Sexual dimorphism often evolves as a response to sexual selection on musculoskeletal traits that improve a male’s ability to compete for access to mates. In addition body mass, adaptations to specific anatomical components that increase strength, stability, and agility may also improve fighting performance. We tested the hypotheses that male voles, as compared to females, are more specialized for fighting performance in their skeletal anatomy and that the degree of sexual dimorphism increases with the intensity of male-male competition. In three species of voles with different mating systems, we found partial support for these hypotheses. In the postcranial anatomy, we found male-biased sexual dimorphism in several skeletal shape indices associated with improved fighting performance. Consistent with predictions, this dimorphism was greatest in the polygynous Microtus californicus, absent in the monogamous Microtus ochrogaster, and intermediate in the promiscuous or socially flexible Microtus oeconomus. In the skull, however, we found results opposite to our predictions. Relative to overall skeletal size, females had larger skulls than males. Larger heads in females may be associated with selection for increased food processing efficiency, which may be highly important because of the compounding effects of increased food intake requirements during gestation and lactation, and the generally low-quality diet of voles. Larger heads in females may also be associated with selection for improved digging performance (voles use their incisors to dig) or for defending offspring. These results suggest disparate selective pressures on the skulls and postcranial skeletons and of female and male voles.