Giant mice on small islands: biogeographic and ecological differences contribute to gigantism in island populations

Island populations of small land vertebrates frequently exhibit insular gigantism, presenting with larger body sizes in comparison to mainland counterparts. While insular gigantism has been observed globally, the effects of biogeographic and ecological factors on body size in island systems are not well understood. Here, we examine the biogeographic and ecological associations of insular gigantism. Deer mice (Peromyscus maniculatus) were live trapped and body mass measured on six of the Gulf Islands and the nearby mainland of British Columbia, Canada. Biogeographic measures of land area and island distance from the mainland and the ecological measure of predator species richness were used in piecewise structural equation modelling to identify associations with insular gigantism. We found evidence of insular gigantism in the Gulf Islands system, with island mice having a larger mean body mass than mainland populations. Land area was positively associated with predator species richness, and predator species richness had a strong negative effect on deer mouse body mass, resulting in the observed pattern of insular gigantism. The concurrent analysis of biogeographic and ecological factors contributes to a better understanding of the evolution of insular gigantism in small vertebrates and its juxtaposition to the phenomenon of insular dwarfism of large vertebrates.