Does Access to Females or Competition among Males Limit Male Home-Range Size in a Promiscuous Rodent?

To maximize fitness, female mammals attempt to maximize offspring survival, whereas males attempt to mate with as many females as possible, which results in differential use of space. The relative influence of male competition versus access to females on space use by males has not been addressed theoretically or empirically. We conducted an experiment in which we manipulated total density, density of females, and density of males to determine relative influence of density of each sex on space use and overlap by male gray-tailed voles (Microtus canicaudus). Home-range size was correlated inversely with total density and was influenced separately by each sex. Home-range sizes of males were significantly smaller in high male-low female populations than in low female-high male populations. Males overlapped 4-5 females and 4-5 other males in populations with low densities of both sexes and high densities of both sexes. When sex ratios were skewed toward females, males still overlapped 4-5 females but only one other male. When sex ratios were skewed toward males, males overlapped only two females while overlapping three other males. Home-range size of a male does not appear to expand beyond an overlap with about five members of either sex. Thus, intrasexual competition with five males or overlap with five females appear to set upper limits to home-range size of male gray-tailed voles. We conclude that space use by males is influenced by intrasexual competition and access to females with an upper limit of overlap with either sex.