Correlates of melanization in multiple high- and low-elevation populations of the lizard, Sceloporus occidentalis: Behavior, hormones, and parasites.

Abstract

Hormones mediate the expression of suites of correlated traits and hence may act either to facilitate or constrain adaptive evolution. Selection on one trait within a hormone-mediated suite of traits may lead to a change in the strength of the hormone signal, causing changes in correlated traits. Growing evidence suggests that melanization, which is in part regulated by hormonal signals, is tightly linked to other traits, such as aggression and stress physiology. Here, we examine six populations of Sceloporus occidentalis lizards differing in degree of melanization (three dark higher-elevation populations and three less-melanized lower-elevation populations) to investigate potential correlations between behavior, hormones, and parasites. We measured aggression by recording behavioral responses of males to staged territorial intrusions; behavior was summarized by two principal components. Analysis revealed that males in the three darker populations signaled aggression less often and made more physical contact than males in the lighter populations. Analyses of plasma steroid hormones (corticosterone and testosterone) revealed significant population differences, but counter to expectation higher aggression was associated with lower testosterone compared across populations. Finally, the three darker populations had higher mean mite loads than the three lighter populations. Overall, this array of phenotypic correlations does not parallel patterns of within-population differences in melanization found in other vertebrates, suggesting that hormonal correlations do not constrain phenotype variation across populations in this species. Given this contradiction between population- and individual-level variation, we urge more study at both levels of variation in traits potentially associated with melanization in other vertebrates.