A Dozen Years of Baleen Hormones: Validations, Discoveries, Surprises, and Next Steps.

Since the first publication in 2014 reporting that progesterone is detectable in whale baleen, numerous studies have confirmed that patterns of hormones in baleen can provide a multiyear time series of continuous endocrine information from individual whales. The field is poised to expand substantially in the near future, and thus it is an opportune time to review findings and identify current knowledge gaps and pathways for future research. A search of baleen-hormone literature reveals 30 publications that, in total, investigate eight steroid hormones and two thyroid hormones in baleen of 10 species of mysticete whale, with the pygmy right whale representing the only mysticete family not yet studied. An early phase of methodological validations optimized the technique, including reduction of sample mass requirement and improvements in hormone yield. Steroid hormones have consistently passed technical and physiological validations; thyroid hormones, however, are still in need of physiological validations. Later literature has entailed a series of descriptive studies, which typically combine endocrine and isotope analyses to elucidate typical hormone ranges and patterns across years in relation to species, sex, age class, and reproductive state. Most descriptive studies have been limited to a small n of individuals (a consequence of the high n of subsamples per whale), yet have been highly informative nonetheless, revealing many unexpected findings (e.g., evidence suggestive of extended gestation, timing and location of breeding, reproductive senescence, early sexual maturity, pregnancy loss, and effects of stressors). Such case-study reports remain of considerable value, but the field is increasingly expanding to include hypothesis-driven research on ecological questions of broad significance, such as influences of oceanographic factors and anthropogenic stressors, and the physiological and behavioral plasticity of individual responses to such environmental drivers. Addressing such questions will require robust statistical frameworks and larger sample sizes of individual whales, a daunting task given that a single baleen specimen can generate > 150 samples requiring months of labor and associated costs. Thus, increased collaborations could be both fruitful and necessary (e.g., a baleen-hormone research consortium wherein datasets can be pooled across research teams). In sum, baleen hormone research has provided unique and invaluable insights into patterns of physiology across time in the great whales, and has great promise to continue advancing understanding of the biology of these vulnerable, long-lived, enigmatic species.