Kin selection and the distribution of altruism in relation to degree of kinship in Japanese macaques (Macaca fuscata)

Using kin selection theory and Hamilton's equation (B/C > 1/r) to determine how group-living individuals should allocate altruism among their various categories of kin has proven extremely difficult. Among the factors respons ible are: (1) the unclear altruistic status of many behavioral categories, (2) the difficulty of assessing the values of B and C, and hence their impa ct on the distribution of altruism according to relatedness; (3) the potent ially confounding effect of reciprocal altruism on kin selection, and (4) t he various time constraints limiting the ability of individuals to favor di stant kin. We report experiments on Japanese macaques (Macaca fuscata) whic h made it possible to assess or control the effect of these factors on the distribution of altruism in relation to degree of kinship. We concentrated on a clearly altruistic behavioral category with a particularly large BIC r atio to maximize the distribution of altruism according to relatedness. The behavior was unilaterally distributed between kin, hence minimizing the co nfounding effect of reciprocal altruism on its distribution. Time constrain ts were eliminated by giving potential donors equal opportunities to behave altruistically with every kin tested. In each experiment, an adult female was given an opportunity to help at low cost one of her juvenile kin outran k its dominant peers. In previous experiments of this sort carried out on j uveniles females, we had tested the impact of four categories of adult fema le kin. In the present experiments carried out on juvenile males, we tested new categories of adult female kin and in creased our sample of kin dyads considerably. Altruism toward young males extended to r=0.125 among direct kin (great-grandmother/great-grandson dyads), and to r=0.25 among collatera l kin (siblings), or inconsistently to r=0.125 (aunt-nephew dyads). These r elatedness limits of nepotism may reflect the limit of the profitability of altruism as defined by the terms of Hamilton's equation and/or the limit o f kin discrimination in our group.