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.

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Evolutionary theory predicts that in metapopulations subject to rapid extinction-recolonization dynamics, natural selection should favour evolution of traits that enhance dispersal and recolonization ability. Metapopulations of field voles (Microtus agrestis) on islands in the Stockholm archipelago, Sweden, are characterized by frequent local extinction and recolonization of subpopulations. Here, we show that voles on the islands were larger and had longer feet than expected for their body size, compared with voles from the mainland; that body size and size-specific foot length increased with increasing geographical isolation and distance from mainland; and that the differences in body size and size-specific foot length were genetically based. These findings provide rare evidence for relatively recent (less than 1000 years) and rapid (corresponding to 100-250 darwins) evolution of traits facilitating dispersal and recolonization in island metapopulations. 1. INTRODUCTION Studies of island populations have played an important role in generating and testing hypotheses central to the ecology and evolution of biological diversity, both historically [1,2] and currently [3-5]. Evolutionary theory predicts that in metapopulations subject to rapid extinction -recolonization dynamics, natural selection should favour evolution of traits enhancing dispersal and recoloni-zation ability [6-8]. However, empirical evidence is relatively scarce. Populations of field voles, Microtus agrestis, on groups of islands in the Stockholm archipelago provide a good model system to test this prediction. These islands emerged from the sea approximately 500-1000 years ago [9] as a result of land elevation following the termination of the latest glaciations, and were colonized by field voles from the mainland [10]. These insular populations are characterized by drastic fluctuations in numbers of individuals and high turnover rates; extinctions and recolonizations from neighbouring islands are frequent [11-13]. There is evidence to suggest increased rates of emigration/dispersal before subpopulation extinctions [11,13], as well as evidence for life-history adaptations in insular environments. Laboratory breeding over several generations show that insular voles produce larger (5.27 versus 4.13) and heavier (17.06 versus 11.02 g) litters; bear larger young, which grow faster; and make a larger reproductive effort compared with mainland voles [10]. However, there is as yet no evidence that selection has favoured the evolution of traits that enhance dispersal and recolonization ability in these insular vole populations. Morphological traits linked to locomotion and

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