Abstract
Reproductive activity of the small Japanese field mouse (Apodemus argenteus) was studied in woodlands at 1,000-1,900 m altitude in the Oze district in northeastern Honshu, Japan. The present surveys were carried out from mid July to early August of each year during 1985 to 1999 except for 1986 and 1991. The specimens investigated were 266 individuals in males and 265 individuals in females. Males and females with body weights of 14.1 g or more were regarded as adults. I estimated that 80.2% (166/207 individuals) of adult males and 60.9% (103/169 individuals) of adult females were reproductively activity. Therefore, the period of the present survey was within the breeding season in both sexes. Moreover, the breeding season of A. argenteus in this survey area seemed to be at least May through August judging from the estimated birth time of the juvenile. Some females became pregnant twice or more during each breeding season. Mean embryo size was 3.57 in all pregnant females and no significant difference was recognized for embryo size per pregnant females among body weight classes.
