Ecological importance of trichromatic vision to primates

Trichromatic colour vision, characterized by three retinal photopigments tuned to peak wavelengths of ∼430 nm, ∼535 nm and ∼562 nm (refs 1, 2), has evolved convergently in catarrhine primates and one genus of New World monkey, the howlers (genus Alouatta)3. This uniform capacity to discriminate red–green colours, which is not found in other mammals, has been proposed as advantageous for the long-range detection of either ripe fruits4,5 or young leaves6 (which frequently flush red in the tropics7) against a background of mature foliage8,9. Here we show that four trichromatic primate species in Kibale Forest, Uganda, eat leaves that are colour discriminated only by red–greenness, a colour axis correlated with high protein levels and low toughness. Despite their divergent digestive systems, these primates have no significant interspecific differences in leaf colour selection. In contrast, eaten fruits were generally discriminated from mature leaves on both red–green and yellow–blue channels and also by their luminance, with a significant difference between chimpanzees and monkeys in fruit colour choice. Our results implicate leaf consumption, a critical food resource when fruit is scarce10, as having unique value in maintaining trichromacy in catarrhines.