Abstract
Researchers from various disciplines have hypothesized a positive correlation between the level of intergroup contest competition (IGCC) and the evolution of behavioural traits, such as cooperation, altruism and friendship, which promote intragroup affiliation. Empirical support for this hypothesis is, however, scarce and mainly available from humans. We tested whether the level of IGCC affects intragroup affiliation (i.e. intragroup grooming exchange) among male and female nonhuman primates. To quantify intragroup affiliation, we used social network measures and a grooming index. Our measure of IGCC combined frequency of intergroup encounters and proportion of aggressive encounters and was calculated separately for males and females. We ran our analyses on 27 wild groups of primates belonging to 15 species (13 Cercopithecinae, one Colobinae and one Cebinae). Our analyses reveal a clear pattern of correlated evolution between grooming network density and interindividual variation in the number of grooming partners on the one hand and the intensity of IGCC on the other in females, but not males. Thus, our results suggest that the exact nature of the relationship between IGCC and intragroup affiliation is sex specific. These results may be explained by the differential costs and benefits males and females experience during aggressive intergroup confrontations and by sex-specific differences in intragroup affiliation.
Highlights
Researchers from various disciplines have hypothesised a positive correlation between the level of inter-group contest competition (IGCC) and the evolution of behavioural traits, such as co-operation, altruism and friendship, which promote intra-group affiliation
There is consistent experimental evidence in humans showing that co-operation increases when groups are competing with one another compared to situations without inter-group competition; this effect is independent from kin relationships amongst group members (Erev et al 1993; West et al 2006; Tan and Bolle 2007; Puurtinen and Mappes 2009)
Females A phylogenetic signal was detected for the standard deviation of the out degree (SDOD; lambda = 0.90) and for density, but not for the Shannon-Wiener index (SWI; Table 1)
Summary
Researchers from various disciplines have hypothesised a positive correlation between the level of inter-group contest competition (IGCC) and the evolution of behavioural traits, such as co-operation, altruism and friendship, which promote intra-group affiliation. Individuals in populations/species where intra-group affiliation is strong should be more likely to take part in collective actions (such as protecting a food source from other groups) than individuals in populations/species where intra-group affiliation is weak (Conradt and List 2009; Miller et al 2013) Following this logic, researchers from a range of different disciplines, including evolutionary biology (Reeve and Hölldobler 2007), behavioural ecology (Wrangham 1980; Sterck et al 1997), psychology (West et al 2006; van Vugt and Park 2009) and anthropology (Haas 1990; Choi and Bowles 2007), have predicted that a high level of IGCC should favour the evolution of behavioural traits which promote intra-group affiliation, such as social bonding and parochialism (Alexander and Borgia 1978; Reeve and Hölldobler 2007). It remains unclear if such effects are only short-lived and temporary (e.g. an emotional response to the stress of the encounter; Radford 2008a, 2011; Polizzi di Sorrentino 2012), if such an increase in affiliative behaviour occurs primarily between already closely bonded partners and/or those who have actively participated to the inter-group encounter (i.e. affiliation is partner specific), or if such behavioural responses to inter-group encounters would lead to an overall more general increase in intra-group affiliation over evolutionary time
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