Catarrhine photopigments are optimized for detecting targets against a foliage background

The colour vision of many primates is trichromatic, whereas that of all oth er mammals is thought to be dichromatic or monochromatic, Moreover, the tri plets of cone pigments in different catarrhines (Old World apes and monkeys ) are strikingly similar in their spectral positions. We ask whether the se lective advantage of trichromacy lies in an enhanced ability to find edible leaves or fruit. Further, we ask whether any factor in these two search ta sks has constrained the particular set of cone spectral sensitivities obser ved in all catarrhines, We measured the spectral properties of the natural environments of six primate species in Uganda: Pan troglodytes, Cercopithec us mitis, Cercopithecus ascanius, Lophocebus albigena, Colobus guereza and Colobus badius, We concentrated on the fruit and leaves in their diets and the leaves of the trees that make up the background against which these die t items must be found. We plotted these measured stimuli in colour spaces a ppropriate for each primate species, and found that both frugivory and foli vory are facilitated by the extra dimension of colour vision found in catar rhines but lacking in most other mammals. Furthermore, by treating the task of searching for food as a signal-detection task, we show that, of all pos sible combinations of cone sensitivities, the spectral positions of the act ual primate pigments are optimal for finding fruit or young leaves against the background of mature leaves. This is because the variance of the chroma ticities of the mature leaves is minimised in one channel of the primate's colour vision, so allowing anything that is not a mature leaf to stand out.