Endocrine traits (e.g., circulating hormone concentrations, receptor expression) can vary considerably among individuals within populations. Here, we develop two evolutionary hypotheses to explain this variation. Under the Optimal Endocrine Phenotype Hypothesis, adaptive plastic responses to environmental variation generate individual variation in endocrine traits and allow individuals to express near-optimal endocrine phenotypes. In contrast, under the Ongoing Selection Hypothesis, individual variation in endocrine traits reflects varying adaptive value, with some individuals expressing suboptimal phenotypes that are selected against. These two hypotheses generate distinct predictions for the effects of hormone manipulations on fitness. Under the Optimal Endocrine Phenotype Hypothesis, all hormone manipulations should incur fitness costs, whereas under the Ongoing Selection Hypothesis, manipulating endocrine phenotypes toward a putative optimum should increase fitness. Using a meta-analysis of findings from experimental field studies that involved manipulation of circulating glucocorticoids or androgens and measurement of fitness effects, we test and find some support for the Optimal Endocrine Phenotype Hypothesis. On average, fitness was reduced across 97 estimates of the effects of experimental hormone manipulations on fitness. However, the fitness effects of glucocorticoid manipulations varied with the sex of the individuals being studied. Fitness was more uniformly reduced by glucocorticoid manipulations in males and when both sexes were considered together. In females, effects on fitness varied from highly positive to highly negative. The effects of androgen manipulations varied across males and females, and depending upon whether fitness was estimated using measures of reproductive success or survival. Reproductive success was consistently decreased by androgen manipulation in females, but was increased almost as often as it was decreased across experiments in males. When survival was estimated as a component of fitness, it was fairly uniformly compromised by exogenous androgens in males. This variation in fitness effects of hormone manipulations across sexes and fitness metrics is consistent with the expectation that hormones differentially regulate life-history investment and that optimal endocrine phenotypes differ between males and females. Overall, our meta-analysis provides some support for the Optimal Endocrine Phenotype Hypothesis, but we await direct tests of the Ongoing Selection Hypothesis to determine the degree to which individual variation in endocrine traits continues to be shaped by natural selection.

中文翻译：

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激素操作会降低健康度吗？最佳内分泌表型假说的荟萃分析测试。

人群中个体之间的内分泌特征（例如，循环激素浓度，受体表达）可能有很大差异。在这里，我们提出了两个进化假设来解释这种变化。在最佳内分泌表型假设下，对环境变化的适应性塑性反应会产生内分泌特征的个体差异，并允许个体表达接近最佳的内分泌表型。相反，在“持续选择假说”下，内分泌性状的个体变异反映了变化的适应性价值，其中一些个体表达了针对其的次优表型。这两个假设对激素操作对健康的影响产生了不同的预测。根据最佳内分泌表型假设，所有激素操作都应产生健身费用，而根据“持续选择假说”，将内分泌表型推向可能的最佳水平应可提高适应性。使用对涉及循环糖皮质激素或雄激素操纵以及健身效果测量的实验田间研究结果进行的荟萃分析，我们测试并发现了对最佳内分泌表型假说的支持。平均而言，实验激素操作对健身的影响的97个估计值都降低了健身水平。然而，糖皮质激素操纵的适应性效应随所研究个体的性别而变化。通过糖皮质激素的操作，男性和两性都一起考虑时，体能会更均匀地降低。在女性中，对健康的影响从高度积极到高度消极不等。雄激素操纵的影响在男性和女性之间各不相同，并且取决于是否使用生殖成功或存活的量度来估计适合度。在雄性动物中，雄激素的操作成功地降低了繁殖成功率，但在雄性动物中，其繁殖成功率几乎与减少一样多。当估计生存是健康的一部分时，男性中外源性雄激素会相当均匀地损害生存率。跨性别和适合度指标进行激素操作的适应性差异与以下预期相符：激素差异调节生活史投资，并且最佳内分泌表型在男性和女性之间有所不同。总体而言，我们的荟萃分析为最佳内分泌表型假说提供了一些支持，