Diet variation driven by color vision phenotype in wild capuchin monkeys

The polymorphic color 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 colorblind) and trichromatic (three-opsin vision, human “normal”) monkeys, raising the potential for dietary niche differentiation. We ask whether color vision type influences diet variation in a population of wild, white-faced capuchins (Cebus imitator) in Costa Rica. To assess this, we record food intake during simultaneous focal follows of adult female dichromats and trichromats, and classify the conspicuity of diet items in capuchin visual space. We assess the degree of dietary and nutritional niche overlap during periods of high and low fruit abundance using nutritional geometry and Pianka’s index. We find that in months of high fruit abundance, trichromats have higher intake of conspicuously-colored fruits, while dichromats have higher intake of camouflaged invertebrates, resulting in lower niche overlap. These differences disappear in months of low habitat-wide fruit abundance, a time when overall dietary breadth collapses. Our results support the hypothesis that niche differentiation in diet contributes to maintaining color vision variation without compromising species-specific nutritional intake.