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.

生活史特征（life history traits）的纬度变异一直是诸多研究的核心议题，但其背后的激素机制（hormonal mechanisms）却未得到足够关注。类固醇激素（steroid hormones）在脊椎动物繁殖过程中发挥核心作用，且可能与生活史权衡（life-history trade-offs）相关。此类激素的循环浓度在脊椎动物中差异极大，但目前仅在鸟类中对雄性激素浓度的种间地理变异开展了详细研究。本研究针对两栖类与爬行类展开此类变异研究，验证了此前在鸟类中观测到的相关模式。本研究采用系统发育比较分析（phylogenetic comparative analyses）方法，结果显示：在两栖类中而非爬行类中，睾酮（testosterone）与基础皮质酮（baseline corticosterone）均与纬度呈正相关。基础皮质酮在两栖类中与海拔呈负相关，而爬行类中无此关联。两类群的睾酮浓度均与繁殖季长度（breeding season length）呈负相关。此外，两类群的睾酮浓度均与基础皮质酮呈正相关。本研究结果可通过“更短的繁殖季加剧雄性间竞争，进而推动睾酮浓度升高以调控次级性征（secondary sexual traits）”这一机制得到最佳解释。更高的繁殖强度（reproductive intensity）所带来的能量需求（energetic demands）提升，可能需要更高的基础皮质酮水平。因此，两类群中睾酮与皮质酮的正相关关系表明，睾酮调控行为存在能量需求，而该需求可通过提升基础糖皮质激素（glucocorticoid）浓度得以满足。