Fertility as a constraint on group size in African great Apes

Abstract

Gorillas and chimpanzees live in social groups of very different size and structure. Here I test the hypothesis that this difference might reflect the way fertility maps onto group demography as it does in other Catarrhines. For both genera, birth rates and the number of surviving offspring per female are quadratic (or ∩-shaped) functions of the number of adult females in the group, and this is independent of environmental effects. The rate at which fertility declines ultimately imposes a constraint on the size of social groups that can be maintained in both taxa. The differences in group size between the two genera seem to reflect a contrast in the way females buffer themselves against this cost. Gorillas do this by using males as bodyguards, whereas chimpanzees exploit fission–fusion sociality to do so. The latter allows chimpanzees to live in much larger groups without paying a fertility cost (albeit at a cognitive cost).

Sex Differences in Intimacy Levels in Best Friendships and Romantic Partnerships

“…In other words, this is a two-step process: we form close relationships not simply to access a direct fitness reward but in order to create coalitions or alliances that in turn allow us to maximise fitness. One possibility, for example, might be to mitigate the fertility costs of group-living (Mesnick 1997;Wilson and Mesnick 1997;Dunbar 2018aDunbar , 2019Dunbar and MacCarron 2019).…”

Structure and function in human and primate social networks: implications for diffusion, network stability and health

“…The problem animals have to solve is how to maintain group stability (i.e. coordination) in the face of the stresses that derive from living in close proximity [ 81 , 83 , 92 ] which would otherwise cause the members to disperse (as happens in herd- and flock-forming species [ 54 , 55 ]. The primate solution to this problem is bonded relationships, since this ensures that individuals will maintain behavioural synchrony and stay together as a group.…”

Religion, the social brain and the mystical stance

“…This gives their groups a degree of cohesiveness and stability through time that is invariably lacking in the societies of other animals and birds (other than in the form of pair-bonded monogamy). The problem for all species is that living in close proximity incurs costs in terms of ecology (groups have to travel further to find sufficient food) as well as social stress, and these have significant negative consequences for the animals involved, and especially the females whose fertility is dramatically affected by stress (Dunbar et al 2009(Dunbar et al , 2018Dunbar, 2019;Dunbar & MacCarron, 2019). These costs offset the advantages of group-living (in most cases, protection from predators), such that group size is always a trade-off between the benefits and costs of living in groups versus living alone.…”

“…Contemporary hunter-gatherers manage this problem in large part through fission-fusion sociality: this form of dispersed social system in which the community is divided between three or four camp groups reduces the number of individuals that anyone has to live with to around 50 while allowing some flexibility for individuals to move between camp groups if they become too stressed by the individuals with whom they happen to be living. Chimpanzees do something similar (Dunbar, 2019). The Neolithic Settlement (itself probably driven by the need for defence against raiders: Johnson & Earle, 2000) disrupted this by forcing the entire community to live in one location with no mechanism for mitigating these costs.…”