Dispersed Male Networks in Western Gorillas

Abstract

Although kin-selection theory has been widely used to explain the tendency of individuals to bias beneficial behaviors towards relatives living within the same social group, less attention has focused on kin-biased interactions between groups. For animal societies in which females emigrate, as is the case for mountain gorillas (Gorilla beringei beringei), encounters between males in different groups often involve aggressive displays that can escalate to physical violence and fatal injuries. However, recent findings on the little-studied western gorilla (Gorilla gorilla) indicate that interactions between social groups occur more frequently than they do in mountain gorillas and are often, although not always, surprisingly nonaggressive. We investigated the pattern of genetic relationships between individuals of different groups and found evidence suggesting a previously unrecognized "dispersed male network" social structure in western gorillas in which the single males leading social groups were usually related to one or more nearby males. We propose that this provides a basis for extra-group, kin-biased behaviors and may explain the reported peaceful intergroup interactions. Furthermore, these results suggest that a patrilocal social structure, in which males remain in their natal region and potentially benefit from kin associations, is a feature unifying African apes and humans.

Sex differences in cooperation: A meta-analytic review of social dilemmas.

“…6 Three lines of evidence suggest men tended to reside throughout life among close consanguineal kin, whereas women transferred residence away from kin at sexual maturity or marriage. First, human's closest living ancestors, chimpanzees, bonobos, and gorillas, all practice male philopatry with moderate female dispersal (e.g., Boesch & Boesch-Achermann, 2000;Bradley, Doran-Sheehy, Lukas, Boesch, & Vigiliant, 2004;Chapais, 2008;Goodall, 1986;Nishida et al, 2003;Watts, 1996), and phylogenetic analyses suggest that primate co-residential patterns are conservative over time (Chapais, 2008;Thierry, Iwaniuk, & Pellis, 2000). Second, behavioral studies on modern forager populations suggest they disproportionately practice sex-biased affiliation among male kin, with a tendency for brothers to co-reside (Hill et al, 2011).…”

Fission‐Fusion Dynamics

“…Orangutans are clearly the least cohesive of the apes, but they are known to aggregate more when ecological conditions are favorable, and there appears to be some form of a community (Delgado and van Schaik 2000). Even the generally cohesive gorilla groups may be part of some form of multilevel society (Bradley, Doran-Sheehy, Lukas, Boesch and Vigilant 2004). Our closest relatives, chimpanzees and bonobos, whose physiology and food choice are more comparable to those of humans, have a social system with a high degree of fissionfusion dynamics but also a very strong level of cooperation and affiliation within communities.…”

Mountain gorilla tug-of-war: Silverbacks have limited control over reproduction in multimale groups

“…Males may stay in their natal groups or emigrate to become solitary and possibly later form a new group (7). A single male is likely to monopolize reproduction in his group, as has been shown to occur in the one-male groups of the closely related western gorilla species (Gorilla gorilla) (8), but it is interesting to ask whether the socially dominant male monopolizes reproduction in multimale groups and, if not, why he might lose reproductive opportunities to his subordinate(s). Tug-ofwar models [also known as ''limited control'' or ''compromise'' models (2, 9)] emphasize relative competitive abilities, and any reproductive sharing is proposed to reflect the inability of individual group members to successfully monopolize reproduction despite their best efforts to do so.…”