Male social relationships among wild Assamese macaques (Macaca assamensis)

General socioecological theories predict that in multi-male multi-female mammalian groups, males and females compete over resources of different nature. While females mainly compete over generally sharable food, males compete over the non-sharable fertilization of each female and the formation of strong affiliative relationships with specific partners might have evolved as a strategy to minimize the costs of this competition. Lifetime co-residency in the same social group enables individuals from the philopatric sex to form strong, equitable and long-lasting affiliative relationships between group members, the so called social bonds. Social bonds in philopatric females are mainly linked to kinship, which influences an individual’s dominance rank position and hence the access to resources. The emergence of social bonds between males is more striking pertaining to the high level of competition over non-shareable fertile females which may hamper the likelihood of the formation of affiliative relationships among males. However, if two males gain mutual benefits from affiliating with each other, for instance if affiliation leads to an increase in the formation of cooperative coalition to outcompete other males and get higher dominance rank positions and the associated reproductive benefits, social bonds might arise. This type of relationship between bond and coalition formation is found in philopatric male chimpanzees. Here social bond formation may serve as an adaptive strategy to maximize the efficiency of these cooperation. In species in which males are not philopatric, the reduced co-residency time represents an additional potential obstacle to the formation of social bonds between males, beside the nature of male-male competition itself. The question therefore remain whether in female philopatric species males still form affiliative relationships with each other of the same nature than philopatric males or females. Yet, it remains unclear so far whether the affiliative relationships between dispersing males share these characteristics as well and can thus be labelled as social bonds. In addition, the proximate behavioural mechanism underlying the formation of affiliative relationships has received less attention in both, philopatric and dispersing living individuals. The overall aim of my thesis was thus, first to shed light on whether three characteristics of social bonds (differentiation in strength, equitability and partner stability) are found between males of the dispersing sex and second to assess whether a behavioural proximate mechanism other than grooming might underlie bond formation and maintenance. To address these aims, I studied two habituated groups of wild Assamese macaques (Macaca assamensis, a species in which females are philopatric and males disperse) at the Phu Khieo Wildlife Sanctuary in North-eastern Thailand. I used data collected for seven years on the AS group and for 1.5 years on the AO group. First, I assessed whether male Assamese macaques form differentiated affiliative relationships by using the composite sociality index (CSI) to examine the distribution of the frequency and duration of spatial proximity, body contact and grooming across all male-male dyads. Further, I investigated whether the strength of a male-male affiliative relationship affected the likelihood of immediate grooming reciprocation. Further, I calculated a grooming symmetry index (GSI) for each male dyad to investigate whether the strength of affiliative relationships had an effect on grooming equitability. In addition, I assessed the most crucial characteristic of social bonds (and so far the most understudied in the dispersing sex), by examining the stability of affiliative relationship over time. I calculated the partner stability index (PSI) of each male with their top 3 preferred affiliative partners and with the following three partners (number 4 , 5, 6) to assess whether stronger affiliative relationships were more stable across years than weaker ones. Finally, I investigated whether male Assamese macaques use male-infant-male interactions (MIMIs, a specific macaque behaviour known to appease the social partners and reduce the likelihood of aggression), as a proximate behavioural mechanism to form and maintain social bonds. I first investigated whether subordinate males benefited from engaging in MIMIs with a higher ranking male upon approach. Second, I assessed whether the strength of male affiliative relationship affected the frequency at which they engaged in MIMIs. Finally, I investigated whether MIMIs may play a role in bond formation by testing if the current rate of MIMIs increased the time two males spent in close proximity in the future and thus had more opportunity to bond. My analyses of about 9000 h of focal animal data show that males form social bonds. Male-male affiliative relationships were highly differentiated with the majority of male dyads sharing weaker affiliative relationships than average and few male dyads sharing stronger affiliative relationships. The stronger an affiliative relationship between two males was, the more likely they reciprocated a grooming act. In those bouts were grooming was reciprocated; males sharing a stronger affiliative relationship were more balanced in their exchange of grooming than those males sharing a weaker affiliative relationship. Partner stability among the three strongest relationships was higher than among weaker relationships, which suggests that top partners were not preserved simply because of a lack of alternatives. Regarding the proximate mechanism underling social bonds, I found that the overall frequency of MIMIs increased with the strength of the dyadic affiliative relationships between males. Further, subordinate males could increase their proximity time with higher-ranking males on average by 50% if they engage in a MIMI upon approach irrespective of the affiliative relationship strength with the male partner. The latter result, increase in proximity time irrespective of affiliative relationship strength, together with the finding that the stronger a male dyads affiliative relationship the more males engage in MIMIs, indicates that male Assamese macaques use this behaviour to maintain their affiliative relationships. In contrast, I did not find support for a role of MIMIs in bond formation since the frequency of MIMIs did not affect the time a male dyad spent in proximity in the consecutive year. However, this result may, may have been caused by a lack of newly immigrating males establishing new bonds during the study period. In summary, my results suggest that male Assamese macaques, the dispersing sex in this species, are able to form social bonds with a similar structure as the bonds reported in the philopatric sex of some other mammalian species and possibly maintain those bonds not just via grooming, but also via male-infant-male interactions. Male Assamese macaques form risky rank-challenging coalitions to rise in rank and get the associated benefit of priority of access to fertile females ultimately leading to a higher reproductive success. In this context, forming social bonds with specific individuals might be mandatory in order to benefit from a reliable coalitionary partner and thus be successful in rank-changing coalition. My findings contribute greatly to our understanding of the pattern of affiliative relationships among individual of the dispersing sex in multi-male multi-female mammalian groups by showing for the first time that social-relationships are not only differentiated across dyads but also extremely stable over time. My specific study on MIMIs provide some insight into alternative mechanisms to grooming which might be involved in bond maintenance and possibly bond formation in social groups. Altogether my results are therefore contributing to the general debate on which behaviours influence social dynamics in group living mammals.