Primate habitats differ dramatically in the intensity and spectral quality (color) of ambient light. However, little research has explored the effects of habitat variation in ambient light on primate and mammalian visual systems. An understanding of variation in nocturnal light environments is particularly lacking, considering the significance of nocturnality and vision in primate evolutionary hypotheses. In this dissertation, I explored effects of habitat variation in light environments on primate visual evolution in three studies. First, I examined how variation in ambient light intensity influenced visual morphology in 209 mammals. Second, I analyzed effects of variation in nocturnal light environments on color vision in nocturnal primates and mammals. For this second objective, I first identified factors influencing variation in nocturnal light environments within and between habitats in Madagascar and explored how nocturnal light spectral quality has influenced mammalian visual pigment spectral tuning. I then analyzed selection acting on the SWS1 opsin gene (coding for blue-sensitive cone visual pigments) between nocturnal lemurs from different habitat types to explore whether nocturnal light environments affect selection for dichromatic color vision. The results of all three studies suggest that habitat variation in light environments has had a significant influence on primate and mammalian visual evolution. In the first study, I found that day-active mammals from forested habitats exhibited larger relative cornea size compared to species from open habitats, reflecting an adaptation to increase visual sensitivity in diurnal forests. The results of the second study revealed that forest and woodland habitats share a yellow-green dominant nocturnal light environment and that nocturnal vertebrates exhibit visual pigments tuned to maximize photon absorption in these environments. Additionally, I observed a potential effect of diet on long-wavelength-sensitive cone spectral tuning among nocturnal mammals. In the third study I sequenced the SWS1 opsin gene in 106 nocturnal lemurs (19 species). Both population genetic and phylogenetic analyses identified clear signatures of differential selection on the gene by habitat type, suggesting that nocturnal light environments has influenced selection for nocturnal dichromacy in nocturnal lemurs. Finally, I discussed the implications of these results for nocturnal primate visual ecology and evolution. / text

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