The biology of iridophores in reptiles

Abstract

Tissue color can have multiple functions in reptiles. Bright ventral color patches are thought to be primarily involved in intraspecific signaling of social status. The generation of bright colors at the level of the chromatophore has not been examined previously in detail. In particular, one type of chromatophore, the iridophore, is poorly understood. The organellogenesis of reflecting platelets within iridophores is shown to be similar to the organellogenesis of other chromatophore organelles. Iridophores produce color structurally, by thin-layer interference, rather than by containing pigments. Iridophore organelles were measured from tissues of several colors in each of three species of lizards and it was found that the calculated reflectances using a thin-layer interference model matched the observed reflectances. Three techniques (qualitative TEM determination of chromatophore types, quantitative TEM of iridophores and thin-layer chromatography of xanthophore pigments) were then used to determine the cellular basis of bright ventral colors in two species of lizards. Male Sceloporus undulatus erythrocheilus exhibit one of three facial color morphs, each of which has blue ventral patches as well. The cellular basis of these various colors was determined. Male Urosaurus ornatus possess blue ventral patches that are sensitive to temperature; when cool, the patches are green and when hot, the patches are vivid blue. This physiological color change is iridophore-dependent and is caused by changes in the spacing between adjacent layers of reflecting platelets. Hue response to temperature was quantified in vivo by warming lizards and quantifying hue with Munsell color chips. It was found that there was considerable individual variability in hue response to temperature. When coupled with information concerning social status in a natural population, it was found that dominant males were more responsive to temperature than subordinate males. Analyses of bright colors at the cellular level (1) allows accurate descriptions of the components that generate a tissue color and (2) facilitates understanding how color changes (either morphological or physiological) are produced. Discussions of the possible functions of pigmentation are ultimately dependent on adequate descriptions of color and color change at the cellular level.