SEXUAL DIMORPHISM IN SIZE, RELATIVE SIZE OF TESTES, AND MATING SYSTEMS IN NORTH-AMERICAN VOLES

Sexual body-length dimorphism and relative size of testes were compared among 16 species (25 subspecies) of North American Microtus and Clethrionomys. Species with polygynous mating systems and territorial males had the greatest degree of sexual size dimorphism with males larger than females, and relatively small testes. Promiscuous species tended to show little or no sexual size dimorphism and relatively large testes. Species considered monogamous had no size dimorphism, and also had large testes. Data support the hypothesis that sexual dimorphism in size is correlated with the intensity of intrasexual competition for mates, but the hypothesis that species with single-male breeding systems (single-male polygyny and monogamy) have relatively smaller testes than species with mating systems characterized by a high potential for sperm competition (promiscuity) was supported only for polygynous species. -Authors