Fluctuations in serum steroid hormone concentrations and body mass during growth and sexual maturation in captive northern fur seals (Callorhinus ursinus).

Northern fur seals (Callorhinus ursinus) have a distinct life history pattern comprising annual terrestrial breeding and oceanic migration, and the physiological changes associated with these patterns are of particular interest for understanding their environmental adaptations. However, owing to their oceanic distribution, limited information is available on the reproductive physiology of wild individuals during the immature stage and the winter migration period. This study aimed to determine the relationships among the seasonal hormone profiles, body growth, age, and pregnancy using monthly serum samples collected over 3-5 years from two male and two female captive individuals during pubescence and sexual maturation. Small increases in the serum testosterone signaled puberty in males aged 3 and 4 years. Thereafter, males showed considerable increases in testosterone during breeding seasons, indicating sexual maturity. Immature female serum progesterone was maintained at low levels, but after pubescence, females showed an increase in serum progesterone in August, the month next to the peak of delivery, followed by a decrease. In non-pregnant females, progesterone did not increase significantly until the next breeding season, but in pregnant females, they increased again from February to March and then gradually decreased. Immature males increased body mass constantly and reached puberty when their body mass exceeded 20 kg, and they showed seasonal weight fluctuations after puberty. These results provide fundamental information for determining sexual maturity and pregnancy in this species based on sex steroid hormones and body mass measurements.

Longitudinal study of northern fur seal (Callorhinus ursinus) hematology.

Although the causes have not been specified yet, wild populations of northern fur seals (Callorhinus ursinus) have been decreasing, which is why conservation techniques such as captive breeding and health maintenance should be established. Hematological parameters can be used to maintain the health status of northern fur seals kept in captivity for artificial reproduction and public education. Year-round fluctuations of blood parameters have not been examined for northern fur seals due to the difficulties in obtaining serial blood samples from wild animals during the oceanic migration period from late autumn to spring. In this study, blood samples were collected from four captive northern fur seals and analyzed monthly for more than three years to clarify the seasonal fluctuation patterns in 14 hematological parameters. Many hematological parameters seemed to be seasonal patterns: summer-autumn and winter-spring; leukocyte-related parameters were higher in summer and autumn than in winter and spring; erythrocyte-related parameters were lower in summer and autumn than in winter and spring. Significant seasonal differences in nine of the 14 parameters were observed using a generalized linear mixed model (GLMM) analysis. These results have improved our understanding of the seasonal patterns of hematological characteristics in the northern fur seal and can contribute to the health care of protected or captive northern fur seals.