Some aspects of the reproductive physiology of otariid pinnipeds

Steller sea lion and northern fur seal populations in Alaska are declining, due in part to a reduction in reproductive output. Though these species have been studied on breeding rookeries for much of the last century, little is known of their reproductive physiology. In order to evaluate reproductive status and potential factors contributing to reduced reproductive output among these species, studies were designed examine the reproductive physiology of otariids pinnipeds. Progesterone was measured in blood and feces of Steller sea lions to evaluate the potential of using non-invasive methods to monitor pregnancy. Excreted levels were correlated with circulating progesterone concentrations, however the identification of a variety of immunoreactive steroid metabolites in samples collected from male and female Steller sea lions animals illustrated the necessity for rigorous validation of fecal assays and suggested more data must be acquired before these techniques may be successfully employed. Examination of the circulating sex steroid metabolites of northern fur seals indicated very low estrogen levels and substantial androgen levels in peripheral circulation of females near the end of the embryonic delay. When contrasted with male and female fur seals sampled during late summer, decreases in levels of specific serum androgens were implicated as possible regulators of uterine implantation. Additionally, elevated progesterone levels in serum of male and female fur seals sampled during summer suggest possible adrenal synthesis of this steroid. Northern fur seal reproductive tissue and adrenal glands were examined for expression and activity of steroidogenic enzymes. The capacity for androgen synthesis was substantial in gonads and adrenal glands of all ages and both genders, however, the term placenta appeared to have limited capacity for sex steroid synthesis. These data suggest participation of the fetal gonad and adrenal and the dependence on the maternal ovary throughout pregnancy. Steroidogenic enzymes capable of converting androgens to estrogens were expressed in vascular tissue. Expression was observed in vessels of all tissues examined, but was particularly high in placental and umbilical vessels. The vasorelaxive effects of estrogens have been well documented and presence of these proteins in pinniped vessels suggests a potential method of maintaining perfusion of the placenta and fetus during dives by local conversion of androgens in the circulating pool to estrogens.