Miles Meredith C, Vitousek Maren N, Husak Jerry F, Johnson Michele A, Martin Lynn B, Taff Conor C, Zimmer Cedric, Lovern Matthew B, Fuxjager Matthew J
Department of Biology, Wake Forest University, Winston-Salem, NC 27109, USA.
Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853, USA.
Integr Comp Biol. 2018 Oct 1;58(4):751-762. doi: 10.1093/icb/icy062.
Circulating steroid hormone levels exhibit high variation both within and between individuals, leading some to hypothesize that these phenotypes are more variable than other morphological, physiological, and behavioral traits. This should have profound implications for the evolution of steroid signaling systems, but few studies have examined how endocrine variation compares to that of other traits or differs among populations. Here we provide such an analysis by first exploring how variation in three measures of corticosterone (CORT)-baseline, stress-induced, and post-dexamethasone injection-compares to variation in key traits characterizing morphology (wing length, mass), physiology (reactive oxygen metabolite concentration [d-ROMs] and antioxidant capacity), and behavior (provisioning rate) in two populations of tree swallow (Tachycineta bicolor). After controlling for measurement precision and within-individual variation, we found that only post-dex CORT was more variable than all other traits. Both baseline and stress-induced CORT exhibit higher variation than antioxidant capacity and provisioning rate, but not oxidative metabolite levels or wing length. Variation in post-dex CORT and d-ROMs was also elevated in the higher-latitude population in that inhabits a less predictable environment. We next studied how these patterns might play out on a macroevolutionary scale, assessing patterns of variation in baseline testosterone (T) and multiple non-endocrine traits (body length, mass, social display rate, and locomotion rate) across 17 species of Anolis lizards. At the macroevolutionary level, we found that circulating T levels and the rate of social display output are higher than other behavioral and morphological traits. Altogether, our results support the idea that within-population variability in steroid levels is substantial, but not exceptionally higher than many other traits that define animal phenotypes. As such, circulating steroid levels in free-living animals should be considered traits that exhibit similar levels of variability from individual to individual in a population.
循环类固醇激素水平在个体内部和个体之间都表现出高度的变异性，这使得一些人推测这些表型比其他形态、生理和行为特征更具变异性。这应该对类固醇信号系统的进化产生深远影响，但很少有研究探讨内分泌变异与其他特征的变异相比如何，或者在不同种群之间有何差异。在这里，我们通过首先探究三种皮质酮（CORT）测量指标——基线、应激诱导和地塞米松注射后——的变异与两种树燕（双色树燕）种群中表征形态（翅长、体重）、生理（活性氧代谢物浓度[d-ROMs]和抗氧化能力）和行为（育雏率）的关键特征的变异如何比较，来进行这样的分析。在控制了测量精度和个体内部变异后，我们发现只有地塞米松注射后的CORT比所有其他特征更具变异性。基线和应激诱导的CORT的变异性都高于抗氧化能力和育雏率，但不高于氧化代谢物水平或翅长。在居住于较不可预测环境的高纬度种群中，地塞米松注射后CORT和d-ROMs的变异性也有所升高。接下来，我们研究了这些模式在宏观进化尺度上可能如何表现，评估了17种安乐蜥中基线睾酮（T）和多种非内分泌特征（体长、体重、社会展示率和运动率）的变异模式。在宏观进化水平上，我们发现循环T水平和社会展示输出率高于其他行为和形态特征。总之，我们的结果支持这样一种观点，即种群内类固醇水平的变异性很大，但并不特别高于许多其他定义动物表型的特征。因此，自由生活动物的循环类固醇水平应被视为在种群中个体间表现出相似变异水平的特征。
