A Colonization Strategy in Field Voles (Microtus Agrestis): Reproductive Traits and Body Size

Abstract

Populations of the field vole (Microtus agrestis) in the Stockholm Archipelago in the Baltic (southern Sweden) are highly dynamic, with frequent extinctions and recolonizations of islands. In order to investigate the demography of such a successful island colonizer, I studied reproductive traits in two breeding colonies of Swedish field voles, one mainland and one from the archipelago, which I kept in a laboratory for 3 yr. Insular voles, as compared to mainland voles, were characterized by a larger adult body size, more rapid growth, larger and heavier litters, and a larger reproductive effort relative to body size. The two colonies were similar in gestation time, litter mortality, and age at sexual maturation. A large body size in combination with a high reproductive investment can be predicted neither from large—scale interspecific allometric relationships, nor from r—K—selection in its wider sense. It is instead argued that both the large body size and the reproductive traits are coevolved components of an island colonization strategy.