‘Impact hunters’ catalyse cooperative hunting in two wild chimpanzee communities

Gilby, Ian C.; Machanda, Zarin; Mjungu, Deus; Rosen, Jeremiah; Muller, Martin N.; Pusey, Anne E.; Wrangham, Richard W.

doi:10.1098/rstb.2015.0005

Cited by 66 publications

(84 citation statements)

References 55 publications

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“…A substantial portion of the population did not participate in any raids, whereas five individuals participated in more than 10 raids and most participated in slightly less than 3 raids. The fact that the initiation of raiding parties appears to depend on leaders who function as nucleation sites for raids and who attract other participants is consistent with research showing how individual variation within a population can contribute to the resolution of collective action problems (17,20), including in risky, intergroup violence in both humans and chimpanzees (46). Leaders may alter the costs and benefits for others-either by reducing the costs of the raid to other participants (e.g., via setting the time of the raid or by scouting) or by exerting social pressure on others to join (18,(47)(48)(49).…”

Section: Discussionmentioning

confidence: 50%

Formation of raiding parties for intergroup violence is mediated by social network structure

Glowacki

Isakov

Wrangham

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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Intergroup violence is common among humans worldwide. To assess how within-group social dynamics contribute to risky, betweengroup conflict, we conducted a 3-y longitudinal study of the formation of raiding parties among the Nyangatom, a group of East African nomadic pastoralists currently engaged in small-scale warfare. We also mapped the social network structure of potential male raiders. Here, we show that the initiation of raids depends on the presence of specific leaders who tend to participate in many raids, to have more friends, and to occupy more central positions in the network. However, despite the different structural position of raid leaders, raid participants are recruited from the whole population, not just from the direct friends of leaders. An individual's decision to participate in a raid is strongly associated with the individual's social network position in relation to other participants. Moreover, nonleaders have a larger total impact on raid participation than leaders, despite leaders' greater connectivity. Thus, we find that leaders matter more for raid initiation than participant mobilization. Social networks may play a role in supporting risky collective action, amplify the emergence of raiding parties, and hence facilitate intergroup violence in small-scale societies.warfare | social networks | collective action | pastoralists | emergence I ntergroup violence is common, worldwide, and harmful. Global annual deaths from large-scale warfare, for example, range from 0.5 to 1 million, and this does not include nonfatal physical and mental injuries (1). A diverse set of approaches has been used to study intergroup violence and warfare. Evolutionary models have credited collective violence with an important role in the development of modern human behavior (2-7), whereas cultural and ecological factors have been shown to influence small and largescale violence (8-13). More recently, there has been increased interest in understanding the dynamics of group-based violence and the social processes that can contribute to it in the setting of insurgent and terrorist groups (14, 15); for example, online records suggests small, self-organizing groups coalesce into larger groups preceding terrorist attacks (16). Warfare has also been studied as a collective action problem-because individuals must mobilize to engage in a group activity with shared gains (e.g., deterrence, territory) and individual risks (e.g., injury, death) (17, 18).Despite these advances, fundamental questions remain about how violent groups are formed, and the extent to which they may self-organize and emerge organically. Theoretical work suggests interindividual differences may be important for initiating and sustaining risky collective action, but empirical evidence in humans supporting this is sparse (19,20). Research in primate behavior provides some clues regarding the emergence of violent intergroup conflict. Wild chimpanzees engage in lethal coalitionary violence against other communities (21), and a few "impact" individuals sh...

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Section: Discussionmentioning

confidence: 50%

Formation of raiding parties for intergroup violence is mediated by social network structure

Glowacki

Isakov

Wrangham

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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“…Thus, one potential underlying proximate cause for individual differences is that both baseline and acute changes in neurohormones like testosterone or OT vary within and between individuals. As mentioned above, it is likely that such hormonally mediated personality differences play a substantial role in shaping cooperation and collective action [121]. Despite sharing the same conserved components of the endocrine architecture with other primates and mammals, humans cooperate in more contexts and at larger scales [1 -3].…”

Section: Resultsmentioning

confidence: 99%

“…In chimpanzees, certain 'impact' males increase the likelihood of a border patrol [120,121], and some of these impact patrollers also went on to become alpha male and achieve high reproductive success. It is not known whether these impact patrollers show acute spikes in testosterone beyond that of other chimpanzees on the same patrol, but such a study would be an interesting comparison given that high impact human soccer players have larger acute increases in testosterone than others on the same team [98].…”

Section: (C) Testosterone and Intergroup Competitionmentioning

confidence: 99%

Evolving the neuroendocrine physiology of human and primate cooperation and collective action

Trumble¹,

Jaeggi²,

Gurven³

2015

Phil. Trans. R. Soc. B

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While many hormones play vital roles in facilitating or reinforcing cooperative behaviour, the neurohormones underlying competitive and cooperative behaviours are largely conserved across all mammals. This raises the question of how endocrine mechanisms have been shaped by selection to produce different levels of cooperation in different species. Multiple components of endocrine physiology—from baseline hormone concentrations, to binding proteins, to the receptor sensitivity and specificity—can evolve independently and be impacted by current socio-ecological conditions or individual status, thus potentially generating a wide range of variation within and between species. Here, we highlight several neurohormones and variation in hormone receptor genes associated with cooperation, focusing on the role of oxytocin and testosterone in contexts ranging from parenting and pair-bonding to reciprocity and territorial defence. While the studies reviewed herein describe the current state of the literature with regard to hormonal modulators of cooperation and collective action, there is still a paucity of research on hormonal mechanisms that help facilitate large-scale collective action. We end by discussing several potential areas for future research.

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“…In addition to behavioural data, the database also contains detailed demographic information. Ages were estimated based on size, appearance, and sightings of the mother (for details, see [37]). …”

Section: Materials and Methods (A) Study Site And Data Collectionmentioning

confidence: 99%

Incomplete control and concessions explain mating skew in male chimpanzees

2016

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Sexual selection theory predicts that because male reproductive success in mammals is limited by access to females, males will attempt to defend access to mates and exclude rivals from mating. In mammals, dominance rank is correlated with male reproductive success; however, the highestranking (alpha) male rarely monopolizes reproduction completely. To explain why, incomplete control models propose that alpha males simply cannot control other males' access to mates. If true, then dominance rank should be a key factor influencing subordinate (non-alpha) male mating success. Alternatively, the concession model states that alpha males can prevent other males from gaining access to mates but posits that they concede matings to subordinates in exchange for social favours. This predicts that a male's grooming interactions with the alpha should mediate his access to females. We test these predictions using 36 years of data, encompassing the tenures of eight alpha male chimpanzees at Gombe National Park, Tanzania. Incomplete control models were most strongly supported. At a given copulation event, the probability that the alpha was the male that mated was negatively associated with the number of males and sexually receptive females in the party. Additionally, as the number of males increased, high dominance rank was associated with an increased likelihood that a particular non-alpha male mated. The concession model, however, was also supported. The amount of time a male spent grooming with the alpha was positively associated with his likelihood of mating when the alpha was present in the party. As grooming is a major affiliative component of male social relationships, our results suggest that social bonds with dominant individuals are leveraged for mating access, particularly in species in which males form coalitions.

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‘Impact hunters’ catalyse cooperative hunting in two wild chimpanzee communities

Cited by 66 publications

References 55 publications

Formation of raiding parties for intergroup violence is mediated by social network structure

Formation of raiding parties for intergroup violence is mediated by social network structure

Evolving the neuroendocrine physiology of human and primate cooperation and collective action

Incomplete control and concessions explain mating skew in male chimpanzees

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