0023-6152

AA00697606

23770

Unrefereed

0023-6152

AA00697606

The correspondence between the external characters commonly used for determining fecundity of the Japanese field vole, Mirrotus wwntebelli, in the field and the presence or absence of testicular and caudal epididymal sperm or histological observations of the female reproductive organ was established. The male breeding condition was grouped into the three types (M-I, M-II and M-III) by the relationship between the testis size and the presence or absence of sperm in both the testis and the caudal epididymis; since males with a visible scrotal bulge had testes of 10mm or more in length and always belonged to the M-III, such a scrotum is regarded as a single criterion for the reproductive activity of the live males in the field. The female breeding condition also was sorted into the three types (F-I, F-II and F-III) by the uterine situation and whether placental scars and corpora lutea are visible to the naked eye; the teats which are sucked by young, perforated vulva and pregnancy detected by palpation (only for the individuals having embryos more than 25 somites) are utilized as useful criteria to determine the reproductive activity of the live females. Basing on the above criteria, the breeding season at Mt. Hitome was fundamentally spring and fall.