Abstract
Although nepotism has been evidenced in various areas of primate behaviour, relatively little is known on how it varies with degree of relatedness (r). For example, nepotism might decrease proportionally and asymptotically with relatedness, or it might decrease up to a certain relatedness threshold, beyond which it plummets. This issue was investigated by analysing one type of beneficent behaviours, agonistic support (third-party interventions in conflicts), among laboratory-housed female Japanese macaques, Macaca fuscata. In previous experiments, mothers and sisters were tested in situations in which they could help their daughters and younger sisters to outrank peers, which they did successfully. In the present experiments, grandmothers and aunts were tested using similar protocols. In the presence of their grandmother, granddaughters outranked all dominant peers, but in the presence of their aunt, nieces could not. These combined results point to a relatedness threshold for effective nepotistic aiding above the aunt–niece level of relatedness. Analysis of 1002 interventions received by juvenile females from various categories of adult female kin in the unmanipulated, complete group confirmed this hypothesis. Finally, 11 years of data on the distribution of female homosexual activity according to relatedness provided complementary evidence for the threshold hypothesis. Non-kin females commonly engaged in homosexual interactions, as did aunts and nieces, but closer kin never did, suggesting that aunts and nieces do not recognize each other as kin for the purpose of sexual interactions. Together, these results point to a relatedness threshold for nepotistic aiding at r=0.25 and suggest that this threshold is not behaviour-specific but generalized.
Published Version
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