Abstract
In multimale multifemale primate groups, the strength and stability of affiliative relationships have been shown to affect an individual's long-term fitness such as offspring survival and longevity. Studies investigating the fitness benefits of close social relationships and the underlying mechanisms have mainly focused on the philopatric sex. The strong relationships of philopatric chimpanzee males and baboon females share important characteristics with human friendships in that increased strength of affiliative relationships is associated with increased equitability in service exchanges and relationship stability. So far, it has remained unclear whether the strong relationships of dispersing males share these characteristics as well and can thus be labelled as social bonds. Here we provide results on the variation in affiliative relationship strength and its relation to equitability and relationship stability from two wild groups of male Assamese macaques, Macaca assamensis, at Phu Khieo Wildlife Sanctuary, Thailand collected over 2 and 7 years, respectively. Our analyses of almost 9000 h of focal animal data show that males formed differentiated affiliative relationships and that the strength of a relationship affected how likely males returned a grooming service within a single bout and how equally males were responsible for the maintenance of close proximity. Partner stability among the three strongest relationships was higher than among weaker relationships which suggests that top partners were not retained simply because of a lack of alternatives. Together, these results suggest that dispersing male Assamese macaques form differentiated affiliative relationships that increase in equitability and stability with increasing relationship strength. This is the first study showing long-term partner stability in males as the dispersing sex. Our results thus add to the growing body of literature indicating that nonhuman animals form close social relationships similar to human friendships.
Highlights
Living in a social group entails costs and benefits for each individual
We have previously shown for the dispersing sex with a smaller sample of 12 adult individuals observed over 2 years that affiliative relationships of male Assamese macaques, Macaca assamensis, are differentiated in strength and that in general the amount of grooming given is correlated with the amount of grooming received across all possible dyads (Schülke et al, 2010)
Our study is based on almost 9000 h of focal animal data collected over 7 consecutive years to test whether dispersing male Assamese macaques form strong, equitable and stable social relationships that qualify as social bonds
Summary
Living in a social group entails costs and benefits for each individual. On the one hand, close proximity to and daily repeated interactions with conspecifics within a group increase feeding and mating competition, the risk of disease transmission and, in some species, the risk of infanticide (Altizer et al, 2003; Krause & Ruxton, 2002; Lukas & Clutton-Brock, 2014; Ostner, Heistermann, & Schülke, 2011; Palombit, Seyfarth, & Cheney, 1997; van Schaik & Aureli, 2000; Smuts, Cheney, Seyfarth, Wrangham, & Struhsaker, 1987; Wittig & Boesch, 2003). The relationship between grooming equitability and the strength of the partners' affiliative relationship, the second characteristic of social bonds, have only been investigated in a few studies (e.g. bonnet macaques, Macaca radiata, Adiseshan, Adiseshan, & Isbell, 2011; chimpanzees, Mitani, 2009; savanna baboons, Silk, Alberts, & Altmann, 2006; chacma baboons, Papio ursinus, Silk et al, 2010b). Females' preferences for association partners were much more stable than preferences for grooming partners (Lehmann & Boesch, 2009) It remains to be shown whether the long-term stability of affiliative relationships varies with their strength in the dispersing sex in which group membership and dominance relationships are more fluid. Our study is based on almost 9000 h of focal animal data collected over 7 consecutive years to test whether dispersing male Assamese macaques form strong, equitable and stable social relationships that qualify as social bonds
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