Reproduction and stress response endocrinology in blue (Balaenoptera musculus) and gray (Eschrichtius robustus) whales

Abstract

Identification of biomarkers that reflect physiological status is fundamental for assessing population health, as well as providing more accurate estimates of life history parameters. Blue (Balaenoptera musculus) and gray (Eschrichtius robustus) whale populations feed on lower trophic levels and migrate between the eastern Tropical and the eastern North Pacific Ocean. With increasing disturbances (e.g., changing environment and human activities), understanding the stress response, resultant coping mechanisms, and the subsequent effects on reproduction, is of growing importance. While extensive knowledge is available on photo-identification and ecology of these two species, information on physiology is limited and what exists is outdated. This dissertation validated and measured a suite of steroid hormones in blubber tissue using an enzyme immunoassay technique to develop physiological biomarkers for reproduction and metabolism in these two species. Coupled with sighting history data, progesterone and testosterone were validated as biomarkers for reproductive physiology. In both species, progesterone concentrations were higher in pregnant females and mixture models were developed to estimate reproductive status for whales of unknown status. Testosterone showed greater variability in adult males and concentrations were higher in samples collected during fall, suggesting physiological preparation for mating. Additionally, progesterone concentrations in gray whales were higher in calves of both sexes, indicating maternal transfer through lactation, while in blue whales, testosterone was detectable only in males and in pregnant whales, suggesting its biosynthesis or metabolism is altered during gestation. Biomarkers for stress response were developed through analytical and biological validation of three corticosteroid hormones: cortisol, corticosterone and aldosterone. First, analytical validations (i.e., parallelism and accuracy tests) were used to determine detectability and measurement accuracy of these hormones using commercially available kits. Hormone concentrations were tested for any relationships with life history parameters (e.g., age and reproductive state) as well as with area and time of sampling within presumably "healthy" (biopsies) whales and "stressed" (stranded) whales. "Stressed" whales, especially those that perished due to trauma and/or nutritional stress, had higher concentrations of all three corticosteroid hormones than "healthy" whales, suggesting ongoing alteration of metabolic status due to a stress response. In female "healthy" whales, reproductive status appeared to be a major factor influencing corticosterone concentrations in blue whales and for cortisol in gray whales. Overall, cortisol was determined to be a valid marker for body conditions in both species. These results set a milestone for assisting to better understand the impact of a changing environment on the physiology of these species and can be used to develop more accurate reproductive and survival rates to use in population dynamics models for management of subsistence resources and for conservation of endangered species.