Data from: Do glucocorticoids predict fitness? Linking environmental conditions, corticosterone and reproductive success in the blue tit, Cyanistes caeruleus

Glucocorticoids, including corticosterone (CORT), have been suggested to provide a physiological link between ecological conditions and fitness. Specifically, CORT, which is elevated in response to harsh conditions, is predicted to be correlated with reduced fitness. Yet, empirical studies show that CORT can be non-significantly, positively and negatively linked with fitness. Divergent environmental conditions between years or study systems may influence whether CORT is linked to fitness. To test this, we monitored free-living blue tits (Cyanistes caeruleus) during breeding over 3 years. We quantified foraging conditions during brood rearing, and examined whether they were correlated with parental baseline CORT and reproductive success. We then tested whether CORT predicted fitness. Elevated parental CORT was associated with lower temperatures, greater rainfall and lower territory-scale oak density. Whereas asynchrony with the caterpillar food peak was correlated with reduced nestling mass and fledging success, but not parental CORT. Only low temperatures were associated with both reduced nestling mass and elevated parental CORT. Despite this, parents with elevated CORT had lighter offspring in all years. Contrarily, in 2009 parental CORT was positively correlated with the number fledged. The absence of a direct link between the foraging conditions that reduce nestling quality and elevate parental CORT suggests that parental CORT may provide a holistic measure of conditions where parents are working harder to meet the demands of developing young. As the positive correlation between parental CORT and fledging success differed between years, this suggests that contrasting conditions between years can influence correlations between parental CORT and fitness. Ultimately, as CORT concentrations are intrinsically variable and linked to the prevalent conditions, studies that incorporate environmental harshness will improve our understanding of evolutionary endocrinology.包括皮质酮（CORT）在内的糖皮质激素（glucocorticoids），被认为是连接生态条件与个体适合度的生理纽带。具体而言，因恶劣环境刺激而升高的皮质酮，理论上应与个体适合度下降呈显著相关。然而，现有实证研究表明，皮质酮与适合度的关联可表现为无统计学显著性、正相关或负相关。年度间或不同研究系统间存在的异质性环境条件，可能会影响皮质酮与适合度之间的关联模式。为验证这一假说，我们在3个繁殖年度内对野生蓝山雀（Cyanistes caeruleus）开展了野外监测。我们量化了育雏阶段的觅食环境条件，并检验其与亲代基础皮质酮水平及繁殖成功率的相关性；随后进一步验证皮质酮能否预测个体适合度。研究结果显示，亲代皮质酮水平升高与环境温度降低、降雨量增加及领地尺度下的橡树密度降低显著相关。雏鸟食物峰值期的物候异步性与雏鸟体重下降及出飞成功率降低相关，但与亲代皮质酮水平无统计学关联；仅低温同时与雏鸟体重下降和亲代皮质酮水平升高存在关联。尽管存在上述结果，但在所有研究年份中，亲代皮质酮水平升高的个体，其雏鸟体重均显著更低。与之相反，2009年时亲代皮质酮水平与出飞幼鸟数量呈正相关。那些会降低雏鸟质量并升高亲代皮质酮的觅食条件之间并未呈现直接关联，这提示亲代皮质酮或可作为亲代为满足雏鸟发育需求而付出更多抚育努力的综合性环境状况指标。鉴于亲代皮质酮与出飞成功率的正相关关系存在年度间差异，这表明年度间迥异的环境条件能够影响亲代皮质酮与适合度之间的关联模式。总而言之，由于皮质酮浓度本身具有内在变异性且与所处的普遍环境条件紧密相关，纳入环境严酷性维度的研究将有助于我们深化对进化内分泌学的理解。