Physiological Shifts in Female Bowhead Whales over Historic Oceanographic and Anthropogenic Regimes.

Long-lived marine mammals such as bowhead whales (Balaena mysticetus) provide rare opportunities to examine physiological responses to past environmental and anthropogenic pressures. Using baleen plates from six adult females, we constructed up to 18 years of longitudinal profiles per individual for eight analytes: corticosterone, cortisol, dehydroepiandrosterone and its sulfated form (DHEA(S)), progesterone, testosterone, triiodothyronine (T3), and stable isotope ratios of carbon (δ13C) and nitrogen (δ15N). These records spanned three distinct periods (regimes): the 1850-1870s, marked by commercial whaling and a warm Pacific Decadal Oscillation (PDO) phase; the 1940-1960s, a cold PDO phase with Arctic stability and multiple La Niña events; and the 1970-1980s, which included a major PDO regime shift and strong El Niño events. We identified 1-8 putative pregnancies per individual based on progesterone profiles, with variation in gestation length (13-21 months), interpregnancy interval (8 months to > 4 years), and apparent pregnancy loss or calf loss. Pregnancy was associated with elevated corticosterone, DHEA(S), and testosterone. Certain hormones varied significantly with regime: DHEA(S) and testosterone were higher in the 1850-1870s relative to at least one later regime, whereas T3 was significantly lower in the 1970-1980s compared to earlier periods. δ¹³C was a significant negative predictor of T3, suggesting a potential link between foraging habitat and metabolic output. These findings highlight baleen as a retrospective biomonitoring tool and reveal potential reproductive plasticity and physiological responses of female bowheads to environmental change.