Visual ecology of true lemurs suggests a cathemeral origin for the primate cone opsin polymorphism

In contrast to the majority of primates, which exhibit dedicated diurnality or nocturnality, all species of Eulemur are cathemeral. Colour vision, in particular, is strongly affected by the spectral composition and intensity of ambient light, and the impact of activity period on the evolution of primate colour vision is actively debated. We studied three groups of wild brown lemurs (Eulemur fulvus) in Ankarafantsika National Park, Madagascar, over a 1-year span. We also used non-invasive faecal DNA collection and analysis to study the opsin genes underlying the colour vision of 24 individuals. We quantified the colour and brightness of dietary fruits and modelled the chromaticity and discriminability of food objects to different visual phenotypes under daylight, twilight and moonlight conditions. We found that E. fulvus possesses routine dichromacy, unlike its congener E. flavifrons, for which polymorphic trichromacy has been reported. Our models suggest that dichromacy is well suited to the foraging ecology of E. fulvus. The performance of modelled dichromats and trichromats is comparable under nocturnal illuminants, and the luminance values of most diet items are detectable across light conditions. The trichromatic phenotype demonstrates a modest advantage under daylight conditions. Our results, taken together with reports of polymorphic trichromacy in E. flavifrons, suggest functional ecological variation in the visual system of the genus Eulemur. Interspecific phenotypic variation in the colour vision of a genus is both unexpected and instructive. Ecological differences between species of Eulemur could reveal thresholds for the origins of polymorphic trichromacy, which preceded the evolution of routine trichromatic vision in humans and other primates.