Maternal kin bias in affiliative behavior among wild adult female blue monkeys

Abstract

Kin-biased cooperative and affiliative behavior is widespread in social mammals and is expected to increase fitness. However, despite evolutionary benefits of cooperating with relatives, demographic circumstances may influence the strength of kin bias. We studied the relationship between maternal kinship and affiliative behavior among 78 wild adult female blue monkeys (Cercopithecus mitis) from 8 groups monitored for 1-5 years. We compared behavior and kinship matrices, controlling for rank differences. Using multivariate models, we examined effects of demographic variables on the extent to which females groomed disproportionately with close adult female kin. Female blue monkeys, like other cercopithecine primates, generally preferred closer maternal kin for grooming and spatial association, although there was also substantial variation. Kin bias was weakest for association (at 7 m) while feeding, intermediate for closer (1 m) association while resting, and most intense for grooming. Grooming kin bias was stronger when a female had more very close relatives (either her mother or daughters), when her group contained more adult females, when she groomed with a lower percentage of group-mates, and when she had fewer total kin. Dominance rank did not predict variation in kin bias. Females generally groomed with all kin, but in larger groups they increased the number of unrelated grooming partners and total grooming time. The increased kin bias intensity in larger groups resulted from the addition of unrelated partners with whom grooming occurred less often than with kin, rather than from time constraints that drove females to select kin more strongly. In natural-sized groups, it may be common that females groom with all their adult female kin, which are present in limited numbers. The addition of grooming partners in larger groups may benefit female blue monkeys who rely on collective action in territorial defense; group-wide cooperation may thus influence grooming decisions in this species.

Keywords: group-size effects; kin preferences; kinship; social affiliation; sociality index; time-constraints.