Data from: Conservation implications of a lack of relationship between baseline glucocorticoids and fitness in a wild passerine

The application of physiological measures to conservation monitoring has been gaining momentum and, while a suite of physiological traits are available to ascertain disturbance and condition in wildlife populations, glucocorticoids (i.e., GCs: cortisol and corticosterone) are the most heavily employed. The interpretation of GC levels as sensitive indicators of population change necessitates that GCs and metrics of population persistence are linked. However, the relationship between GCs and fitness may be highly context-dependent, changing direction, or significance, depending on the GC measure, fitness metric, life history stage, or other intrinsic and extrinsic contexts considered. We examined the relationship between baseline plasma corticosterone (CORT) levels measured at two periods of the breeding season and three metrics of fitness (offspring quality, reproductive output, and adult survival) in female tree swallows (Tachycineta bicolor). Specifically, we investigated whether: i) a relationship between baseline CORT metrics and fitness exists in our population; ii) whether the inclusion of energetic contexts such as food availability, reproductive investment, or body mass could alter or improve the strength of the relationship between CORT and fitness; iii) whether energetic contexts could better predict fitness compared to CORT metrics. Importantly, we investigated these relationships in both natural conditions and under an experimental manipulation of foraging profitability (feather clipping) to determine the influence of an environmental constraint on GC-fitness relationships. We found a lack of relationship between baseline CORT and both short- and long-term metrics of fitness in control and clipped birds. In contrast, loss in body mass over reproduction positively predicted reproductive output (number of chicks leaving the nest) in control birds; however, the relationship was characterized by a low R2 (5%), limiting the predictive capacity, and therefore the application potential, of such a measure in a conservation setting. Our results stress the importance of ground-truthing GC-fitness relationships and indicate that baseline GCs will likely not be easily employed as conservation biomarkers across many species and life history stages. Given the accumulating evidence of temporally-dynamic, inconsistent, and context-dependent GC-fitness relationships, placing effort towards directly measuring fitness traits, rather than plasma GC levels, will likely be more worthwhile for many conservation endeavours.

将生理指标应用于保护监测的研究正日益受到关注；尽管目前已有一系列生理性状可用于评估野生动物种群的受扰状况与健康状态，但糖皮质激素（glucocorticoids, GCs：皮质醇与皮质酮）仍是应用最为广泛的一类指标。将糖皮质激素水平作为种群变化的敏感指示指标，前提是糖皮质激素与种群存续相关指标之间存在关联。然而，糖皮质激素与个体适合度之间的关系可能高度依赖于具体情境：其作用方向或显著性会随所采用的糖皮质激素测定方式、适合度指标、生活史阶段，以及其他内在与外在情境的不同而发生变化。本研究以雌性双色树燕（Tachycineta bicolor）为研究对象，检测了繁殖期两个时段的基础血浆皮质酮（corticosterone, CORT）水平与三类适合度指标（后代质量、繁殖产出以及成体存活率）之间的关联。具体而言，我们探究了三个问题：其一，本研究种群中基础皮质酮指标与适合度之间是否存在关联；其二，纳入食物可获得性、繁殖投入或体质量等能量情境因素，是否能够改变或增强糖皮质激素与适合度之间关联的强度；其三，相较于糖皮质激素指标，能量情境因素是否能更好地预测适合度。尤为重要的是，我们同时在自然条件与人为操纵觅食收益（剪羽处理）的条件下开展了上述关联研究，以明确环境约束对糖皮质激素-适合度关系的影响。研究结果显示，无论对照组还是剪羽处理组，基础皮质酮水平与适合度的短期及长期指标之间均未发现显著关联。与之相对，对照组个体在繁殖过程中的体质量损失可正向预测繁殖产出（即离巢雏鸟数量），但该关联的决定系数R²仅为5%，预测能力有限，进而限制了该指标在保护实践中的应用潜力。本研究结果强调了验证糖皮质激素-适合度关系的重要性，并表明在多数物种与生活史阶段中，基础糖皮质激素或许难以直接作为保护生物标志物加以应用。鉴于越来越多的研究表明糖皮质激素与适合度之间的关系具有时间动态性、不一致性以及情境依赖性，相较于测定血浆糖皮质激素水平，投入精力直接测量适合度性状，或许能为更多保护工作带来更切实的价值。