Evolved psychology in a novel environment : Male macaques and the "seniority rule"

The human "environment of evolutionary adaptedness" can only be inferred indirectly. In contrast, the behavior of some nonhuman animals can be compared among "natural" and various altered environments. As an example, male immigration tactics in unprovisioned versus provisioned macaque (Macaca) populations are compared using Tooby and Cosmides's (1992) framework for evolutionary functional analysis. In unprovisioned populations, social groups contain few males, and immigrant male takeovers of alpha rank occur frequently. In provisioned populations, groups contain many males, and males almost invariably enter social groups at very low rank and rise in rank only as more dominant males emigrate or die. Male conformity to the "seniority rule" is hypothesized to represent the behavioral output of an evolved decision-making algorithm (psychological mechanism) that takes into account (1) the net payoff of each rank in the dominance hierarchy and (2) the power of male group size as a predictor of the likelihood of successful immigrant takeover.

Infant handling in wild Cebus capucinus: testing bonds between females?

The evolved functions, if any, of infant handling (IH) by female primates remain unclear for many species. I tested a new hypothesis, that IH tests social bonds between adult females, using data on a group of wild white-faced capuchins. I also tested the nonadaptive, learning-to-mother, reciprocity, harassment and alliance-formation hypotheses. Focal subjects were the mothers of 10 infants that ranged in age from 0 to 90 days. The behaviours comprising IH (inspecting, nuzzling, sniffing, and a distinctive vocalization) differed from typical maternal behaviour. Nulliparous females engaged in no more IH than did parous females. The median frequency of rough handling was zero, kidnapping was not observed, and maternal restraint of infants was extremely rare. Infant handling rate was unrelated to the relative dominance ranks of the mother and the handler. Females tended to handle the infants of females with which they groomed and formed aggressive coalitions more frequently. Dyadic IH rates were not correlated with rates of allonursing when the infants were older than 90 days of age. There was no evidence for reciprocal exchanges of IH between females. Mother-terminated IH bouts were longer when the mother had groomed the handler more frequently before the infant's birth. Based on these findings, the learning-to-mother, reciprocity and harassment hypotheses are unsupported for this species. Some support exists for the nonadaptive hypothesis. Contrasting predictions of the bond-testing and alliance-formation hypothesis are presented for future testing. Copyright 1999 The Association for the Study of Animal Behaviour.

Inbreeding avoidance in rhesus macaques: whose choice?

Whether nonhuman primates avoid copulating with close kin living in their social group is controversial. If sexual aversion to relatives occurs, it should be stronger in females than in males because of females' greater investment in each offspring and hence greater costs resulting from less viable offspring. Data presented here show that adult male rhesus macaques breeding in their natal groups at Cayo Santiago experienced high copulatory success, but copulated less with females of their own matrilineages than with females of other matrilineages. Adult females were never observed to copulate with males of their own matrilineage during their fertile periods. Although natal males sometimes courted their relatives, examination of two measures of female mate choice showed that females chose unrelated natal males over male kin. Female aversion to male kin was specific to the sexual context; during the birth season, females did not discriminate against their male relatives in distributing grooming. Evolved inbreeding avoidance mechanisms probably produce different outcomes at Cayo Santiago than in wild rhesus macaque populations. Gender differences in sexual aversion to relatives may be partly responsible for differences between studies in reported frequency of copulations by related pairs.