Abstract
Intraspecific color vision variation is prevalent among nearly all diurnal monkeys in the neotropics and is seemingly a textbook case of balancing selection acting to maintain genetic polymorphism. Clear foraging advantages to monkeys with trichromatic vision over those with dichromatic "red-green colorblind" vision have been observed in captive studies; however, evidence of trichromatic advantage during close-range foraging has been surprisingly scarce in field studies, perhaps as a result of small sample sizes and strong impacts of environmental or individual variation on foraging performance. To robustly test the effects of color vision type on foraging efficiency in the wild, we conducted an extensive study of dichromatic and trichromatic white-faced capuchin monkeys (Cebus capucinus imitator), controlling for plant-level and monkey-level variables that may affect fruit intake rates. Over the course of 14 months, we collected behavioral data from 72 monkeys in Sector Santa Rosa, Costa Rica. We analyzed 19,043 fruit feeding events within 1,602 foraging bouts across 27 plant species. We find that plant species, color conspicuity category, and monkey age class significantly impact intake rates, while sex does not. When plant species and age are controlled for, we observe that trichromats have higher intake rates than dichromats for plant species with conspicuously colored fruits. This study provides clear evidence of trichromatic advantage in close-range fruit feeding in wild monkeys. Taken together with previous reports of dichromatic advantage for finding cryptic foods, our results illuminate an important aspect of balancing selection maintaining primate opsin polymorphism.