Data from: Heritable variation in circulating glucocorticoids and endocrine flexibility in a free-living songbird

Phenotypic flexibility is a central way that organisms cope with challenging and changing environments. As endocrine signals mediate many phenotypic traits, heritable variation in hormone levels, or their context-dependent flexibility, could present an important target for selection. Several studies have estimated the heritability of circulating glucocorticoid levels under acute stress conditions, but little is known about the potential for either baseline hormone levels or rapid endocrine flexibility to evolve. Here we assessed the potential for selection to operate on the elevation (circulating hormone levels) and flexibility of glucocorticoid reaction norms to acute restraint stress. Multivariate animal models revealed low but significant heritability in baseline (h2=0.13-0.14) and stress-induced glucocorticoids (h2=0.18), and moderate heritability in glucocorticoid flexibility in response to acute stress (h2=0.38) in free-living juvenile tree swallows (Tachycineta bicolor; n=408). Baseline glucocorticoids were not genetically correlated with either stress-induced glucocorticoids or glucocorticoid flexibility. These findings indicate that baseline glucocorticoids and the acute stress response are distinct traits that can be independently shaped by selection. Microevolutionary changes that influence the expression or flexibility of these endocrine mediators of phenotype may be an important way that populations adapt to changing environments and novel threats.

表型灵活性（phenotypic flexibility）是生物体应对挑战性与动态变化环境的核心策略。由于内分泌信号（endocrine signals）介导了诸多表型性状，激素水平的遗传变异（heritable variation）或其环境依赖性灵活性，或可成为自然选择（selection）的重要作用靶点。已有多项研究对急性应激（acute stress）条件下循环糖皮质激素（circulating glucocorticoid）水平的遗传力（heritability）进行了估算，但目前对于基础激素水平（baseline hormone levels）或快速内分泌灵活性（rapid endocrine flexibility）的进化潜力仍知之甚少。本研究评估了自然选择作用于糖皮质激素针对急性束缚应激的反应规范（glucocorticoid reaction norms）的水平（即循环激素水平）与灵活性的潜力。通过多变量动物模型（multivariate animal models），我们在野生幼年双色树燕（Tachycineta bicolor；样本量n=408）中发现：基础糖皮质激素的遗传力较低但显著（h²=0.13~0.14），应激诱导的糖皮质激素遗传力为0.18，而响应急性应激的糖皮质激素灵活性的遗传力处于中等水平（h²=0.38）。基础糖皮质激素与应激诱导的糖皮质激素、糖皮质激素灵活性均无遗传相关（genetic correlation）。上述研究结果表明，基础糖皮质激素与急性应激反应（acute stress response）属于可被自然选择独立塑造的不同性状。影响这些介导表型的内分泌因子（endocrine mediators of phenotype）的表达或灵活性的微进化变化（microevolutionary changes），或为种群适应动态变化环境与新型威胁的重要途径。