Primate nepotism: What is the explanatory value of kin selection?

Kin selection theory (KS) is widely invoked to account for the preferential treatment of kin-nepotism-in primate societies: Because this idea is so pe r vasive the role of KS is often unquestioned and optional mechanisms are o ften ignored. I first examine the potential role of some other nepotism-gen erating mechanisms by concentrating on the effect of the proximity correlat e of matrilineal kinship. This correlate of kinship may bias the developmen t of mutually selfish interactions among relatives-kin-biased mutualism-and that of reciprocally altruistic interactions-kin-biased reciprocal altruis m-two mechanisms that have been given little weight compared to KS and whos e impact on the evolution of nepotism is therefore unknown. However, these two options to KS cannot account for the existence of unilaterally altruist ic interactions among kin, which provide, therefore, the best type of evide nce to test KS. But such evidence is difficult to obtain because many behav iors 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 c onsidering, in addition to the variables of Hamilton 's equation, the durat ion of behaviors, the size of kin classes and their differential availabili ty. In general, altruism is expected to be allocated at a fairly constant r ate among kin categories and to drop markedly past the degree of relatednes s beyond which altruism is no more profitable Very little data allow one to test conclusively this prediction, as well as some other significant predi ctions. Overall, there is ample evidence for the role of KS in shaping moth er-offspring interactions in various areas. But the evidence for kin-select ed altruism beyond the mother-offspring bond (r < 0.5), though qualitativel y solid, is much less abundant. Kin altruism drops markedly beyond r = 0.25 (half-siblings and grandmother-grandoffspring dyads).