The annual reproductive pattern of Apodemus semotus was studied. A. semotus specimens were trapped at monthly intervals from October 1984 through March 1986 at about 2 300 m elevation in the Hohuanshan area of central Taiwan (24? latitude). Reproductive parameters of 267 mice were measured. Seasonal changes in weights of male gonads showed peaks in April-May and September-October, whereas gonadal weights were sharply reduced and there was histological evidence of regression in June-July and December-February. Ovarian monthly mean weights also showed 2 peaks in April and September. Pregnant or lactating females were found in April-May and August-December. Breeding was more frequent in autumn than in spring. The mean litter size of 23 pregnant mice was 3.83 ? 0.88 (range 2-6). The difference between litter size and ovulation rate (4.24 ? 0.22) indicates that prenatal mortality was 12.8%, including 7.5% pre-implantation mortality and 5.3% post-implantation mortality. No more than 2 sets of placental scars were found in any individual. Analyses of age structure and the time of appearance of juveniles give further evidence of a bimodal annual reproductive pattern for A. semotus.
