Primate Nepotism: What is the Explanatory Value of Kin Selection?

Abstract

Kin selection theory (KS) is widely invoked to account for the preferential treatment of kin—nepotism—in primate societies. Because this idea is so pervasive the role of KS is often unquestioned and optional mechanisms are often ignored. I first examine the potential role of some other nepotism-generating mechanisms by concentrating on the effect of the proximity correlate of matrilineal kinship. This correlate of kinship may bias the development of mutually selfish interactions among relatives—kin-biased mutualism—and that of reciprocally altruistic interactions—kin-biased reciprocal altruism—two mechanisms that have been given little weight compared to KS and whose impact on the evolution of nepotism is therefore unknown. However, these two options to KS cannot account for the existence of unilaterally altruistic interactions among kin, which provide, therefore, the best type of evidence to test KS. But such evidence is difficult to obtain because many behaviors considered altruistic may in fact be selfish, and because kin altruism is seldom unilateral; it is most often bilateral, as expected by reciprocal altruism theory. For these reasons, one should be extremely cautious before equating nepotism exclusively with KS. Next, I examine the predictions of KS regarding the deployment of altruism according to degree of kinship by considering, in addition to the variables of Hamilton's equation, the duration of behaviors, the size of kin classes and their differential availability. In general, altruism is expected to be allocated at a fairly constant rate among kin categories and to drop markedly past the degree of relatedness beyond which altruism is no more profitable. Very little data allow one to test conclusively this prediction, as well as some other significant predictions. Overall, there is ample evidence for the role of KS in shaping mother-offspring interactions in various areas. But the evidence for kin-selected altruism beyond the mother-offspring bond (r < 0.5), though qualitatively solid, is much less abundant. Kin altruism drops markedly beyond r = 0.25 (half-siblings and grandmother-grandoffspring dyads).