Trichromatic colour vision, characterized by three retinal photopigments tu
ned to peak wavelengths of similar to 430 nm, similar to 535 nm and similar
to 562 nm (refs 1, 2), has evolved convergently in catarrhine primates and
one genus of New World monkey, the howlers (genus Alouatta)(3). This unifo
rm 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 fruits(4,5) or young leaves(6) (which frequently flush red in
the tropics(7)) against a background of mature foliage(8,9). Here we show t
hat four trichromatic primate species in Kibale Forest, Uganda, eat leaves
that are colour discriminated only by red-greenness, a colour axis correlat
ed with high protein levels and low toughness. Despite their divergent dige
stive systems, these primates have no significant interspecific differences
in leaf colour selection. In contrast, eaten fruits were generally discrim
inated from mature leaves on both red-green and yellow-blue channels and al
so by their luminance, with a significant difference between chimpanzees an
d monkeys in fruit colour choice. Our results implicate leaf consumption, a
critical food resource when fruit is scarce(10), as having unique value in
maintaining trichromacy in catarrhines.