Data from: Variation in testosterone and corticosterone in amphibians and reptiles: relationships with latitude, elevation, and breeding season length

Latitudinal variation in life history traits has been the focus of numerous investigations, but underlying hormonal mechanisms have received much less attention. Steroid hormones play a central role in vertebrate reproduction and may be associated with life-history trade-offs. As such, circulating concentrations of these hormones vary tremendously across vertebrates, yet inter-specific geographic variation in male hormone concentrations has only been studied in detail in birds. We here report on such variation in amphibians and reptiles, confirming patterns observed in birds. Using phylogenetic comparative analyses, we found that in amphibians, but not reptiles, testosterone and baseline corticosterone were positively related to latitude. Baseline corticosterone was negatively related to elevation in amphibians, but not reptiles. For both groups, testosterone concentrations were negatively related with breeding season length. Additionally, testosterone concentrations were positively correlated with baseline corticosterone in both groups. Our findings may best be explained by shorter breeding seasons increasing male-male competition, which may favor increased testosterone concentrations that modulate secondary sexual traits. Elevated energetic demands resulting from greater reproductive intensity may require higher baseline corticosterone. Thus, the positive relationship between testosterone and corticosterone in both groups suggests an energetic demand for testosterone-regulated behavior that is met with increased baseline glucocorticoid concentrations