Abstract
Sexual body-length dimorphism and relative size of testes were compared among 16 species (25 subspecies) of North American Microtus and Clethrionomys by use of data collected from museum specimens. 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. These data support the hypothesis that sexual dimorphism in size is correlated with the intensity of intrasexual competition for mates. However, 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.
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