Age-structure and reproduction investigation on Niviventer confucianus living on islands at Qiandao Lake in autumn

Niviventer confucianus,the main pest rat at Qiandao Lake,were caught in their preferred woodland habitat on islands in the lake.We captured 123 individuals from September to November,2007,and brought them back to our laboratory where they were marked,measured for morphological characteristics,and then necropsied to determine sex,age and reproductive condition.Specimens were assigned to four age classes on the basis of body length: juvenile group(body length≤110 mm),sub-adult group(body length from 111-125 mm),adult group(body length from 126-150 mm) and elder-adult group(body length≥151 mm).Males and females demonstrated no difference in body size and could be classified with the same standard.Most of the rats bred in autumn with a reproductive peak from September to October.Most reproduction occurred in the adult and elder-adult groups.Reproductive condition was significantly influenced by vegetation coverage and human disturbance.The index of reproduction and the average number of embryos were highest in the medium-sized islands(3.45,5.17±0.0.42) and lowest in the small islands(0.75,4.00±1.53),with values from the large islands in between(1.96,4.9±0.35).By contrast,the rates of scrotal testes decreased as the size of island increased,with 68.4% in the small islands,64.7% in the medium islands and 50.0% in the large islands.Although age structure declined in the medium island populations,adults were the main reproductive group,and the population size increased after the reproductive season.The sex ratio(M/F) of adults was maladjusted in the small islands.Therefore,the population was stable in the large islands,higher in the medium islands,and fluctuating in the small islands.