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

Abstract

Highlights• Wild male Barbary macaques formed stable, strong, differentiated social bonds.• Strong bonds endured even through intensely competitive mating seasons.• Rank and bond strength predicted which bystander males were recruited to coalitions.• Males rejected recruitment from weakly bonded and low-ranked recruiters.• Supporter selection is based on two independent criteria. 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. Keywords bystander; coalition; Macaca sylvanus; male-male competition; recruitment; social bonds Social relationships among animals living in stable social groups are competitive as well as affiliative, since group members compete for access to resources while establishing social bonds with both kin and nonkin (Cords, 1997, Hinde, 1976, Hinde, 1983, Massen et al., 2010, Silk, 2005and Silk, 2007. If dyadic affiliative relationships are differentiated within a social group, those relationships that are characterized by high frequencies of affiliative interactions, a relatively balanced directionality of exchange and temporal stability can be construed as...

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

“…Male macaques cooperate frequently with bonded male partners in agonistic coalitions (35,37,71,72). Thus, as in humans and other mammals, individuals with strong social bonds may benefit from greater social support from other group members (4,6,7,59).…”

“…bonds between males can yield a number of adaptive benefits, including increased social status and mating and paternity success (34,35,37,39,70). Male macaques cooperate frequently with bonded male partners in agonistic coalitions (35,37,71,72).…”

“…Parallel dispersal has been observed (43), and in large provisioned groups maternal relatedness partly drives agonistic support (44), but the strength of male social bonds is not decreased in maternally unrelated males in the wild (35). Males vary in the number and strength of social bonds they form (37), which may partly be guided by age (36,45) and may additionally be affected by personality (46,47).…”

“…Similar to philopatric female baboons and male chimpanzees (38,40,41) macaque males of some species, including Barbary macaques (Macaca sylvanus), form strong social relationships with a few male partners (35,36,39,42) that are stable over consecutive years and characterized by equitability in exchanges of affiliation (37). The mechanisms guiding partner selection for the formation of social bonds in male macaques are currently unknown.…”

Adaptations for social cognition in the primate brain

“…Less attention has been paid to correlates of cooperation and fitness among primate males, for whom reproductive success is often more strongly correlated with dominance rank, which in turn is related to condition and fighting ability [26]. In recent years, however, it has become evident that the presence of allies can, in some instances, also influence the dominance ranks, tenure and reproductive success of primate males (Assamese macaques, Macaca assamensis [27]; Barbary macaques, Macaca sylvana [28]; geladas, Theropithecus gelada [29]). This observation also holds true for chimpanzees (Pan troglodytes [30,31]), whose male-bonded, fission -fusion society is very different from that of most Old World monkeys.…”