Sex-specific Endocrine Bioindicators Reflect Metabolic Well-being and Reproductive Loss in Humpback Whales from...

Migratory whales are excellent sentinels of the health of marine ecosystems as they traverse whole ocean basins. Their metabolic balance, body condition, and reproductive suppression during marine heatwaves reflect their sensitivity to ocean conditions. A pilot study used corticosteroids as biomarkers of metabolic stress responses. Routine validations were conducted for cortisol, corticosterone, and aldosterone enzyme immunoassays, with differing success for each sex. High-pressure liquid chromatography (HPLC) profiles for the same hormones were analyzed on extracted blubber for each sex. No immunoreactivity of female blubber corresponded to the corticosterone peak, while 60% total immunoreactivity in the males was corticosterone. Conversely, no immunoreactivity of male blubber corresponded to cortisol, whereas 41% total immunoreactivity in females was cortisol. Both females and males had immunoreactive peaks that overlapped with aldosterone. The disparity in glucocorticoids between sexes likely reflected differential metabolism of the hormones in blubber. In a follow-up study, reproductive and metabolic biomarkers were concurrently monitored and pregnancy was detected from elevated blubber progesterone concentrations in biopsies from adult female humpback whales from 2020 to 2024 with long-term sighting histories near Juneau, Alaska. Annual pregnancy rates were 0.36 to 0.75 and minimum calving rates were predictably lower (0.18-0.50). An annual realized population growth rate (1.001-1.059) was estimated using an age-structured Lotka model, where data from the literature were used for age-specific survival rates and average age of first birth, and year-specific data from this study to estimate reproductive rates. The difference between the pregnancy rate and the minimum calving rate in late spring to summer, post oceanic migration, indicated this is a key period when pregnancy may be disrupted and perinatal mortality may occur. Annual monitoring of unique subpopulations will improve knowledge of individual's physiological well-being, and assessing pregnancy, calving, and realized population growth rates aids our understanding of endocrine adaptations that influence population's stability during changing ocean conditions.