Color patterns in amphibians

All vertebrate classes exhibit a broad diversity of colors and color patterns. Amphibians are the only group that exhibits its colors on bare skin as opposed to hairs, feathers or scales. They are also unique in undergoing metamorphosis and changing media between their larval and adult stages. We reviewed the literature to identify associations between these properties and the diversity of color patterns observed in the group. Frog skin has specific structures named chromatophores that generate their color patterns. Three types of chromatophores are present in most frogs. Melanophores and xanthophores contain pigments while iridophores produce structural coloration. Differences in the structures of the pigments and in chromatophore position account for most color variation seen in frog skin. Additionally, movement of chromatophores in the skin can result in dynamic changes of color that can be regulated by behavior or occur along development. The regulation of melanin production develops during neurulation while expression of the dermal chromatophore unit tends to ramp up during or after metamorphosis. Exposure of juveniles to sex hormones can trigger early expression of adult coloration through changes in chromatophore expression affecting both pigments and structural coloring. Color pattern polymorphism is common among frogs and the causes range from single locus dominant allele determination to polygenic inheritance. Changes in background color, temperature, geographic location, light intensity, and predation risk can trigger adjustments in color pattern. Evidence indicates that amphibian color patterns are selected primarily to promote sexual signaling and predator avoidance. In conclusion, the exposed skin of amphibians allows for expression of a variety of pigment-based and structural colors. These can be modified behaviorally or developmentally to adjust to environmental conditions and cope with the signaling or crypsys needs of the animals