What limits male range sizes at different population densities? Evidence from three populations of water voles

In many mammalian species, females compete with each other for food and space to raise offspring, while males compete with each other for access to females. Few studies have examined the factors which limit male range sizes or the degree of overlap between male ranges. We deduced four possible responses of the range sizes of non-territorial male small mammals to increasing population density and/ or levels of forage abundance. These were: (1) male range sizes might remain the same irrespective of population density; (2) at high population densities males may become territorial, and their ranges small and non-overlapping; (3) at high forage abundance/population densities, males' ranges may become smaller but remain intra-sexually overlapping; (4) at high forage abundance, male range sizes may increase. We examined the relationship between population density, range lengths and range overlaps and body weights of both sexes in a comparative study of three populations of water voles Arvicola terrestris. Male range sizes were smaller at higher population densities, but their ranges remained both inter-and intrasexually overlapping. Heavier males had larger ranges than did lighter males at all sites. These results comply with what would be expected if male range sizes were at least partially restricted by the number of ranges of other individuals with which they overlapped. Although we could not discount the hypothesis that forage abundance may also have had a direct effect on male range sizes, our results implied that male range sizes were at least partially determined by social factors.