What normal used to be: Longitudinal adrenal, reproductive, and thyroid hormone profiles from four bowhead whales during a period of Arctic stability

Abstract Assessment of large whale physiological response and resiliency to environmental change requires information about historical baseline physiological data to compare with current trends. Multi-year studies of historic and modern individual bowhead whales are possible by utilizing baleen, a keratin matrix tissue that captures a time series of physiological data across multiple years. We assessed six hormones in bowhead baleen informative of stress, reproduction, and metabolism: corticosterone, cortisol, progesterone, testosterone, triiodothyronine (T3), and dehydroepiandrosterone (and its sulfated form, collectively DHEA(S)). We also performed assay validations for DHEA(S), as this hormone has not been assessed in keratin. Through examination of longitudinal endocrine profiles of two adult males and two adult females with data from the 1940’s-1960’s, a period of relative Arctic climatic stability, we determined that reproductive cycles could be identified in adult bowhead whale baleen more than sixty years old. Testosterone cycling was observed in males while three putative pregnancies, and a fourth confirmed pregnancy, were observed in females. Strong correlations were observed between DHEA(S) and testosterone in males and pregnant females. Pregnancy displayed the strongest correlations among hormones, indicating that pregnancy is likely a form of physiological stressor that should be controlled for when studying adrenal hormones. However, in non-pregnant females, cortisol was strongly negatively correlated with progesterone and testosterone, indicating that physiological stress may influence ovarian cycling and/or likelihood of future pregnancy. Our findings underscore the importance of utilizing museum specimens to establish historical baselines, as well the value of panels of multiple hormones (representing different physiological axes) when assessing multiple stressors in free-ranging wildlife. Understanding how age, sex, and life-history stage (e.g. pregnancy) influence those hormone patterns is useful for addressing greater conservation-relevant questions, particularly related to environmental change.