Fruits, foliage and the evolution of primate colour vision

Primates are apparently unique amongst the mammals in possessing trichromat ic colour vision. However, not all primates are trichromatic. Amongst the h aplorhine (higher) primates, the catarrhines possess uniformly trichromatic colour vision, whereas most of the platyrrhine species exhibit polymorphic colour vision, with a variety of dichromatic and trichromatic phenotypes w ithin the population. It has been suggested that trichromacy in primates and the reflectance func tions of certain tropical fruits are aspects of a coevolved seed-dispersal system: primate colour vision has been shaped by the need to find coloured fruits amongst foliage, and the fruits themselves have evolved to be salien t to primates and so secure dissemination of their seeds. We review the evi dence for and against this hypothesis and we report an empirical test: we s how that the spectral positioning of the cone pigments found in trichromati c South American primates is well matched to the task of detecting fruits a gainst a background of leaves. We further report that particular trichromat ic platyrrhine phenotypes may be better suited than others to foraging for particular fruits under particular conditions of illumination; and we discu ss possible explanations for the maintenance of polymorphic colour vision a mongst the platyrrhines.