Sulphur bellied rats (Niviventer confucianus) were collected from Taoyuan County in 1998 to 2001, from YueYan County in 1992 to 1994, and in 2003 to 2004. The animals were caught primarily in their preferred woodland habitat. A total of 109 specimens were captured, and 90 animals were autopsied to determine reproductive condition. Position, size and weight of testes were noted in males. For females, the reproductive tract was examined for tubal condition of the uterus, number and size of embryos, and placental scars. Specimens were assigned to four age classes based on body mass: juvenile (body weight≤35.0g), sub-adult (body weight from 35.1～50.0g), adult (body weight from 50.1～80.0g), and old-adult (body weight>80.0g). Males with testes had dropped into the scrota, and females for which the uterus was tubal were considered sexually mature. Females with embryos or visible uterine scars were considered as reproductive animals. The total sex ratio (M/F) is approximately 1:1, but it varies in different seasons and with different age class. The proportion of females is higher than that of males except during spring and winter, and is higher in juvenile (66.7%) and but lower in old-adults (33.3%).
The total percentage of pregnancy in captured females was 31.1%, which also varied with season and age classes. Rates of pregnancy in spring, summer and autumn were 38.1%, 37.5% and 30.0%, respectively, with no breeding in winter. Juveniles were immature with no reproductive activity, but the sub-adult group (20.0%) had begun to reproduce. Proportion of pregnanacy was highest for the adult group (40.0%), and dropped (33.3%) in the old-adult group. The average number of embryos in a litter was 3.7±1.3 (Mean±SD), and ranged from 2 to 8. Litter size varied with changes in the seasons, being lowest in spring (3.0±0.8), highest in autumn (5.0±1.0), and intermediate in summer (4.3±1.5). The litter size of the animals increased with age class (r=0.680, d_f=13, p=0.015), and also showed a significant positive correlation with body weight (r=0.631, d_f=13, p=0.021). The old-adult group had the highest litter size (6.0), which might be a common phenomenon in rodents generally.
The index of reproduction for all females was either 0.58 (as total litter size/all animals) or 1.15 (as total litter size/females). Among seasons, the reproductive indices in summer (0.86 or 1.61, respectively) and autumn (0.88 or 1.50, respectively) were higher than those in spring (0.53 or 1.14, respectively) and winter (with no pregnant females). The reproductive index of the adult group was the highest (0.74), followed by the old group (0.67) and sub-adult (0.26). The main group to reproduce was the adult class.
For males, the rate of testes in scrota for all males was 75.6%. Almost all captured males had scrotal testes in summer and autumn, whereas the rates of scrotal testes in spring (58.3%) and winter (85.7%) were lower. Among age classes, the rates of scrotal testes increased gradually from 0.0% in juveniles to 100.0% of old-adults. The size and weight of testes of mature males changed significantly among seasons (weight: ANOVA, F3,33=11.458, p<0.001; long diameter of testes: F_3,33=3.479, p<0.05; short diameter of testis: F_3,33=4.145, p<0.05). Size and weig ht of testes in winter were always lowest. From juveniles to old-adults, the weight, long diameter, and short diameter of testes increased gradually. There were statistically significant differences among age classes (weight: ANOVA, F3,44=13.291, p<0.001; long diameter of testis: F_3,44=19.458, p<0.001; short diameter of testis: F_3,44=16.965,p<0.001).
Reproductive traits of males were similar to those of females by season and age classes, so it can be concluded that the rat breeds during the spring to autumn, and stops breeding in winter. The most favorable breeding season of the animals is summer and autumn, and the adult group is the main reproductive class. The reproductive potential peaks in summer and autumn, when the index of reproduction, percentage of pregnancy, and percentage reproductive females reach a crest, and when the rate of scrotal testes in males is 100%.