Abstract
Reproductive activity of the Japanese long-tailed field mouse (Apodemus argenteus) was studied in northern and southern parts of woodland at 1,000-2,018m in alt. on Mt. Kumotori, Chichibu Mountains. Trappings were done at Sites I (1,000-1,400m in alt.), II (1,400-1,750m in alt.) and III (1,750-2,018-1,700m in alt.). Males with the length of the major axis of testis and seminal vesicle at 8.5mm and more had sperms in the caudal epididymis. The majority of these males weighed over 14.0g, hence males over this weight class were regarded as adults. Females who had perforated vaginas, were pregnant, and/or lactating, and weighed over 12.0g were regarded as adults. Reproduction took place between spring and autumn at each site. In regions of low altitude, sitesI and II, however, reproductive activity tended to decrease in summer. The mean embryo count was 4.00 through the year. No significant difference was recognized in the mean embryo count among the sites. From Hokkaido to Kyushu, the regional variation of the embryos per dam was small. Some females delivered twice during each breeding season.
