Experimental manipulation has established testosterone as a potent, pleiotropic regulator coordinating morphology, physiology and behavior. However, the relationship of field-sampled, unmanipulated testosterone concentrations with traits of interest is often equivocal. Circulating testosterone varies over the course of the day, and recent reports indicate that testosterone is higher during the night in diurnal songbirds. Yet, most field studies sample testosterone during the morning. Sampling at times when levels and individual variation are low may be one reason relationships between testosterone and other traits are not always observed. Testosterone is regulated by the hypothalamic-pituitary–gonadal axis, with gonadotropin-releasing hormone (GnRH) initiating the endocrine cascade. Research has examined GnRH-induced testosterone levels with traits of interest, yet the relevance of these induced levels and their relationship with endogenously produced levels are not fully clear. Using photostimulated male great tits (Parus major) we tested the hypotheses that circulating testosterone levels peak during the night and that GnRH-induced testosterone concentrations are positively related to nightly testosterone peaks. Blood was sampled during first, middle or last third of night. One week later, baseline and GnRH-induced testosterone levels were sampled during mid-morning. Morning baseline testosterone levels were low compared with night-sampled levels that peaked during the first third of the night. Further, GnRH-induced testosterone was strongly positively correlated with levels observed during the first third of the night. These data suggest that morning testosterone samples likely do not reflect an individual’s endogenous peak. Instead, GnRH-induced testosterone levels do approximate an individual’s nightly peak and may be an alternative for birds that cannot easily be sampled at night in the field. These findings are likely to have implications for research aimed at relating traits of interest with natural variation in sex steroid hormone levels.

实验操作已确定睾酮是一种有效的、多效性的调节剂，可协调形态、生理和行为。然而，现场采样的、未经处理的睾酮浓度与感兴趣的特征之间的关系通常是模棱两可的。循环睾酮在一天中变化，最近的报告表明，昼夜鸣禽的夜间睾酮水平较高。然而，大多数实地研究在早上对睾酮进行采样。在水平和个体差异较低的时候取样可能是睾酮与其他特征之间的关系并不总是被观察到的原因之一。睾酮受下丘脑-垂体-性腺轴调节，促性腺激素释放激素 (GnRH) 启动内分泌级联反应。研究已经检查了具有感兴趣特征的 GnRH 诱导的睾酮水平，但这些诱导水平的相关性及其与内源性产生水平的关系尚不完全清楚。使用光刺激的男性大山雀 (大山雀) 我们测试了以下假设：循环睾酮水平在夜间达到峰值，并且 GnRH 诱导的睾酮浓度与夜间睾酮峰值呈正相关。在第一个、中间或最后三分之一的晚上采集血液。一周后，在上午中旬对基线和 GnRH 诱导的睾酮水平进行采样。与夜间前三分之一达到峰值的夜间采样水平相比，早晨基线睾酮水平较低。此外，GnRH 诱导的睾酮与前三分之一晚上观察到的水平呈强正相关。这些数据表明，早晨的睾酮样本可能不会反映个体的内源性峰值。反而，GnRH 诱导的睾酮水平确实接近个体的夜间峰值，并且可能是夜间在野外不易取样的鸟类的替代方案。这些发现可能对旨在将感兴趣的特征与性类固醇激素水平的自然变化联系起来的研究产生影响。