Sexual selection and mating patterns in a mammal with female-biased sexual size dimorphism

In mammals, species with highly male-biased sexual size dimorphism tend to have high variance in male reproductive success. However, little information is available on patterns of sexual selection, variation in male and female reproductive success, and body size and mating success in species with female-biased size dimorphism. We used parentage data from microsatellite DNA loci to examine these issues in the yellow-pine chipmunk (Tamias amoenus), a small ground squirrel with female-biased sexual size dimorphism. Chipmunks were monitored over 3 years in the Kananaskis Valley, Alberta, Canada. We found evidence of high levels of multiple paternity within litters. Variation in male and female reproductive success was equal, and the opportunity for sexual selection was only marginally higher in males than females. Male and female reproductive success both depended on mating success. We found no evidence that the number of genetic mates a male had depended on body size. Our results are consistent with a promiscuous mating system in which males and female mate with multiple partners. Low variation in male reproductive success may be a general feature of mammalian species in which females are larger than males.