Effects of Life-History Stage and Environmental Change Observed in Longitudinal Hormone Profiles of Bowhead Whales (Baleana mysticetus)
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 may be 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 2-4 cm along each baleen specimen, producing between 58-130 samples per individual for assay of for corticosterone, cortisol, dehydroepiandrosterone (DHEA(S)), progesterone, testosterone, and triiodothyronine (T3). Hormones were extracted from baleen powder with a methanol vortex method and quantified with enzyme immunoassays, with stable isotope analyses to confirm baleen growth rate and timeline. Our aims were to determine: 1) 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. Results demonstrated that DHEA(S) and corticosterone were especially informative for interpreting reproductive patterns, while cortisol and T3 showed less consistent associations. Female inter-calving intervals were highly variable both within and across individuals, including evidence of back-to-back pregnancies and extended gaps between reproductive events. Males generally exhibited consistent annual testosterone cycles, though occasional deviations were observed. Hormone and stable isotope profiles from the 1970s–1980s differed significantly from earlier time periods, suggesting altered endocrine patterns possibly related to the 1977 Pacific Decadal Oscillation (PDO) regime shift. This work confirms that steroid and thyroid hormones remain detectable in baleen more than 150 years old and highlights the utility of natural history specimens for reconstructing long-term physiological histories. By combining life-history endocrinology with ecological context, this dissertation offers novel insights into bowhead whale biology and provides a framework for understanding how long-lived species may respond to environmental change across centuries.
biomarkers, environmental change, marine mammals, Pacific Decadal Oscillation, reproduction, stress
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