Bowhead whales have the longest baleen of any mysticete, with a single baleen plate capturing up to 20 years of continuous physiological data. Thus, bowhead baleen is an ideal sample type for retrospective study of reproductive cycles and physiological responses to environmental stressors. We studied patterns of six hormones across the full length of ~2.5m-long baleen specimens from ten individual adult bowhead whales representing different eras (1850s-1870s, n=2; 1940s-1960s, n=4; and 1970s-1980s, n=4). We subsampled baleen every 2.5-4 cm, extracting and assaying samples for corticosterone, cortisol, dehydroepiandrosterone (DHEA), progesterone, testosterone, and triiodothyronine (T3). Hormones were quantified with enzyme immunoassays, with stable isotope analyses to confirm baleen growth rate and timeline. Our aims were to: 1) determine if this panel of hormones could provide information regarding life-history stage and reproductive state, 2) if Alaskan bowhead adult females experience the expected 3-year inter-calving interval across all eras, 3) if males in the same population experience annual testosterone cycles, and lastly 4) if museum specimens 150 years old may provide insight into historic populations. We found that DHEA and corticosterone added substantial information for the interpretation of adult bowhead reproductive patterns, while cortisol and T3 did not. Inter-calving interval (estimated from progesterone patterns) was highly variable both within individual females and across time periods, with some cases of apparent back-to-back pregnancies in successive years, sometimes followed by an extended inter-calving interval. Males had more consistent annual testosterone cycles, but they also experienced occasional deviations in annual cycles. We confirmed that steroid and thyroid hormones are readily detectable in baleen approximately 175 years old. Our results underscore the importance of examining hormone concentrations with reference to reproductive state, the utility of historic specimens from natural history museums, and the value in evaluating several hormones concurrently in free-ranging wildlife studies.

Understanding how individual animals respond to stressors behaviourally and physiologically is a critical step towards quantifying long-term population consequences and informing management efforts. Glucocorticoid (GC) metabolite accumulation in various matrices provides an integrated measure of adrenal activation in baleen whales and could thus be used to investigate physiological changes following exposure to stressors. In this study, we measured GC concentrations in faecal samples of Pacific Coast Feeding Group (PCFG) gray whales (Eschrichtius robustus) collected over seven consecutive years to assess the association between GC content and metrics of exposure to sound levels and vessel traffic at different temporal scales, while controlling for contextual variables such as sex, reproductive status, age, body condition, year, time of year and location. We develop a Bayesian Generalized Additive Modelling approach that accommodates the many complexities of these data, including non-linear variation in hormone concentrations, missing covariate values, repeated samples, sampling variability and some hormone concentrations below the limit of detection. Estimated relationships showed large variability, but emerging patterns indicate a strong context-dependency of physiological variation, depending on sex, body condition and proximity to a port. Our results highlight the need to control for baseline hormone variation related to context, which otherwise can obscure the functional relationship between faecal GCs and stressor exposure. Therefore, extensive data collection to determine sources of baseline variation in well-studied populations, such as PCFG gray whales, could shed light on cetacean stress physiology and be used to extend applicability to less-well-studied taxa. GC analyses may offer greatest utility when employed as part of a suite of markers that, in aggregate, provide a multivariate measure of physiological status, better informing estimates of individuals’ health and ultimately the consequences of anthropogenic stressors on populations.