Variation in kin bias: Species differences and time constraints in macaques

Recent evidence suggests that female grooming kin bias in rhesus and Tibetan macaques varies within groups over time in a manner that is consistent with the time constraints model. Here we test the same hypothesis for Tonkean macaques, using data from a corral-living group collected between 1987 and 2002. We also use published data to ask whether time constraints can explain variation across the three macaque species and/or whether these species appear to show inherent differences in kin bias based on their species' social styles, as predicted by the phylogenetic model. Several predictions of the time constraints model were supported for Tonkean macaques: as the number of potential grooming partners increased, females groomed similar amounts, but displayed decreased per capita amounts of grooming, focused their grooming on a subset of potential partners, and displayed more intense grooming kin bias. Although numbers of potential partners affected all three species similarly, species differences in kin bias were sustained and were related to species social styles independently of numbers of potential partners. We suggest that grooming kin bias represents a social reaction norm, in which species display qualitatively similar, but inherently different ranges of response to the same current conditions.