Melanization, α-melanocyte stimulating hormone and steroid hormones in male western fence lizards from nine populations

Abstract

Hormones can mediate suites of correlated traits. Melanocortins regulate melanin synthesis and elements of the melanocortin system can directly, and indirectly, affect a number of other traits, such as stress reactivity. Trait correlations within the melanocortin system have been studied mainly in birds and mammals but less so in reptiles. We examined adult male western fence lizards (Sceloporus occidentalis) and if melanization was correlated with plasma levels of three hormones, including peptide hormone α-melanocyte stimulating hormone (α-MSH), testosterone and corticosterone, and ectoparasite loads. This lizard is darker at higher elevations in California, and we compared five high-elevation and four low-elevation populations during comparable periods of the breeding season at each site. We first validated use of an α-MSH assay kit with lizard plasma. Since Anolis carolinensis is one of the few species with published values for α-MSH plasma levels, we assayed both Anolis and Sceloporus plasma and compared hormone values to those we generated for Anolis to the publish values. We also evaluated effects of different methods of storing spiked plasma pools on resulting α-MSH concentrations. Plasma levels of α-MSH did not differ significantly, but some populations differed significantly in mean corticosterone and mean testosterone. Combining all individuals from the nine populations, we found that individual variation in α-MSH was not associated with individual variation in melanization, but levels of α-MSH were positively associated with plasma testosterone and negatively associated with corticosterone. The lack of association between individual levels of melanization and expression of most other traits differs from a growing number of within-population studies of melanization, and we discuss what differences in physiological mechanisms could produce different hypothetical patterns. Circulating levels of -MSH are only one element of the melanocortin system; in situ synthesis of α-MSH by the skin and the diversity of melanocortin receptors could also contribute to variation in traits mediated by the melanocortin system and should be examined.