Another genetically promiscuous ‘polygynous’ mammal: mating system variation in Neotoma fuscipes

Polygyny is widely thought to be the dominant mating system in mammals. However, more recent genetic work casts doubt on this view. Variation in mating systems has been found in both males and females within and across mammalian species. The causes and consequences of mating system variation have important implications for understanding the population and evolutionary dynamics of species. To better understand mating system variation, both in mammals and more generally, this study analyses genetic mating system variation in dusky-footed woodrats, Neotoma fuscipes. Contrary to expectation, there was little support for polygyny at the genetic level. Instead, the study populations were characterized by promiscuity and monogamy, in both males and females. At higher densities, variance in the numbers of mates and offspring were higher in breeding males than in females, as is often observed. However, this trend was reversed in low-density, coniferous forest habitat. Model selection revealed that the best model of successfully mated pairs includes population density, operational sex ratio and individual pairwise distances as predictors. Higher densities coupled with male-biased sex ratios appear to decrease the probability of mating and decrease opportunities for polygamy, particularly in females. Although woodrats display sexual size dimorphism, male body size had no detectable effect on mating success. This study questions the prevalence of polygyny in mammals and demonstrates the need for more detailed, genetic investigations of mating systems. Future studies are needed to explore the complex interactions among mating system determinants and test hypotheses of sex-specific mating system variation.