Fitness benefits of coalitionary aggression in male chimpanzees

Abstract

Coalitionary aggression occurs when at least two individuals jointly direct aggression at one or more conspecific targets. Scientists have long argued that this common form of cooperation has positive fitness consequences. Nevertheless, despite evidence that social bond strength (which is thought to promote coalition formation) is correlated with fitness in primates, cetaceans, and ungulates, few studies have directly examined whether coalitionary aggression improves reproductive success. We tested the hypothesis that among free-ranging chimpanzees (Pan troglodytes schweinfurthii), participation in coalitionary aggression increases reproductive output. Using 14 years of genetic and behavioral data from Gombe National Park, Tanzania, we found that coalitionary aggression increased a male’s chances of A) siring offspring, compared to other males of similar dominance rank, and B) ascending in rank, a correlate of future reproductive output. Because male chimpanzees form coalitions with many others within a complex network, we used social network analysis to identify the types of connections correlated with these fitness benefits. The beneficiaries of coalitionary aggression were males with the highest ‘betweenness’ – that is, those who tended to have coalition partners who themselves did not form coalitions with each other. This suggests that beyond simply recognizing third-party relationships, chimpanzees may use this knowledge to choose coalition partners. If so, this is a significant step forward in our knowledge of the adaptive value of social intelligence. Regardless of mechanism, however, this is the first evidence of genetic benefits of coalitionary aggression in this species, and therefore has important implications for understanding the evolution of cooperation.

Social connectedness is associated with fibrinogen level in a human social network

“…Though social dominance hierarchies are known to affect the health of their members, most notably in non-human primates [1], recent evidence has also demonstrated affiliative, non-hierarchical and non-kin relationships in primates, birds, ungulates and cetaceans [42]. Such relationships lend themselves to sociocentric network analyses of the type we deploy here [43]. Because the coagulation cascade itself is highly conserved across mammals [44], non-human friendship networks may also modulate fibrinogen and other biomarkers of coagulation and inflammation, although as non-human ties are ascertained through observation rather than nomination, determining the potential directedness of these ties (in order, for instance, to distinguish between indegree and outdegree, as we do here) may require novel ethological methods.…”

Coalitions in theory and reality: a review of pertinent variables and processes

“…As yet, the most direct evidence for the benefits of rank-changing coalitions was provided in two studies of Assamese macaque and chimpanzee males, where coalitionary aggression increased a male's chances of ascending in rank, which in turn was associated with increased reproductive success Gilby et al, 2013). Similarly, fight intervention was associated with an increase in rank early in the rut and accounted for increased mating success in male fallow deer (Jennings et al, 2011).…”

Precursors to language: Social cognition and pragmatic inference in primates

“…Long-term field studies demonstrate that an important predictor of a baboon's or a chimpanzee's reproductive success is an individual's ability to form close, long-term bonds. Doing so, moreover, would seem to require recognition of the relationships that exist among others (Gilbey et al, 2013;Mitani, 2009;Silk et al, 2009Silk et al, , 2010. Similar results apply to many nonprimate mammals (see Seyfarth & Cheney, 2012, for a review).…”

“…was undoubtedly the linking up of individual bits of conceptual structure to individual vocalizations^(see also Hurford, 2003;Jackendoff, 1987;Kirby, 1998). The present proposal is new, however, in its emphasis on social cognition (Worden, 1998, makes a similar argument), and because we can now link social cognition with reproductive success (Cheney & Seyfarth, 1990, 2007Gilbey et al, 2013;Silk et al, 2009Silk et al, , 2010.…”