Does a trade‐off between fertility and predation risk explain social evolution in baboons?

Abstract

The distribution of group sizes in woodland baboons forms a pair of demographic oscillators that trade fertility off against predation risk. Fertility rates, however, set an upper limit on group size of around 90–95 animals. Despite this, two species of baboons (hamadryas and gelada) have groups that significantly exceed this limit, suggesting that these two species have been able to break through this fertility constraint. We suggest that they have done so by adopting a form of social substructuring that uses males as ‘hired guns’ to minimize the stresses of living in the unusually large groups required by high predation risk habitats.

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

“…As a result, social evolution in primates [74] occurs as a result of a stepwise increase in group size [71] achieved by bolting together several basal subgroups to create successive layers rather than through a continuous adjustment of group sizes as in most birds and mammals. Primate species achieve larger groups by delaying group fission that would normally act as a nonlinear oscillator to keep group size within a defined range around the local mean [81][82][83]. The process thus seems to behave more like a series of phase transitions triggered by a natural fractionation process.…”

Laughter and its role in the evolution of human social bonding

“…This timing coincides with the point at which the hominin lineage evolved a more nomadic lifestyle in open, terrestrial, high predation-risk habitats—a problem that primates generally solve by living in larger, better bonded social groups [ 26 , 56 , 57 ]. This coincided with a dramatic increase in the size of the species' biogeographic range, including, for the first time, expansion into hotter lowland habitats and the crossing of the landbridges out of Africa into Eurasia.…”

“…It is notable that the two taxa which lie on the extreme right-hand side of the graph ( Papio hamadryas and Theropithecus gelada ) both have a form of social structure (small, stable, semi-independent reproductive units or harems, nested within large unstable bands) that is unique among the primates [ 26 , 53 – 55 ]. It seems that they evolved this system in response to the stresses created by having to live in very large social groups [ 56 ], a problem they solve by substructuring the group so as to create a multilevel social system that exploits the capacity to form temporary herds and at the same time allows the animals to defuse the stresses of group-living by dispersing when it is safe to do so [ 26 , 57 ].…”

“…This limit seems to occur at approximately 50 individuals, roughly the upper limit on the mean size of social groups in nonhuman primates [ 26 ]. Note that this does not mean that individual groups cannot be larger: primate [ 46 , 62 , 63 ], and human [ 64 ], groups are dynamic nonlinear oscillators designed to target a particular mean size dictated by the trade-off between fertility and the demands of the local habitat [ 57 , 62 , 63 ]. Although each species has a target size suited to the habitats it is adapted to, its groups cannot undergo fission the moment they exceed the optimal size: they have to wait until their size is at least twice the minimum set by the habitat's riskiness [ 46 ].…”

“…Although each species has a target size suited to the habitats it is adapted to, its groups cannot undergo fission the moment they exceed the optimal size: they have to wait until their size is at least twice the minimum set by the habitat's riskiness [ 46 ]. As a result, groups oscillate within a range either side of the optimum value, and thus unavoidably incur significant costs in terms of both fertility [ 56 , 57 , 62 , 63 ] and group stability [ 46 , 65 ] during the time they spend at either end of the oscillator.…”