Male and female Brandt's voles have higher reproductive success when they have more mating partners regardless of population density

Although population density and body mass are known to alter the mating strategies of individuals, their impacts on reproductive success have not been fully investigated. We examined genetic mating system variation (estimated number of mating partners), reproductive success (offspring number), and sperm competition intensity (sires per litter) in semi-natural populations of Brandt's voles (Lasiopodomys brandtii) under low-, medium- and high-densities. We used 12 microsatellite genetic markers and parentage assignment of 3516 voles, with the aim of identifying the estimated number of mating partners and offspring produced by founder individuals. We provided strong evidence that individuals with more mating partners experienced increased reproductive success in all density groups, as measured by the production of more offspring, compared with individuals that had fewer mating partners. Further, we found that large males had more mating partners (especially in high-population density enclosures) and also produced more offspring relative to smaller males. In high-density (HD) enclosures, the average reproductive success per female vole was lower (a negative density-dependent effect), but voles had more mating partners, more sires per litter, but lower paternity skew compared to low-density enclosures. Our results suggests that Brandt's voles increased reproductive success through increasing mating partners under HD stress which may have significant implications in offsetting the negative density-dependency effect on population growth. We provide strong evidence that both males and females with more mating partners experienced increased reproductive success, as measured by the production of more offspring, compared with individuals that had fewer mating partners. Further, we found that large males had more mating partners (especially in high-population density enclosures) and also produced more offspring relative to smaller males. A high density of founder individuals led to more sires per litter, but lower paternity skew across litters. Our results suggests that a polygamous mating system and increased body size should be favored by natural selection, and the increasing number of mating partner with more offspring in high-density would be an important adaption in offsetting the negative density-dependency of reproductive performance in the Brandt's voles.