Abstract
Cooperative behaviors are promoted by kin selection if the costs to the actor are smaller than the fitness benefits to the recipient, weighted by the coefficient of relatedness. In primates, cooperation occurs primarily among female dyads. Due to male dispersal before sexual maturity in many primate species, however, it is unknown whether there are sufficient opportunities for selective tolerance and occasional coalitionary support for kin selection to favor male nepotistic support. We studied the effect of the presence of male kin on correlates of male reproductive success (residence time, duration of high dominance rank) in non-natal male long-tailed macaques (Macaca fascicularis). We found that "related" (i.e., related at the half-sibling level or higher) males in a group have a significantly higher probability to remain in the non-natal group compared to males without relatives. Moreover, males stayed longer in a group when a relative was present at group entry or joined the same group within 3months upon arrival. Males with co-residing relatives also maintained a high rank for longer than those without. To our knowledge, this is the first demonstration of a potential nepotistic effect on residence and rank maintenance among non-natal males in a social system without long-term alliances.
Highlights
Much animal cooperation is in the form of nepotism, in which individuals support relatives
This result is not driven by natal dispersers, who have been shown to disperse into neighboring groups with peers, as neither the type of dispersal, nor the interaction between the presence of relatives and dispersal type had a significant effect on residence time (MECM, v2ML Mode of Dispersal: P = 0.40, v2ML Interaction: P = 0.61, Fig. 1, Table 1)
We found that males appeared to be more likely to stay if a related male was present at group entry compared to males who did not have a male relative upon entry into a group (GLMM, N = 18 without related males, 66% stayed for a year, N = 17 with related males, 94% stayed for a year, v2ML: P = 0.060, Table 2), this was marginally nonsignificant
Summary
Much animal cooperation is in the form of nepotism, in which individuals support relatives (kin). Long-lasting supportive alliances in primates occur more often among females than among males (van Schaik 1996). This sex difference has traditionally been attributed to the fact that in many species, females are philopatric: They remain in their natal group for life. Their pervasive social alliances, with their positive fitness consequences (Silk et al 2009, 2010), are generally built on kinship.
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