Breeding season in urban population of Norway rats in Yokohama estimated from age composition

Abstract

We calculated recruitment in a Norway rat (Rattus norvegicus) population in a 21-hectare business district in Yokohama from compositions of ages estimated from eye lens weight. We set live or snap traps outside buildings in February or March for three years from 2014 to 2016. We made age compositions from 27, 43 and 27 rats of 12 months old or less, collected in 2014, 2015 and 2016, respectively. Peaks in recruitment occurred in the hot season from August to October 2014, in the cold season in January 2015, and in the spring and autumn in 2016. These data show that factors other than air temperature affected the breeding activities of the rats.