The world in color presents a dazzling dimension of phenotypic variation. Biological interest in this variation has burgeoned, due to both increased means for quantifying spectral information and heightened appreciation for how animals view the world differently than humans. Effective study of color traits is challenged by how to best quantify visual perception in nonhuman species. This requires consideration of at least visual physiology but ultimately also the neural processes underlying perception. Our knowledge of color perception is founded largely on the principles gained from human psychophysics that have proven generalizable based on comparative studies in select animal models. Appreciation of these principles, their empirical foundation, and the reasonable limits to their applicability is crucial to reaching informed conclusions in color research. In this article, we seek a common intellectual basis for the study of color in nature. We first discuss the key perceptual principles, namely, retinal photoreception, sensory channels, opponent processing, color constancy, and receptor noise. We then draw on this basis to inform an analytical framework driven by the research question in relation to identifiable viewers and visual tasks of interest. Consideration of the limits to perceptual inference guides two primary decisions: first, whether a sensory-based approach is necessary and justified and, second, whether the visual task refers to perceptual distance or discriminability. We outline informed approaches in each situation and discuss key challenges for future progress, focusing particularly on how animals perceive color. Given that animal behavior serves as both the basic unit of psychophysics and the ultimate driver of color ecology/evolution, behavioral data are critical to reconciling knowledge across the schools of color research.

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We tested the prediction of the sensory drive hypothesis using four allopatric populations of the lizard Anolis cristatellus from two distinct environments (i.e., mesic and xeric conditions). For each population, we measured habitat light characteristics and quantified signal design by measuring the spectral and total reflectance and transmittance of the dewlap. We used these data to calculate dewlap detectability using an empirically based model of signal detection probability. We found that populations from mesic and xeric conditions occupy two distinct habitats with respect to light intensity and spectral quality and that dewlap design has diverged between populations in a way that increases signal detectability in each habitat. The major difference in dewlap design was in total reflectance and transmittance, making dewlaps from xeric habitats darker and dewlaps from mesic habitats brighter. Furthermore, dewlap detection decreased significantly when a dewlap from a xeric habitat is detected under the spectral conditions of a mesic habitat. The converse is true for a dewlap from a mesic habitat. We propose that sensory drive has promoted divergence in dewlap design in distinct habitat light conditions, and we discuss the possibility that selection might promote early stages of reproductive isolation as a by-product of selection on dewlap design to distinct habitat light conditions.

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Terrestrial habitats exhibit a variety of light environments. If species exhibit evolutionary adaptations of their visual system or signals to habitat light conditions, then these conditions can directly influence the structure of communities. We evaluated habitat light characteristics and visual-signal design in a pair of sympatric species of lizards: Anolis cooki and Anolis cristatellus. We found that each species occupies a distinct microhabitat with respect to light intensity and spectral quality. We measured the relative retinal spectral sensitivity and found significant differences between the species that correlate with differences in habitat spectral quality. We measured the spectral reflectance of the dewlaps (colourful throat fans used in communication), and found that the A. cooki dewlap reflects little ultraviolet (UV), while that of A. cristatellus reflects strongly in the UV. For both species downwelling light (irradiance) is rich in UV. However the background light (radiance) is rich in UV for A. cooki, but low in UV for A. cristatellus. Thus, the dewlap of each species creates a high contrast with the background in the UV. Our findings strongly suggest that these two species are partitioning their habitat through specializations of the visual system and signal design to microhabitat light conditions.