Mating systems and sexual dimorphism in mass in microtines

Sexual dimorphism in body size is expected to vary as a function of mating system. Heske and Ostfeld (1990, J. Mammal. 71:510-519) found no difference in body-length dimorphism between the majority of promiscuous and monogamous microtines. In five species we used body mass and in three species, length of body to test the robustness of these findings. We found no significant dimorphism in length of body in any species, but significant dimorphism in body mass in all species, independent of mating system. We propose that 1) mass rather than length is the measure needed to assess dimorphism in microtines, and 2) the bias in favor of large males should rank as follows with respect to mating system: facultative monogamy = polygyny> promiscuity> obligate monogamy