Effect of Group Size and Individual Characteristics on Intergroup Encounters in Primates

Abstract

Intergroup encounters are common in nonhuman primates and can vary from affiliative to aggressive. We extracted data from the literature to test five different hypotheses: 1) where there are group size differences between opposing groups, whether the larger group is more likely to win an intergroup encounter than the smaller group; 2) whether the likelihood of a group engaging in aggressive intergroup encounters increases with group size; and 3-5) whether dominant, older individuals, and/or males are more likely to participate aggressively in intergroup encounters than subordinate, younger individuals and/or females. Our data set comprised 52 studies on 31 primate species (3 lemur species, 5 New World monkeys, 19 Old World monkeys, and 4 apes). We found that the larger group is more likely to win an encounter against a smaller group than vice versa. We found no significant relationship between group size and propensity to be aggressive during intergroup encounters. We found weak/no support for the effect of age, dominance rank, and sex on the frequency of aggression displayed toward outgroup individuals during intergroup encounters. Species-and population-specific differences in inter-and intragroup competition and in the degree of the unequal distribution of resources across group members may explain why age, dominance rank, and sex are not strong predictors of aggression during intergroup encounters.

Intergroup Competition Enhances Chimpanzee (Pan troglodytes verus) In-group Cohesion

“…Thus, collective group defense may present a collective action problem (Langergraber et al 2017;Willems et al 2013Willems et al , 2015, especially with increasing group size and in species that live in a social system with high fission-fusion dynamics in which group members do not permanently associate. However, in many species, including humans, collective action and strength in numbers are key during intergroup conflicts (Majolo et al 2019;Willems et al 2015;Wrangham 1999), suggesting that species may overcome the collective action problem. Indeed, in humans, intergroup competition is known to engender solidarity, affiliation, and cohesion within a group (Bernhard et al 2006;Gneezy and Fessler 2012;Puurtinen and Mappes 2009;Yamagishi and Mifune 2009).…”

Assessment during Intergroup Contests

“…The best-supported single RHP proxy in intergroup contests is group size: the number of members in each group. A recent meta-analysis affirmed this in primates [12], and group size is relevant in other taxa from ants (e.g., wood ants Formica rufa) [24], to lions (Panthera leo) [25], to birds (green woodhoopoes Phoeniculus purpureus) ( [26]; cf [27] where group size does not predict outcomes in greater anis Crotophaga major). However, proxies other than absolute numbers might be important.…”

An Evolutionary Explanation for the Female Leadership Paradox

“…In the context of intragroup conflict, observations of both chimpanzees and bonobos also suggest females tend to recruit coalition partners primarily to defend kin and friends against male aggression, whereas males tend to build coalitions primarily to compete for high rank and the mating opportunity it affords (Newton-Fisher, 2006;Tokuyama and Furuichi, 2016). However, a recent metanalysis of relevant primate studies revealed only weak support for the effect of sex on the frequency of aggression displayed toward outgroup individuals during intergroup encounters (Majolo et al, 2020). Moreover, the meta-analysis found significant variation in female participation in intergroup aggression across and within species (Majolo et al, 2020).…”