The evolution of XY females in mammals

Some lemmings have a large proportion of XY females in the population, as well as XX females and XY males. In this paper we first review the biological literature on XY females in mammals, with particular emphasis on the genetics and cytogenetics of this trait. We then consider population genetics models of the behaviour of this trait, and we show that there are serious discrepancies between predictions and observations regarding the sex ratio and the frequency of XY females; in several lemming populations the reported sex ratio is lower and XY females are more common than expected. Finally we consider evolutionary models to understand why XY females persist in these populations and how selection might act on other parts of the reproductive biology to modify the system.