Sato Y, Garcia T M S, Low J S, Bauer C M
Department of Biology, Swarthmore College, Swarthmore, PA 19081, USA.
Integr Org Biol. 2025 Jul 29;7(1):obaf033. doi: 10.1093/iob/obaf033. eCollection 2025.
The steroid hormone testosterone is important for stimulating male reproductive processes including territory acquisition, mating displays, and spermatogenesis. When examining the relative effects of testosterone on reproductive processes, it is most reasonable to focus on peak baseline testosterone levels, especially for reproductive processes that may occur during specific times of day, such as mating and spermatogenesis. However, some studies have not found consistent positive relationships between circulating testosterone levels and reproductive variables. These nonsignificant relationships could be driven by methodology, as most studies in wild, free-living animals collect blood samples during an animals' active period, yet many species show peak baseline testosterone levels during their inactive period. This may be the case for the common degu (), as field and laboratory studies have exclusively sampled these diurnal rodents during their active period and have found little correlation between testosterone levels and reproductive success. In this study, we measured testosterone levels in captive male degus every 4 h across a 24-h cycle to test the hypothesis that male degus demonstrate diel variation in their baseline testosterone levels. We saw significant variation in male degu baseline testosterone levels over a 24-h period, and our prediction that baseline testosterone levels would be higher during nighttime (inactive period) timepoints compared to daytime (active period) timepoints was supported. However, nighttime baseline testosterone levels were still several magnitudes lower than testosterone levels after a gonadotropin-releasing hormone (GnRH) injection. While GnRH injections significantly increased circulating plasma testosterone levels during any daytime time period, we found no significant correlation between nighttime baseline testosterone levels and post-GnRH testosterone levels, which suggests GnRH-challenges during the daytime cannot be used to approximate or estimate nighttime baseline testosterone levels. These findings expand our knowledge surrounding testosterone dynamics and suggest that future studies should take into account the time of day when sampling testosterone and other hormone levels.
类固醇激素睾酮对于刺激雄性生殖过程非常重要，这些过程包括领地获取、求偶展示和精子发生。在研究睾酮对生殖过程的相对影响时，最合理的做法是关注睾酮的峰值基线水平，特别是对于可能在一天中特定时间发生的生殖过程，如交配和精子发生。然而，一些研究并未发现循环睾酮水平与生殖变量之间存在一致的正相关关系。这些不显著的关系可能是由方法学导致的，因为大多数针对野生自由活动动物的研究在动物的活跃期采集血样，然而许多物种在其不活跃期显示出峰值基线睾酮水平。普通草原鹿可能就是这种情况，因为野外和实验室研究仅在其活跃期对这些昼行性啮齿动物进行采样，并且发现睾酮水平与生殖成功率之间几乎没有相关性。在本研究中，我们在24小时周期内每4小时测量一次圈养雄性草原鹿的睾酮水平，以检验雄性草原鹿的基线睾酮水平存在昼夜变化这一假设。我们发现雄性草原鹿的基线睾酮水平在24小时内有显著变化，并且我们的预测得到了支持，即与白天（活跃期）时间点相比，夜间（不活跃期）时间点的基线睾酮水平会更高。然而，夜间基线睾酮水平仍比促性腺激素释放激素（GnRH）注射后的睾酮水平低几个数量级。虽然GnRH注射在任何白天时间段都显著提高了循环血浆睾酮水平，但我们发现夜间基线睾酮水平与GnRH注射后的睾酮水平之间没有显著相关性，这表明白天的GnRH刺激不能用于近似或估计夜间基线睾酮水平。这些发现扩展了我们对睾酮动态的认识，并表明未来的研究应考虑在采样睾酮和其他激素水平时的时间。
