Frugivory and colour vision in Alouatta seniculus, a trichromatic platyrrhine monkey.

It is a long-standing hypothesis that primate trichromacy evolved to help fruit-eating primates find fruits amongst leaves. We measured the reflectance spectra of fruits eaten by a trichromatic primate, Alouatta seniculus, in the rainforest of French Guiana, as well as those of the leaves that form the natural background to fruits. We develop a method of specifying these natural colour signals in a chromaticity diagram appropriate for A. seniculus. By treating the task facing frugivorous monkeys as a signal detection task, we show that the spectral tuning of the L and M cone pigments in A. seniculus is optimal for detecting fruits amongst leaves.