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.

糖皮质激素（Glucocorticoids）包括皮质酮（corticosterone, CORT），被认为是连接生态环境条件与个体适合度（fitness）的生理纽带。具体而言，会在恶劣环境条件下升高的皮质酮，被预测与降低的个体适合度存在关联。然而，现有实证研究表明，皮质酮与适合度之间可呈现无显著相关性、正相关以及负相关三种关联模式。不同年份或研究体系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方向与强度。
为验证这一假说，我们在3个繁殖季内对野生自由栖息的蓝山雀（Cyanistes caeruleus）开展了监测工作。我们量化了亲鸟育雏阶段的觅食环境条件，检验了这些条件是否与亲鸟基础皮质酮水平以及繁殖成功率相关，并进一步验证了皮质酮能否预测个体适合度。
研究结果显示，亲鸟皮质酮水平升高与更低的环境温度、更高的降雨量以及更低的领地尺度橡树密度显著相关。而与鳞翅目幼虫食物峰值出现时间的异步性，与雏鸟体重降低和出飞成功率下降显著相关，但与亲鸟皮质酮水平并无关联。仅低温环境同时与雏鸟体重降低以及亲鸟皮质酮水平升高相关。
尽管存在上述关联，皮质酮水平升高的亲鸟，其所有研究年份的雏鸟体重均显著更低。与之相反，在2009年，亲鸟的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鸟皮质酮水平的觅食环境条件之间并未呈现直接关联，这提示亲鸟的皮质酮水平或许可作为一项综合性指标，反映亲鸟为满足雏鸟生长发育需求而承受的抚育压力。由于亲鸟皮质酮与繁殖成功率之间的正相关关系存在年际差异，这表明不同年份间的环境条件差异，能够影响亲鸟皮质酮与适合度之间的关联模式。
归根结底，由于皮质酮浓度本身具有内在变异性且与当前的环境状况相关，纳入环境恶劣程度因素的研究，将有助于我们深化对进化内分泌学的理解。