Physiological Color Changes in Reptiles

SYNOPSIS. The physiological regulation of color changes in reptilesas studied in the lizard, Anolis carolinensis, is discussed.In Anolis, the ability to adapt to a background is dependentupon the level of circulating MSH, therelease of which is dependenton information received through the eyes. Blinded (or intact)lizards are brown under conditions of strong illumination andgreen under conditions of lower light intensities, and, again,these color changes are regulated by MSH. According to Kleinholz,color changes in the blinded lizard are regulated by dermalphotoreceptors. High or low temperatures directly affect thecolor of Anolis skins and alter the rate at which skins respondto hormones. Aggregationof melanin granules within Anolis melanophoresin response to sympathomimetic stimulation is regulated throughalpha adrenergic receptors whereas dispersion of melanin granulesin response to such stimulation is controlled through beta adrenergicreceptorspossessed by the melanophores. Most Anolis melanophores possessboth alpha and beta adrenergic receptors, but some melanophorespossess only beta adrenergic receptors. In the normal physiologyof the lizard, under conditions of stress, stimulation of alphaadrenergic receptors by catecholamines leads to an "excitement—pallor"followedby an "excitement—darkening" resulting from stimulationof beta adrenergic receptors which causes dispersion of melaningranules within localized populations of melanophores. Thus,in Anolis, dispersion of melanin granules within melanophoresis regulated by both MSH and by catecholamines. Evidence ispresented that the intracellular level of cyclic 3', 5'-AMPwithin melanophores may be responsible for the regulation ofmovement of melanin granules.