Density‐dependent changes of mating system and family structure in Brandt's voles (Lasiopodomys brandtii)

A mating system is an important life history for animals dealing with changing environments. Population density affects the plasticity of a mating system and subsequently the family structure of animals, but its impacts on mating systems and social structures are rarely investigated by using molecular markers in field conditions. In this study, using microsatellite genetic markers, we examined the changes in the social and genetic mating system and family structure of Brandt's voles in the grassland of Inner Mongolia, China, under low‐, medium‐, and high‐density enclosures (each enclosure 0.48‐ha with 4 replicates.) We found, with the increase in population density of the founder voles introduced into the enclosure in early spring, both sexes increased their number of genetic mating partners, while males increased their social partners, resulting in a more promiscuous mating system. The number of genetic fathers and mothers per family, the number of social offspring per founder male or female, and the proportion of extra‐group offspring increased with increased density, indicating that the response of the family structure to density change, which is consistent with the change in mating system observed in our study. Both male and female voles had multiple mates, but males had a larger number of social and genetic mates, the number of social and genetic offspring, and the number of social and genetic families. Our study highlights the significance of density‐dependent plasticity of a mating system and family structure in affecting the population change of small mammals. Our study indicates that with increased population density, Brandt’s voles showed a shift in the mating system to a higher male‐biased promiscuous system and more extra‐group mating behaviors. The plasticity of mating system may play a significant role in regulating population dynamics of Brandt’s voles through altering their family structure and then fitness under field condition.