Lethal aggression in Pan is better explained by adaptive strategies than human impacts

Abstract

increasing their access to key resources, such as food or mates.1-5 Alternatively, it has 5 been argued to be a non-adaptive result of human impacts, such as habitat destruction 6 or provisioning of food. [6][7][8][9] To discriminate between these hypotheses we compiled long-7 term information from 18 chimpanzee communities and 4 bonobo communities. Our 8 data include 152 killings (N=58 observed, 41 inferred, and 53 suspected killings) by 9 chimpanzees in 15 communities and one suspected killing by bonobos. We found that 10 males had the greatest involvement as attackers (92% of participants) and victims 11 (73%); most killings (66%) involved intercommunity attacks; and attackers greatly 12 outnumbered their victims (median 8:1 ratio). Variation in rates of killing among 13communities depended on demographic variables but was unrelated to measures of 14 human impacts. These results from all major study populations over the last five 15 decades are consistent with previously proposed adaptive explanations for killing by 16 chimpanzees but not with the human impact hypothesis. 17 18Conspecific killing has been documented at multiple chimpanzee study sites, 2-5,10-12 but rates 19 vary greatly among sites. The human impact hypothesis and the adaptive strategies 20 hypothesis yield contrasting predictions, which we test here (Tables 1, 2). The human impact 21 hypothesis states that killing occurs mainly as an incidental outcome of aggression, 22 exacerbated by human activities such as providing a concentrated food resource, 23 deforestation-induced crowding, anthropogenic diseases or hunting. Accordingly, lethal 24 aggression should be high where human disturbance is high. In contrast, the adaptive strategies hypothesis views aggression as an evolved strategic 27 response by which aggressors tend to increase their fitness through increased access to 28 territory, food, mates or other benefits. [1][2][3][4][5][10][11][12][13][14][15][16][17] 45Intracommunity infanticide by females may result from intense competition among females 46 for the best feeding areas.17 Population differences in rates of killing are accordingly 47 expected to result from socioecological factors such as differences in grouping patterns 2,11 48 and/or demography.14 Lethal aggression thus occurs within a diverse set of circumstances, 49 but is expected to be most commonly committed by males; directed towards males; directed 50 6 towards non-kin, particularly members of other groups; and committed when overwhelming 51 numerical superiority reduces the costs of killing. 52 53Previous studies have developed and tested these specific hypotheses 2,5,[11][12][13][14][15][16][17] ; the present study 54 represents the first effort to test multiple hypotheses simultaneously with a comprehensive 55 dataset. To do so, we assembled data from 18 chimpanzee communities from both eastern 56 (N=12) and western (N=6) clades 24 of chimpanzees studied over 426 years (median = 21 57 years; range: 4-53) and from 4 bonobo communities studied for 92 years (media...

Coalitions in theory and reality: a review of pertinent variables and processes

“…Coalitions for competition against conspecifics from other groups (hereafter 'group-wide coalitions') occur when the opponents belong to different social groups (or family units, e.g., Scheiber et al, 2005), whereas within-group coalitions occur when two or more members of the same group compete against at least one other 'ingroup' member. Group-wide coalitions can be offensive if individuals join forces to oust incumbent male(s) and take over a group (e.g., Pope, 1990;Rood, 1990;Packer et al, 1991;Waser et al, 1994;Ostner & Kappeler, 2004), or to conduct a lethal attack on members of a neighbouring group (reviewed by Wrangham, 1999;Wilson et al, 2014). They can also be defensive, as when group members form a coalition to repel single challengers and/or an out-group coalition (e.g., Feh, 1999).…”

Physically strong men are more militant: A test across four countries

“…males, Wilson et al 2014) ii) coalitional aggressors frequently exempt adult females from aggression and may focus instead on males and infants (Wrangham & Glowacki 2012;Wilson et al 2014; though see Pradhan, Pandit & van Schaik 2014) iii) lethal aggression is timed to periods of asymmetric fighting ability such as when one group greatly outnumbers another (Wilson et al 2014), the result being that attackers are rarely seriously injured (Watts et al…”

Lethal coalitionary aggression and long-term alliance formation among Yanomamö men

“…Surprisingly few detailed analyses exist concerning the social composition of lethal coalitions in small-scale societies. This lack is problematic, because the particular form that lethal coalitions take sheds light on the adaptive function of this behavior and the phylogenetic roots of coalitionary aggression with chimpanzees (Pan troglodytes).Humans, like chimpanzees, demonstrate a capacity to coordinate behavior with others to kill conspecifics (11)(12)(13)(14)(15). Although the scope of lethal coalitionary aggression is far greater in humans, a number of similarities exist between the two species, namely coordinated groups of adult males defending home territories and aggressing against individuals from other communities with low-cost but lethal intergroup killings (12-17).…”

“…Humans, like chimpanzees, demonstrate a capacity to coordinate behavior with others to kill conspecifics (11)(12)(13)(14)(15). Although the scope of lethal coalitionary aggression is far greater in humans, a number of similarities exist between the two species, namely coordinated groups of adult males defending home territories and aggressing against individuals from other communities with low-cost but lethal intergroup killings (12-17).…”

“…These similarities have led some researchers to hypothesize that lethal coalitionary aggression represents a homology, with the last common ancestor between humans and chimpanzees having a similar capacity for coordinated violence (11-15). In chimpanzees, the primary proximate mechanism driving lethal coalitionary aggression appears to be local imbalances of power which lead to group-level benefits such as larger territories, more food, and greater reproductive opportunities to the aggressors (11,12,15). Anthropologists, on the other hand, have postulated a number of mechanisms responsible for lethal intergroup conflict in small-scale societies that span a variety of causal levels and empirical support, such as population regulation (18,19), within-culture individual rewards (status and reproductive opportunities) (20,21), between-group competition (22, 23), and novel response to contact with the Western world (24).…”