Abstract
The male-female affinitive relationships among non-kin which persist beyond the estrus period have been known in some primates, mainly savanna baboons, Japanese macaques and rhesus macaques. I call this friendship, and review the benefits of friendships for each sex. For females, proximity and alliance of male friends disrupts antagonism by opponents, more or less, in all three species. On the other hand for the males, the effect of enhanced reproductive success seems to be negative. It is necessary to fully consider male benefits via infants in savanna baboons. Since it is highly possible that baboon males take care of a female friend's infant that he sired in the past, such friendships, especially in infanticidal baboons, would evolve through kin-selection rather than reciprocal altruism. When it comes to reciprocal exchange of benefits between a male and female, it seems that females play a positive role in the maintenance of friendships, as overall benefits seem to be female-biased. Socio-ecology can offer explanations for the ultimate cause of friendships. However, its explanatory power is not enough to explain the following observations: two non-troop males visit a group of Japanese macaques in coastal forest, Yakushima on separate days during a mating season. They received grooming one-sidedly by a female and soon left the group without copulating. These two cases seemed to be interactions between old friends, but the reciprocity cannot be formed. As the reunion with an old female friend of a male after immigration rarely occurs, mal-adaptive behavior would not be a problem. Socio-ecology does not explain such rarely occurring events. However, we can not ignore such cases just because they rarely happen. It is one aspect of the Japanese macaque society that monkeys “renews” old friendship.
