Historical contingency in the evolution of primate color vision.

Primates are unique among eutherian mammals for possessing three types of retinal cone. Curiously, catarrhines, platyrrhines, and strepsirhines share this anatomy to different extents, and no hypothesis has hitherto accounted for this variability. Here we propose that the historical biogeography of figs and arborescent palms accounts for the global variation in primate color vision. Specifically, we suggest that primates invaded Paleogene forests characterized by figs and palms, the fruits of which played a keystone function. Primates not only relied on such resources, but also provided high-quality seed dispersal. In turn, figs and palms lost or simply did not evolve conspicuous coloration, as this conferred little advantage for attracting mammals. We suggest that the abundance and coloration of figs and palms offered a selective advantage to foraging groups with mixed capabilities for chromatic distinction. Climatic cooling at the end of the Eocene and into the Neogene resulted in widespread regional extinction or decimation of palms and (probably) figs. In regions where figs and palms became scarce, we suggest primates evolved routine trichromatic vision in order to exploit proteinaceous young leaves as a replacement resource. A survey of the hue and biogeography of extant figs and palms provides some empirical support. Where these resources are infrequent, primates are routinely trichromatic and consume young leaves during seasonal periods of fruit dearth. These results imply a link between the differential evolution of primate color vision and climatic changes during the Eocene-Oligocene transition.