Seasonal variation in population characteristics and management implications for brown rats (Rattus norvegicus) within their native range in Harbin, China

The brown rat (Rattus norvegicus) originated in north-eastern China, Siberia and Japan and subsequently spread worldwide. However, despite its importance to agriculture, public health and scientific and medical research, surprisingly few studies have focused on wild brown rat populations. There are four subspecies in China, but little is known about their original distributions. In the present study, we investigated the seasonal biological and ecological characteristics of brown rats in their native range in Harbin, north-eastern China. Trapping campaigns were conducted in June and November 2006 at a farm site and a rice site, and seasonal variation was analysed. The sex ratio was male biased at the farm site and female biased at the rice site in both seasons. Although juvenile, sub-adult and young-adult rats comprised over 80% of the population in both seasons, the age composition displayed seasonal differences, with higher proportions of juvenile rats in the summer and sub-adults in the winter. There were no significant morphological differences between different sexes or seasons, or between sites. Heavy, female and pregnant rats were captured first and heavier male rats maintained relatively higher reproductive activities than lighter ones, reflecting the link between social dominance and feeding priority. Rats had heavier reproductive organs in summer than in winter. The relative masses of the spleen and adrenal glands also showed seasonal and gender differences. This study demonstrates that brown rats in their native region have similar seasonal biological and ecological characteristics to American and European populations. This information on brown rat in north-east China will contribute to the development of management strategies for controlling this agricultural pest.