Photopigments and the dimensionality of animal color vision

•The dimensionality of color vision is defined by color matching; it is a fundamental feature of human vision and its biology.•Long known that the dimensionality of human color vision links to the number of active photopigments.•Information about photopigment number is often used to draw inferences about color vision in other species.•These inferences frequently fail to account for the wide variations in the uses of photopigment information.•Cases where photopigment number links directly to dimensionality in nonhuman species and those where it does not are reviewed. Early color-matching studies established that normal human color vision is trichromatic. Subsequent research revealed a causal link between trichromacy and the presence in the retina of three classes of cone photopigments. Over the years, measurements of the photopigment complements of other species have expanded greatly and these are frequently used to predict the dimensionality of an animal’s color vision. This review provides an account of how the linkage between the number of active photopigments and the dimensions of human color vision developed, summarizes the various mechanisms that can impact photopigment spectra and number, and provides an across-species survey to examine cases where the photopigment link to the dimensionality of color vision has been claimed. The literature reveals numerous instances where the human model fails to account for the ways in which the visual systems of other animals exploit information obtained from the presence of multiple photopigments in support of their behavior.