Variation in fecal testosterone levels in male gray whales on a foraging ground relative to maturity and timing

Understanding wildlife reproductive seasonality is crucial for effective management and long-term monitoring of species. This study investigates the seasonal variability of testosterone in male Pacific Coast Feeding Group (PCFG) gray whales, using an eight-year dataset (2016-2023) of individual sightings, drone-based photogrammetry, and endocrine analysis of fecal samples. We analyzed the relationship between testosterone levels and total body length (TL), body condition (body area index, BAI), and day of the year (DOY) using generalized additive mixed models (GAMMs). Our findings reveal a significant increase in fecal testosterone levels in mature males (MM) towards the end of the foraging season. This increase was not observed in juvenile males (JM), highlighting age-dependent development of sexual characteristics. No significant relationship was found between testosterone levels and TL. Additionally, BAI did not significantly influence testosterone levels. Our results suggest that the increasing testosterone levels in MM gray whales indicate preparation for mating before the southbound migration. These findings provide valuable insights into the reproductive biology of PCFG gray whales and underscore the importance of non-invasive fecal sampling for studying reproductive seasonality in large whales. Our approach not only provides further insights into the seasonality of male reproduction for the PCFG gray whales but also offers tools for enhancing the understanding of male reproduction in baleen whales broadly with non-invasive approaches.