Male aggression and the population dynamics of the vole, Microtus townsendii

Abstract

Male behaviour was manipulated by drug and hormone implants in field populations of Microtus townsendii on Westham Island near Vancouver, B.C. Small (≤45 g) males in one population were made precociously aggressive by subcutaneous Silastic capsules of testosterone. Large (≥46 g) males in another population were made less aggressive with implants of scopolamine HBr. Experiments were conducted over a 9-week period that coincided with a spring decline in population size and repeated for a 6-week period during the summer breeding season.A population with dummy implants indicated that neither the process of implantation nor the initial dose of either chemical affected survival of implanted voles. The rate of spring decline in male numbers was significantly lower in a scopolamine (passive large male) population than in control populations. Female breeding started earlier and more females bred in the scopolamine population than in the control populations during the first experimental period; however, this did not lead to significant differences in recruitment. Thus, although there were detectable demographic differences between scopolamine and control populations during the first experiment, they were not as pronounced as had been predicted. We conclude that male spacing behaviour alone is not sufficient to control the demography of Microtus townsendii populations.