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Hormonal biomarkers are useful indicators of mammalian reproductiveand metabolic states. The present study validated and applied the use ofprogesterone and cortisol blubber assays for studies of blue whales fromthe Gulf of California, Mexico. In a validation study for pregnancy detection,blubber progesterone concentrations were correlated with pregnancystatus for four female blue whales: three resighted with a calf theyear following sampling and the fourth stranded with a fetus. The progesteroneconcentrations were significantly higher than those measuredin juvenile whales (n = 3). In the application study, blubber samples fromblue whales (51 noncalf females, 2 female calves, 48 noncalf males, and1 male calf) with known sighting histories were analyzed. Putative pregnantfemales had elevated progesterone concentrations. Cortisol concentrationsdid not differ between male and female blue whales, or amongfemales in different reproductive classes. After correcting for uncertainages, hence maturity status, the pregnancy rate of noncalf females was33.4% (95% CI 32.2%–34.3%). Although interpretation of hormone biomarkersmust consider all physiological states that may influence progesteroneconcentrations, these results demonstrate the utility of pairinghormone biomarkers with sighting histories to help assess environmentalor anthropogenic impacts on reproduction in blue whales.

Palabras clave: blue whale, Balaenoptera musculus, pregnancy rates; progesterone, cortisol, biomarkers

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