ABSTRACT
One group of hypotheses relate pigment distribution to physiological (e.g., hydroregulation, vitamin D synthesis) requirements of plants or animals. These hypotheses depend on chemical and physical properties of pigments. Color is a by-product and an animal's pattern of color is interpreted not as an optical signal, but as the visible evidence of a physiological need. Predictions from these hypotheses depend on the particular characteristics (e.g., spectral sensitivity) of the photoreceptors and on environmental variables (e.g., spectral quality of ambient illumination, transmission properties of air or water) that affect the appearance of an optical signal. A third group of hypotheses, which has received very little attention, relate to color as it affects the animal's own vision. Polyak suggested that lines through the eyes act as sighting lines. Formulation of predictive hypotheses often suggests important variables that were formerly unrecognized. We are far from a comprehensive theory that predicts the coloration and pattern of color of animals.
