In contrast to the majority of primates, which exhibit dedicated diurnality or nocturnality, all species of Eulemur are cathemeral. Color 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 color vision is actively debated. We studied three groups of wild brown lemurs (Eulemur fulvus) in Ankarafantsika National Park, Madagascar over a one-year span. We also used non-invasive fecal...