Concessions of an alpha male? Cooperative defence and shared reproduction in multi-male primate groups

Abstract

By living in social groups with potential competitors, animals forgo monopolizing access to resources. Consequently, debate continues over how selection might favour sociality among competitors. For example, several models exist to account for the evolution of shared reproduction in groups. The 'concession model' hypothesizes that dominant reproducers benefit from the presence of subordinates, and hence tolerate some reproduction by subordinates. This mutual benefit to both dominants and subordinates may provide a foundation for the formation of social groups in which multiple members reproduce-a necessary step in the evolution of cooperation. To date, however, the concession model has received virtually no support in vertebrates. Instead, the vast majority of vertebrate data support 'limited control models', which posit that dominant reproducers are simply unable to prevent subordinates from reproducing. Here we present the most comprehensive evidence to date in support of the concession model in a vertebrate. We examined natural variation in the number of adult males in gelada (Theropithecus gelada) reproductive units to assess the extent of reproductive skew in multi-male units. Dominant ('leader') males in units that also had subordinate ('follower') males had a 30 per cent longer tenure than leaders in units that did not have followers, mainly because followers actively defended the group against potential immigrants. Follower males also obtained a small amount of reproduction in the unit, which may have functioned as a concession in return for defending the unit. These results suggest that dominants and subordinates may engage in mutually beneficial reproductive transactions, thus favouring male -male tolerance and cooperation.

Adaptations for social cognition in the primate brain

“…Less attention has been paid to correlates of cooperation and fitness among primate males, for whom reproductive success is often more strongly correlated with dominance rank, which in turn is related to condition and fighting ability [26]. In recent years, however, it has become evident that the presence of allies can, in some instances, also influence the dominance ranks, tenure and reproductive success of primate males (Assamese macaques, Macaca assamensis [27]; Barbary macaques, Macaca sylvana [28]; geladas, Theropithecus gelada [29]). This observation also holds true for chimpanzees (Pan troglodytes [30,31]), whose male-bonded, fission -fusion society is very different from that of most Old World monkeys.…”

Constraints and flexibility in mammalian social behaviour: introduction and synthesis

“…By contrast, compromise models assume that reproductive skew is the outcome of a struggle over reproduction between group members, the intensity of which is mediated only by the detrimental effects it imposes on group productivity. Most studies of reproductive skew in mammalian societies have found support for compromise models or a less formalized class of models that focus on priority-of-access [35 -37] (but see [107,108]). …”

Evidence for tactical concealment in a wild primate

“…Typically, males gain dominance through aggressive takeovers 32 , although other avenues have been suggested 33 . Although these units aggregate into large, highly concentrated herds that provide ample opportunity for extra-pair copulation (EPC) 34 , extra-unit paternity has never been documented 31 . However, some units have 1-5 subordinate 'follower' males that sire a small percentage (17%) of the unit's offspring 31 .…”

“…Although these units aggregate into large, highly concentrated herds that provide ample opportunity for extra-pair copulation (EPC) 34 , extra-unit paternity has never been documented 31 . However, some units have 1-5 subordinate 'follower' males that sire a small percentage (17%) of the unit's offspring 31 . Despite taking away some reproduction from the leader male, follower males provide a net benefit to leaders through shared unit defence, and thus a longer tenure for the leader 31 .…”

“…Here, we examined the mating behaviour of a wild primate with known levels of extra-pair paternity 31 . Male geladas, Theropithecus gelada, obtain reproductive success by becoming the dominant 'leader' male of a reproductive unit comprising 1-12 adult females (hereafter, 'unit').…”