Hypotheses to explain the emergence and maintenance of trichromacy in primates have long focused on the role of diet, often suggesting trichromats are better able to detect ripe fruits and nutritious leaves. However, many neotropical primate species also need to avoid eating conspicuous food items like insects that have evolved to warn potential predators of their unprofitability. This factor has largely been ignored in work on primate colour vision. We suggest here that dichromatic and trichromatic individuals vary in their ability to learn about conspicuous but unprofitable food and that trichromats could be more effective than dichromats at detecting conspicuous unprofitable prey. To test this hypothesis, three females and one male white-faced saki (Pithecia pithecia) participated in behavioral experiments at Korkeasaari zoo (Helsinki, Finland) where choice boxes were presented marked with two symbols against a green background. Only trichromats could discriminate orange symbols that provided an unprofitable food reward from the more profitable green symbols. Each saki made 80 choices over 10 trials. While we did not detect any evidence for learning about symbol profitability, we found two females significantly avoided the conspicuous prey meaning they received greater food rewards than the dichromat male and one putative dichromat female. Further analysis is needed of the opsin genes of the females in this study to confirm the role of unprofitable food in Saki colour vision.
