Aggression, testosterone, and the spring decline in populations of the vole Microtus townsendii

Abstract

Many vole populations decline sharply in size at the start of the spring breeding season. We attempted to test the hypothesis that aggressiveness controls the magnitude of the spring decline in Microtus townsendii by implanting male voles with testosterone pellets in January 1975. The spring decline was very slight on the control grid and nearly absent on the experimental area, the opposite effect to what we predicted. The amount of skin wounding increased at the same rate on both experimental and control areas as voles entered breeding condition. Aggression tests in a neutral arena showed no differences in aggressive behavior scores for testosterone-implanted males compared with control males. In a more carefully controlled laboratory test we were unable to alter aggressive behavior by either of two levels of testosterone injections. We concluded that aggressiveness in Microtus townsendii could not be altered by testosterone treatments on intact males. Our field test of the aggression hypothesis thus failed.