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.