Colour vision as an adaptation to frugivory in primates.
Most mammals possess two classes of cone, sensitive to short and to long wavelengths of light, but Old World primates (Catarrhini) have distinct medium and long wavelength sensitive classes. The sensitivities of these cones photopigments are alike in all catarrhines with peaks at about 440 nm ('blue'), 533 nm ('green') and 565 nm ('red'). One possible reason for the evolution and conservatism of catarrhine trichromacy is that colour vision is a specialization for finding food. A model of retinal coding of natural spectra, based on discrimination thresholds, is used to examine the usefulness of dichromatic and trichromatic vision for finding fruit, and for identifying fruit and leaves by colour. For identification tasks the dichromat's eye is almost as good as a trichromat's, but the trichromat has an advantage for detecting fruit against a background of leaves.