results indicated that this method showed a wide linearity range (0.1-100 ng/mL for cortisol and 0.025-100 ng/mL for testosterone), good linearity (r2 > 0.999), low limit of quantification (LLOQ values of cortisol and testosterone were 0.1 ng/mL and 0.025 ng/mL, respectively), good recovery (90.4% - 108.0%), suitable repeatability (CV values were in the range of 2.7% - 14.8%), good stability in different conditions. The validated method was applied to quantify cortisol and testosterone levels in several rat blood samples.
 Keywords: LC-ESI-MS/MS, cortisol, testosterone, rat serum, solid-phase extraction.
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Testosterone and cortisol are negatively associated with ritualized bonding behavior in male macaques

Neuroendocrine research on the formation of social bonds has primarily focused on the role of nonapeptides. However, steroid hormones often act simultaneously to either inhibit or facilitate bonding. Testosterone is proposed to mediate a trade-off between male mating effort and nurturing behavior; therefore, low levels are predicted during periods of nurturing infant care and social bonding. In species where social bonding and support regulates hypothalamic-pituitary-adrenal axis activity, we also expect glucocorticoid levels to be low during bonding periods. We investigated how immunoreactive urinary testosterone (iuT) and cortisol (iuC) were related to triadic male-infant-male interactions – a ritualized male bonding behavior – as well as infant care in male Barbary macaques (Macaca sylvanus). We collected >3000 h of behavioral observation data during full-day focal animal follows from 14 adult males and quantified iuT and iuC from 650 urine samples. iuT was negatively correlated with rates of triadic interactions within subjects, but positively correlated between subjects. iuC was negatively correlated with triadic interactions both within and between subjects. Time spent caring for infants was positively correlated to both iuT and iuC within subjects, but not between subjects. The observed negative relationship between iuT and triadic interactions within subjects may be beneficial to lower competitive tendencies between adult males and to not inhibit bond formation. However, the positive correlation of iuT with triadic interactions between subjects was unexpected. We speculate that it could be due to a link between triadic interactions and coalition formation. A negative relationship between triadic interactions and iuC could reflect increased bonding and perceived social support as triadic interactions predict future coalition formation in this species, or reflect buffered tensions between males. The positive relationship of iuT and iuC with infant care suggests that the handling of infants may be less nurturing but rather protective or competitive in this species. Measuring steroid hormones in relation to bonding and nurturing can help us interpret behaviors within the ecological contexts that they occur.

Oxytocin increases after affiliative interactions in male Barbary macaques

Mammals living in stable social groups often mitigate the costs of group living through the formation of social bonds and cooperative relationships. The neuropeptide hormone oxytocin (OT) is proposed to promote both bonding and cooperation although only a limited number of studies have investigated this under natural conditions. Our aim was to assess the role of OT in bonding and cooperation in male Barbary macaques (Macaca sylvanus). First, we tested for an effect of affiliation - grooming and triadic male-infant-male interactions - with bond and non-bond partners on urinary OT levels. Second, we tested whether grooming interactions (and thus increased OT levels) increase a male's general propensity to cooperate in polyadic conflicts. We collected >4000 h of behavioral data on 14 adult males and measured OT levels from 139 urine samples collected after affiliation and non-social control periods. Urinary OT levels were higher after grooming with any partner. By contrast, OT levels after male-infant-male interactions with any partner or with bond partners were not different from controls but were higher after interactions with non-bond partners. Previous grooming did not increase the likelihood of males to support others in conflicts. Collectively, our results support research indicating that OT is involved in the regulation of adult affiliative relationships. However, our male-infant-male interaction results contradict previous studies suggesting that it is affiliation with bond rather than non-bond partners that trigger the release of OT. Alternatively, OT levels were elevated prior to male-infant-male interactions thus facilitating interaction between non-bond partners. The lack of an association of grooming and subsequent support speaks against an OT linked increase in the general propensity to cooperate.

Complex patterns of grooming and sexual activity in Barbary macaques (Macaca sylvanus).

Grooming in primates is often considered a "currency" that can be exchanged for other "services" or "commodities" such as reciprocal grooming, coalitionary support, infant handling, tolerance around food sources, active food sharing, or mating opportunities. Previous studies on primate grooming-for-sex exchange viewed the males as the demanding class, with the females as suppliers of mating opportunities. In this study, we examine the broader context of grooming-for-mating exchange in Barbary macaques in Gibraltar. Our data show that Barbary macaque males groom females with whom they are mating more frequently and for longer periods than other females, and the relationship between grooming and mating remains significant in both sexual and nonsexual contexts. In addition, females groomed males with whom they were mating more frequently and for longer periods than other males. In both sexes, grooming was observed to be far more frequent and to occur for longer durations in sexual compared to nonsexual contexts. We did not find any difference in grooming behavior between presexual and postsexual contexts. Our data suggest that there is no simple model to describe Barbary macaque grooming patterns in sexual contexts. Although our results are partly consistent with male use of grooming as payment for mating, broadly assessed grooming-mating patterns cannot be solely explained by a male-driven grooming-for-mating exchange.

Measuring urinary cortisol and testosterone levels in male Barbary macaques: A comparison of EIA and LC–MS

The development of methods to quantify hormones from non-invasively collected samples such as urine or feces has facilitated endocrinology research on wild-living animals. To ensure that hormone measurements are biologically meaningful, method validations are strongly recommended for each new species or sample matrix. Our aim was to validate three commonly used enzyme immunoassays (EIA), one for analysis of cortisol and two for analysis of testosterone, to assess adrenocortical and gonadal endocrine activity, respectively, from the urine of male Barbary macaques. We compared EIA and liquid chromatography–mass spectrometry (LC–MS) results to determine if the EIA measurements truly reflect levels of the target hormone and to determine if antibody cross-reactivities with other steroids were potentially confounding results. Furthermore, we conducted a biological validation of testosterone to ensure that both EIA and LC–MS were able to capture physiologically meaningful differences in hormone levels. We found that cortisol measured by EIA correlated strongly with cortisol measured by LC–MS in both adult and immature males, without the need for deconjugation of steroids in the urine. Both testosterone EIAs correlated strongly with LC–MS in adult males, but only if steroids in the urine were deconjugated by enzymatic hydrolysis prior to analysis. However, in immature males, EIA and LC–MS results did not correlate significantly. Further correlation analyses suggest this is likely due to cross-reactivity of the testosterone antibodies with other adrenal steroids such as cortisol, DHEA, and likely others, which are present at much higher concentrations relative to testosterone in immature males. Testosterone levels were significantly higher in adult compared to immature males as measured by LC–MS but not as measured by EIA. Taken together, our results suggest that the testosterone EIAs are suitable to assess gonadal activity in adult but not immature males, and only if a hydrolysis of the urine is conducted prior to analysis.

Correlates of androgens in wild male Barbary macaques: Testing the challenge hypothesis.

Investigating causes and consequences of variation in hormonal expression is a key focus in behavioral ecology. Many studies have explored patterns of secretion of the androgen testosterone in male vertebrates, using the challenge hypothesis (Wingfield, Hegner, Dufty, & Ball, 1990; The American Naturalist, 136(6), 829-846) as a theoretical framework. Rather than the classic association of testosterone with male sexual behavior, this hypothesis predicts that high levels of testosterone are associated with male-male reproductive competition but also inhibit paternal care. The hypothesis was originally developed for birds, and subsequently tested in other vertebrate taxa, including primates. Such studies have explored the link between testosterone and reproductive aggression as well as other measures of mating competition, or between testosterone and aspects of male behavior related to the presence of infants. Very few studies have simultaneously investigated the links between testosterone and male aggression, other aspects of mating competition and infant-related behavior. We tested predictions derived from the challenge hypothesis in wild male Barbary macaques (Macaca sylvanus), a species with marked breeding seasonality and high levels of male-infant affiliation, providing a powerful test of this theoretical framework. Over 11 months, 251 hr of behavioral observations and 296 fecal samples were collected from seven adult males in the Middle Atlas Mountains, Morocco. Fecal androgen levels rose before the onset of the mating season, during a period of rank instability, and were positively related to group mating activity across the mating season. Androgen levels were unrelated to rates of male-male aggression in any period, but higher ranked males had higher levels in both the mating season and in the period of rank instability. Lower androgen levels were associated with increased rates of male-infant grooming during the mating and unstable periods. Our results generally support the challenge hypothesis and highlight the importance of considering individual species' behavioral ecology when testing this framework.

Age-dependent change of coalitionary strategy in male Barbary macaques.

Inter- and intra-specific variation in the propensity to form coalitions has been explained by variation in the availability of suitable partners, distribution of fighting ability, coalition profitability, and costs of coordination. Male coalition formation can be an alternative reproductive strategy to one-on-one competition to maximize male reproductive success. Here we focus on age as a state variable to explain within-group variation in individual propensity to form coalitions against other group members. We specifically test the prediction that males conditionally switch from a solo strategy for achievement of high mating success to a cooperative strategy after reaching post-prime age in male Barbary macaques (Macaca sylvanus). We combined new observations with data collected in 2006 and 2008 on the same individuals from one captive group living in semi-natural conditions at Affenberg Salem, Germany, and found that in all years males between 5 and 13 years formed significantly fewer coalitions than males 14 years and older (post-prime). More importantly, we found those males that aged into the post-prime phase to have switched their reproductive strategy and to form significantly more coalitions in 2014 compared to 2008. These first longitudinal data together with earlier cross-sectional analyses in this and other primate species suggest that group-level measures of coalition propensity may be strongly affected by the age composition of groups and that male coalition formation can be a conditional reproductive strategy.

Metabolic strategies in wild male Barbary macaques: evidence from faecal measurement of thyroid hormone.

Selection is expected to favour the evolution of flexible metabolic strategies, in response to environmental conditions. Here, we use a non-invasive index of basal metabolic rate (BMR), faecal thyroid hormone (T3) levels, to explore metabolic flexibility in a wild mammal inhabiting a highly seasonal, challenging environment. T3 levels of adult male Barbary macaques in the Atlas Mountains, Morocco, varied markedly over the year; temporal patterns of variation differed between a wild-feeding and a provisioned group. Overall, T3 levels were related to temperature, foraging time (linked to food availability) and intensity of mating activity, and were higher in the provisioned than in the wild-feeding group. In both groups, T3 levels began to increase markedly one month before the start of the mating season, peaking four to six weeks into this period, and at a higher level in the wild-feeding group. Our results suggest that while both groups demonstrate marked metabolic flexibility, responding similarly to ecological and social challenges, such flexibility is affected by food availability. This study provides new insights into the way Barbary macaques respond to the multiple demands of their environment.

The functions of non-reproductive mounts among male Barbary macaques (Macaca sylvanus).

Same-sex, non-reproductive mounts have been observed in a number of primate species and in various social contexts. However, the function of non-reproductive mounts is still largely unknown. We aimed to test whether non-reproductive mounts function to assert dominance and as appeasement behavior in male Barbary macaques. We analyzed post-mount behavior in 54 macaques belonging to two captive groups at Trentham Monkey Forest in Staffordshire, using 10 min post-mount/matched-control focal sessions collected either on the mounter or the mountee. In support of the dominance assertion hypothesis, the higher-ranking male within a mounting pair was more likely to be the mounter than the mountee, and to mock bite the lower-ranking male. In support of the appeasement hypothesis, the former mounting partners were more likely to exchange grooming and to have a lower frequency of self-scratching (a measure of social tension) after a non-reproductive mount than in control sessions. Our study indicates that non-reproductive mounts have different and not mutually exclusive functions and can modulate the quality of social interactions among group members. We discuss the possible factors that can affect the occurrence of non-reproductive mounts within and between species.

Responses to social and environmental stress are attenuated by strong male bonds in wild macaques.

In humans and obligatory social animals, individuals with weak social ties experience negative health and fitness consequences. The social buffering hypothesis conceptualizes one possible mediating mechanism: During stressful situations the presence of close social partners buffers against the adverse effects of increased physiological stress levels. We tested this hypothesis using data on social (rate of aggression received) and environmental (low temperatures) stressors in wild male Barbary macaques (Macaca sylvanus) in Morocco. These males form strong, enduring, and equitable affiliative relationships similar to human friendships. We tested the effect of the strength of a male's top three social bonds on his fecal glucocorticoid metabolite (fGCM) levels as a function of the stressors' intensity. The attenuating effect of stronger social bonds on physiological stress increased both with increasing rates of aggression received and with decreasing minimum daily temperature. Ruling out thermoregulatory and immediate effects of social interactions on fGCM levels, our results indicate that male Barbary macaques employ a tend-and-befriend coping strategy in the face of increased environmental as well as social day-to-day stressors. This evidence of a stress-ameliorating effect of social bonding among males under natural conditions and beyond the mother-offspring, kin or pair bond broadens the generality of the social buffering hypothesis.

Male social bonds and rank predict supporter selection in cooperative aggression in wild Barbary macaques

Cooperation in coalitions against coresident males has been shown to increase male reproductive success directly via increased mating success (levelling coalitions) or indirectly via increased dominance success (rank-changing coalitions). Two mechanisms guiding coalitionary supporter selection have been proposed. First, supporter selection may depend on the supporters available, whereby an animal chooses the highest ranking supporter present to maximize their chance of winning. Second, males may also select supporters based on the strength of the social bond they share with them. Different studies on male Barbary macaques, Macaca sylvanus, have produced support for both mechanisms but crucial assumptions and predictions remained untested. The aim of this study was to test predictions derived for both mechanisms after establishing whether Barbary macaque males formed social bonds. We observed two wild groups of macaques in Morocco (>2000. h focal animal data) and recorded the identity of males recruited to join a coalition, of all bystanders, and of the coalitionary target. We demonstrate for the first time that male Barbary macaques formed strong, equitable social bonds that were stable for 2 years. Corroborating earlier studies we found that males selected supporters by more than one criterion, namely by the strength of their social bonds to the potential ally and by their dominance rank position among potential supporters. The animals who received recruitment signals were more likely to reject the recruitment invitation the weaker their social bond to the recruiter was and if the target was higher ranking than the recruiter. In a subset in which we examined only levelling coalitions that would flatten the mating skew, males only used the mechanism that would maximize the feasibility of the coalition by more frequently selecting the highest ranked bystander. These results suggest that males flexibly apply different criteria for supporter selection depending on the context of the conflict. © 2014 The Association for the Study of Animal Behaviour.

Impacts of tourism on anxiety and physiological stress levels in wild male Barbary macaques

Wildlife tourism is a burgeoning global industry with the potential to make a significant contribution to the conservation of endangered species. However, a number of studies have provided evidence that tourists’ presence and behaviour may impact negatively on the animals involved, with potentially harmful consequences for their health, reproduction and population viability. Here, we investigate impacts of tourism on wild male Barbary macaques ( Macaca sylvanus) in Morocco, quantifying a behavioural index of animals’ anxiety (self-scratching) and a measure of their physiological stress levels (faecal glucocorticoid concentrations – FGCs). Four measures of tourist presence, number or proximity were explored: maximum number, percentage of time present, mean number while present, and closest proximity to the macaques. In addition, rates of three types of interactions between tourists and macaques – neutral (e.g. photographing), feeding and aggressive – were quantified. Males’ rates of self-scratching were positively related to the mean number of tourists present and to rates of all three human-macaque interactions, but were unrelated to the other three measures of tourist pressure. FGCs were positively related to rates of aggressive interactions between humans and macaques, but unrelated to any of the other six measures of tourist pressure. These findings suggest that while tourist presence and interactions (even apparently innocuous ones) with the macaques elevate the study animals’ anxiety levels, only aggressive interactions are sufficient to elicit a detectable increase in our measure of physiological stress. These results can be used to inform management of tourism both at this site, and at other locations where tourists view and can interact with wild primates.

Social bonds predict future cooperation in male Barbary macaques, Macaca sylvanus

Social bonds have been construed as mental representations mediating social interactions among individuals. It is problematic, however, to differentiate this mechanism from others that assume more direct exchanges and interchanges of behaviour such as reciprocity, market effects or mutualism. We used naturally occurring shifts in rates and patterns of social interactions among male Barbary macaques to test whether affiliation in the nonmating season predicts coalition formation in the mating season. We carried out 1377 h of observation of all 23 males living in a semifree-ranging bisexual group at Affenberg Salem, Germany. The mating season was characterized by significantly increased rates of aggression, male coalition formation and spontaneous submission as well as decreased rates of affiliation. Rates of coalition formation in the mating season were predicted by affiliation between these males several weeks earlier in the nonmating season after we controlled for affiliation in the mating season. We conclude that social bonding in the nonmating season may build reputations among partners that are crucial for cooperation in risk-prone agonistic coalitions against other males. From the pronounced time lag between bonding and cooperation and the fact that males largely ignored affiliation as well as agonistic patterns in the time-matched mating season, we conclude that short-term reciprocity, mutualism or market effects cannot explain these observations. Instead, we conclude that long-term reciprocity mediated by emotional book keeping may be the basic mechanism. Building on that, males may employ a mental representation of their social bonds when choosing partners for cooperative interactions.

Coalitions destabilize dyadic dominance relationships in male Barbary macaques (Macaca sylvanus)

Abstract Dyadic agonistic dominance relationships are thought to result from asymmetries in both intrinsic and extrinsic power. One form of extrinsic power is the ability to solicit agonistic support from other individuals. In extreme cases extrinsic power differences may override intrinsic power differences so that physically inferior individuals attain rank positions above stronger competitors. In other cases superior extrinsic power in physically inferior individuals may destabilize the otherwise clear dominance relationships. We tested this prediction with observational data on adult males in one of three free-ranging groups of Barbary macaques at Affenberg Salem, Germany. All prime males that were subjects of this study were at least 5–8 years (average 10 years) younger than the old post-prime males that were all subordinate to them. Assuming large age differences to reflect large intrinsic/physical power differences, interactions between these prime and the old males allowed investigation of the separate effects of intrinsic and extrinsic power asymmetries on dominance relationships. We estimated relationship instability using four different estimates (counter aggression, conflicts initiated by aggression from the subordinate, spontaneous submissions by dominants, and decided conflicts won by the subordinate). Relationship instability did not decrease with increasing asymmetry in intrinsic power. Instead, all four measures of relationship instability were positively related to the number of times the dominant in a dyad was target of destabilizing coalitions. Destabilizing coalitions targeted dominant males in old male–prime male dyads more often than in old male–old male dyads. Consequently, old males had less stable relationships with the much stronger prime males than with each other. We conclude that extrinsic power asymmetries predicted the instability of dyadic dominance relationships and may systematically override intrinsic power asymmetries.

Coalition formation among Barbary macaque males: the influence of scramble competition

Owing to the nature of male competition over a nonshareable resource, cooperation and coalition formation are unexpected among mammalian males. Nevertheless, coalitions of unrelated males are widespread across mammals. Recently, a mathematical model for the evolution of male coalitions within groups was developed, predicting patterns and outcomes of coalitions depending on the degree of contest competition for mates. We tested this model in a species with presumably low-contest competition. Accordingly, under low contest, coalitions should be large, formed by mid- to low-ranking males targeting top-rankers and aimed at decreasing reproductive skew (all-up levelling coalitions). We also investigated the effect of scramble competition on coalitions, as its impact was not included in the model. Scramble competition is expected to be pronounced in our study population of Barbary macaques, Macaca sylvanus, and thus we predicted that coalitions should target mainly young immigrants, irrespective of rank, to isolate them and eventually reduce male group size and, consequently, scramble competition. Data on social interactions, including coalitions, were collected on 23 males of a group of Barbary macaques living at Affenberg Salem, Germany. Coalition size was small, coalitions rarely targeted top-ranking males and all-up levelling coalitions were rare. Overall, the model was not confirmed. In contrast, more than three-quarters of coalitions targeted young immigrants, which were isolated from the group. Thus, male coalition formation in our group seemed to be influenced by scramble rather than contest competition. Both modes of competition should thus be taken into account if payoffs of coalition formation are investigated.

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.

Infants as costly social tools in male Barbary macaque networks

In mammals, females are the main caretakers of offspring. In a number of species, however, males also care for young, and may spend considerable time with infants. In various nonhuman primates, male interactions with infants appear to benefit mainly the males, who use them to regulate their relationships, particularly with other male group members. We studied whether interactions with infants were related to glucocorticoid output in male Barbary macaques, Macaca sylvanus, at the enclosure ‘La Foret des Singes’, France. We used a combination of behavioural observations, social network analysis and measures of faecal glucocorticoid metabolites as an indicator of physiological stress, and compared the results for the birth season in spring, during a time of intense interactions with infants, with a period in autumn, when males interacted less frequently with infants. Social network analyses revealed that infant carriers had stronger ties with other males than noncarriers. Overall, male relationships were stronger in spring than in autumn. Infant care was unevenly distributed among males and not related to rank, suggesting that the motivation to invest in infant carrying depends on the position in the social network and the current constellation of relationships. Despite the correlation of infant carrying with male interactions, the time spent with infants alone explained most of the variation in glucocorticoid levels, suggesting that access to the male–male network is paid for by an increase in stress hormone levels. Taken together, these results show that Barbary macaque infants can be viewed as costly social tools that facilitate networking among males.

Coalitions in male Barbary macaques, Macaca sylvanus: strength, success and rules of thumb

Several quantitative models of coalition formation assume that a coalition is successful if the strength of the coalition is greater than the strength of the target, but unsuccessful otherwise. However, strong empirical evidence in favour of this hypothesis is still lacking. In this study, we provide an empirical test of this assumption in Barbary macaque males, by using a field-based estimate of individual competitive ability from which coalition strength is derived. Coalition success was determined for 90 coalitions composed of two partners and targeted at one male. Of these, 72.2% were behaviourally successful and 27.8% were unsuccessful. Asymmetry in strength was a significant predictor of coalition success, as this factor alone could explain up to 78.6% of coalition outcomes in the study group. Males behaved as if they were at least partially informed about the nature of this asymmetry. The targets of attacks by coalitions were more likely to counterattack as asymmetry in strength decreased, and coalition partners formed coalitions that produced on average a greater asymmetry in strength than would be expected by chance. However, we provide evidence that males may have used simple rules of thumb based on their knowledge of dyadic and third-party relationships, rather than estimates of asymmetry in strength per se. We conclude that competitive ability is an important factor in coalition formation in Barbary macaque males and discuss additional factors not included in this study, which may account for the unexplained outcomes.

Male Barbary macaques eavesdrop on mating outcome: a playback study

Female copulation calls have been viewed as being addressed to the mating partner, to ensure male protection and mate guarding or, alternatively, to other males, to incite male competition. In a previous study, we found that copulation calls of female Barbary macaques, Macaca sylvanus ; given during ejaculatory matings differed from those given during nonejaculatory matings and hence provided potential information about mating outcome to listeners. In this playback study, we tested whether this variation is salient to male listeners. In addition, we assessed the influence of cues from other modalities that are related to female receptivity on male responses to copulation calls. The study was conducted on Barbary macaques living in the enclosure ‘La Foret des Singes’, at Rocamadour, France. Males responded more rapidly and more strongly, and spent more time walking and in the vicinity of females, after playbacks of calls given during ejaculatory copulations than after those given during nonejaculatory copulations. Responses, however, were not influenced by the cycle stage of the female whose calls were played back. The results suggest that male Barbary macaques appear to discount contextual information in favour of information related to mating outcome when allocating their mating effort.