Evidence of the island rule and microevolution in white-footed mice (Peromyscus leucopus) in an urban harbor archipelago

The island rule generally states that larger species are dwarfed on islands while smaller species exhibit gigantism. Among the smaller species in which this pattern has been observed, rodents have been a focus of numerous studies. Through our long-term trapping on the Boston Harbor Islands, USA, we have revealed that the white-footed mice on Bumpkin and Peddocks Islands exhibit a significantly larger body size than their mainland counterparts. On Bumpkin Island, adult mice averaged 28.2 g (n = 187, SE ± 0.35) and on Peddocks Island adult animals averaged 31.2 g (n = 85, SE ± 0.42). Published average masses for this species range from 15 to 25 g for adults. Additionally, the mice on Bumpkin Island have shown an increase in mass over the course of our study and this increase was significant between 2011 and 2014 when no trapping occurred on that island. The large size suggests that these animals have been isolated on these islands for a sufficient amount of time for divergence to occur. Additionally, the changes in mass over time, in a population with annual turnover, suggests that microevolution in response to environmental factors may be taking place.