Meeting Abstract
The concept of hormonal pleiotropy suggests facilitated, or constrained, evolution among a suite of hormone-mediated traits, and the melanocortin system has been one focus of work motivated by this endocrine concept. Melanocortins can regulate melanin synthesis and many other traits, and we studied relationships between melanization and plasma levels of three hormones: α-melanocyte stimulating hormone (α-MSH), testosterone and corticosterone. We compared both between- and within-population differences of adult male western fence lizards, Sceloporus occidentalis in California where individuals are increasingly darker at higher elevations. We studied five high- and four low-elevation populations, and worked during comparable periods of the breeding season at each site. Baseline plasma levels of α-melanocyte stimulating hormone (α-MSH) did not differ significantly among populations, but populations differed in means for both corticosterone and testosterone, although there was no consistent pattern with elevation or mean melanization for any hormone. Combining all individuals from the nine populations, we found that variation in α-MSH was not associated with variation in melanization, but that plasma α-MSH levels were positively associated with baseline plasma testosterone and negatively correlated with baseline corticosterone. Our results comparing across populations differ from a growing number of within-population studies of melanization, and we discuss hypothetical differences in endocrine mechanisms that could produce different trait correlation patterns. Our data suggest that hormonal pleiotropy does not constrain phenotypic variation, especially when considering the melanocortin system, with in situ synthesis of α-MSH by the skin and the diversity of melanocortin receptors.
