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14 How Intelligent Is Machiavellian Behavior? Redouan Bshary, Felice Di Lascio, Ana Pinto, and Erica van de Waal Abstract The hypothesis that the complexity of social life selects for large brains is currently very prominent. Somewhat surprisingly, this functional hypothesis has been mainly tested by experiments which aim to identify the cognitive processes/mechanisms that may underlie social behavior. Such research is inherently challenging because it is extremely difficult to design experiments that conclusively allow the exclusion of simple cognitive processes as an explanation for successful behavior. Here it is argued that cognitive scientists should not focus on processes only but rather test quantitatively what animals can do with their brain: how fast, how precise, how much can they learn depending on the problem at hand. Many differences between species concerning cognitive tasks in the social domain are quantitative in nature: the number of group members and their past behavior that an individual has to recognize respectively, the number of opportunities for social learning or cooperation that arise per time unit, etc. Tests on how such quantitative differences between species translate into quantitative cognitive performances should be addressed in many species to permit a comparative approach, where predictions about relative performance can be made based on detailed knowledge of each study species’ecology. Comparative approaches are methodologically challenging but can be tackled through large-scale cooperation between scientists. Introduction The evolution of sociality has been a key research focus in evolutionary biology for a long time. One likely reason why so many evolutionary biologists find this question so interesting is that humans are highly social animals. In social species, an individual’s main challenge for successful survival and reproduction is competition with fellow group members over access to limited resources like food and mates. At the same time, group members can be important alliance partners against predators and neighboring groups. Cooperation 210 R. Bshary et al. and conflict are thus the two opposing forces that affect virtually any decision of individuals, selecting for cognitive abilities that allow individuals to cope with the complexity of decision making in this dynamic social world. Two related hypotheses, the Machiavellian intelligence hypothesis and the social brain hypothesis, propose that the challenges linked to social life caused the evolution of increased cognitive abilities and, correspondingly, an increase in relative (allometric) brain size in social species, more precisely of particular parts of the brain (e.g., the neocortex). The social brain hypothesis (Dunbar 1992; Barton and Dunbar 1997) stresses a link between social complexity and neocortex size evolution in mammals without specifying what aspects of social life might be particularly cognitively demanding. A basic assumption of this approach is that relative brain size is a good proxy for the developmental and running costs of a brain, while accepting that there will be unexplained variation because changes in cell density, connectivity, receptor density, and neurotransmitter concentration may provide alternative ways to increase or reduce costs. Initial analyses focused on group size as a correlate of social complexity and obtained positive correlations between group size and neocortex ratio (neocortex size regressed against the size of the rest of the brain) in primates, carnivores, and bats (Barton and Dunbar 1997). However, several other variables correlate positively with neocortex ratio and hence it remains unclear what aspects of social life may cause an enlargement of the neocortex (Healy and Rowe 2007). The Machiavellian intelligence hypothesis, in its original form (Byrne and Whiten 1988; Whiten and Byrne 1997), took a wide, permissive perspective on the variety of sociocognitive adaptations through which an individual may exploit the potential benefits of its social world, as well as dealing with the hostile aspects of it: social knowledge, discovery techniques, social curiosity, social problem solving , innovation, flexibility, social expertise, social play, mind reading, selfawareness , imitation, and culture were all explicitly included. However, a more refined hypothesis emerged which focuses on the importance of gaining influence and power for an individual’s fitness. In this view, key cognitive abilities of individuals are (a) the ability to understand and remember relationships between other group members in order to form strategic alliances and (b) skills in manipulation and deception of group members. The refined version corresponds well to several key questions that were addressed at the Forum: • What do different kinds of animals know about the relationships between others? • What do they know about each other’s intentions and motivations? • What do they know about each other’s knowledge and beliefs? • How...

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