UC Santa Cruz

Hormones provide a direct measure of the physiological state of wild animals. Yet, the natural variation that underlies endocrine biomarkers remains poorly characterized in North Pacific humpback whales (Megaptera novaeangliae), a population recovering from commercial whaling while facing increasingly variable ocean conditions. In this dissertation, I characterized variation in reproductive steroids and glucocorticoids measured from minimally invasive skin–blubber biopsies collected on the Hawaiʻi breeding grounds (2014–2025) and the Southeast Alaska feeding grounds (2023–2025), spanning sex, reproductive class, season, social context, and region.In male humpback whales (Chapter 1), androgens and progesterone were elevated on the breeding grounds, and androgens declined across the breeding season, consistent with males arriving in a heightened reproductive state and exhibiting reduced androgen production as the breeding season progressed. Reproductive hormone profiles did not differ among males occupying different social roles on the breeding grounds, indicating that endocrine profiles reflect broader reproductive physiology than short-term breeding behavior. In females (Chapter 2), progesterone identified presumed pregnant females on the feeding grounds, while progesterone and estrone differentiated mothers from females without calves on the breeding grounds. Multihormone profiles further distinguished these reproductive classes, demonstrating that coordinated endocrine variation provides greater insight into reproductive physiology than individual hormones alone. For glucocorticoids (Chapter 3), cortisol was the predominant hormone measured in blubber and was elevated on the breeding grounds. Seasonal glucocorticoid trajectories differed among reproductive classes, with mothers maintaining relatively constant concentrations while females without calves declined across the breeding season. In males, cortisol was positively associated with testosterone and varied with social-group composition.Across all three chapters, reproductive state and seasonal timing—rather than sex or social role—were the primary drivers of endocrine variation. Taken together, this work establishes a multihormone framework for interpreting endocrine variation in North Pacific humpback whales and providing the biological context needed to interpret endocrine responses to environmental change in this recovering population.