Strategic social behaviour in wild vervet monkeys
In comparison to other vertebrates, primates have a large brain in relation to their body size. It has been hypothesised that the degree of social complexity is the major predictor for such variation. In group living species, individuals face various social challenges which can include finding the right balance between cooperation and competition with other group members. Thus to survive and reproduce individuals would have to show an adapted cognitive flexibility. Following this argument, two parallel hypotheses emerged; the “Machiavellian intelligence” and the “Social brain” hypotheses propose that the social complexity of group living selected respectively for advanced cognitive abilities and an increase in relative brain/neocortex size (i.e. complexity). However, finding a positive correlation between the complexity linked to social life, corresponding advanced cognitive processes and brain size/complexity remains challenging. First, adequate proxies of social complexity that could be applied to various taxa remain to be found. Second, examples of strategic social behaviour such as proposed by the Machiavellian intelligence have been described in many taxa suggesting that more comparative studies are needed to distinguish between advanced cognitive processes and those that could rely on associative learning. Finally, a potential link between cognitive abilities and brain/neocortex size remains largely unexplored. <br> By studying wild vervet monkeys (<i>Chlorocebus aethiops</i>) in South Africa, the aim of this thesis was to test for the presence of some social knowledge facets in their behaviour. I also wanted to assess their ability to use such knowledge strategically in both cooperation and competition contexts. Vervet monkeys represent an ideal species as they are highly social, have a strict linear female and male hierarchies and are usually very willing to participate in set-up experiments involving food. <br> In Chapter I, I tested the effect of natural migration, births and deaths on the individual centrality and strength of dyadic relationships within the grooming, 1m and 5m proximity social networks (i.e. method 1). I also used a new method (i.e. SIENA; method 2) to test both the network structure and the relationships dynamics. With both methods, I found a strong among-group variation. In addition, results suggest that females and juveniles have more influence than males on the stability at both the individual and dyadic levels, especially within the grooming network. Social relationships might be subject to frequent and significant changes often linked to natural demographic variation. Thus, social network analyses have the potential to capture important aspects of the cognitive social challenges an individual has to cope with. In Chapter II, I conducted rank reversal playbacks to test vervet monkeys’ knowledge about the entire group’s hierarchy. I found that females know about both female and male hierarchies while males and juveniles seem to lack such knowledge about the female hierarchy. Results therefore suggest sex and developmental differences in the extent of third party rank relationships. In Chapter III, I first trained females to consistently approach their personalised boxes to obtain a food reward, which allowed staging potential conflicts by placing two boxes next to each other. With such experiments I could show that subordinates trade grooming for tolerance and coalitionary support and that such trading is modified by the composition of the audience (i.e. individuals within 10m). However data also suggest that subordinates are not able to incorporate the effect of their grooming on dominants’ decision-making to their own advantage. <br> In summary, the results of this thesis provide important insights on vervet social strategies and underlying cognitive processes. The introduced methodological advances regarding social network analyses and experimentation to reveal social strategies offer a basis for future research on other primate species for comparison. Such data would then be amenable for correlative studies that link the results to brain evolution. In such a way, one can hope to make important progress regarding the major quest: to assess how social complexity, strategic social behaviours and brain size are interlinked.
Thèse de doctorat : Université de Neuchâtel, 2016
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