Can radio collars affect dominance relationships in Microtus?

We measured the effects of radio-collar mass (3.9–17.3% of live body mass) on dominance relationships between adult female meadow voles (Microtus pennsylvanicus). Fifty individuals of known dominance status were fitted with dummy transmitters and their status was measured 2 and 14 days later. There was no significant change in dominance when collar mass was < 10% of live body mass. However, we registered a significant loss of dominance after voles received collars of > 10% of live body mass. Body mass and activity levels of voles decreased after collar attachment, but these reductions were not correlated with collar mass. Control voles did not experience such decreases. The radiotelemetry technique as it is generally used in microtine research is not put in doubt by our results, but we demonstrate social costs associated with the use of heavier transmitters.