Investigating colour vision-based niche differentiation in wild, white-faced capuchins (Cebus imitator)

Abstract

The polymorphic colour vision of platyrrhine monkeys is a fascinating example of balancing selection acting on multiple genetic and phenotypic morphs. Yet, the mechanism of natural selection maintaining this variation remains elusive. Past research has demonstrated task-specific foraging advantages to dichromatic (two-opsin vision, red-green colour-blind) and trichromatic (three-opsin vision, human “normal”) monkeys, raising the potential for dietary niche differentiation. In this thesis, I investigate whether colour vision type leads to foraging niche differentiation in a population of wild, white-faced capuchins (Cebus imitator) in Costa Rica. I ask whether colour vision phenotype contributes to variation in foraging frequency, food intake rates, and nutritional outcomes, and how this is impacted by habitat-wide fruit abundance. To assess these multidimensional questions, I analyze a multi-year dataset of behavioural scan samples alongside paired focal follows of dichromat and trichromat adult female capuchins. I integrate these behavioural data with models of fruit conspicuity, estimations of fruit abundance, and quantification of both dietary and nutritional niche overlap. I find that while colour vision phenotype does not lead to differences in foraging frequency for different resources, dichromats take more bites of surface-dwelling invertebrates and trichromats take more bites of conspicuously coloured fruits, but this does not result in significantly different nutritional intake. These intake differences are apparent only in high fruit months, possibly indicating that dietary niche differentiation allows dichromats and trichromats to maximize energy and/or nutrient gain during times of relative fruit abundance. These results provide some preliminary evidence for niche differentiation as a mechanism contributing to the maintenance of polymorphic colour vision in platyrrhine primates over deep evolutionary time.