Age , Reproduction , and Demography of the Spiny Rat ( Muridae : Niviventer coxingi ) in Subtropical Central Taiwan

Hon-Tsen Yu and Yao-Sung Lin (1999) Age, reproduction, and demography of the spiny rat (Muridae: Niviventer coxingi) in subtropical central Taiwan. Zoological Studies 38(2): 153-163. Annual reproductive cycles and demographic changes in a wild population of the spiny rat (Niviventer coxingi) in central Taiwan were studied. Reproductive status of the rat was investigated by autopsy and histological examination of the testes and ovaries. Age of each animal was categorized into one of 5 age classes by examining wear patterns of the upper molars. Using the 5 age classes as a priori groupings, a discriminant function analysis on 4 body and 23 cranial measurements was used to crosscheck the reliability of the age criteria by tooth wear. The rat shows sexual dimorphism in body weight and timing of sexual maturity. However, the dimorphism is age dependent: only males of age class III or older become heavier than females, and yet females gain sexual maturity at a younger age than males. Despite climatic seasonality in its natural habitats, the rat does not have a limited breeding season. Males remain potent after sexual maturity and large proportions of males in the population breed in every season. Females breed in all seasons. Nonetheless, fewer breeding females were present in the coldest months of the year. Demographic analysis across 15 mo indicates that young rats are recruited in all seasons, supporting the conclusion that the spiny rat can breed year round. Finally, we compare features of life histories and ecological characteristics between N. coxingi and Apodemus semotus.