1999 年 2 巻 1 号 p. 11-19
The reproductive tracts of 29 female Steller sea lions Eumetopias jubatus collected in the Nemuro Strait, Hokkaido, Japan, during the winters of 1995 and 1996, were examined to estimate their reproductive histories and status. After sectioning ovaries at intervale of 250μm thickness, a light microscope was used to examine 23 of the 57 ovaries for small retrograde corpora lutea (RCLs). None of the RCLs that were more than three years old were found. The regression process of an RCL is presumed to occur rapidly because of the pressure exerted by a large corpus luteum of pregnancy (PCL) formed during the subsequent pregnancy. No more than three placental scars were found in any one female, probably because the scars disappeared during placentation during subsequent pregnancies. Thus, estimations of the reproductive histories of Steller sea lions were not possible beyond three years. Females became sexually mature from four to seven years of age, which was one year later than in previous studies (Perlov 1971). Among those sampled during January and February, the pregnancy rate for females that had ovulated was 88.5% (23/26).