Two kinds of lemming have evolved genetic modifications of sex determination that result in surpluses of daughters. Female‐biased sex ratios can evolve when mating occurs between neighbouring individuals who are more related than if mating occurred randomly. Two proposed sources of such ‘viscous’ gene flow in lemmings arc cyclical changes in population density and mosaic habitat. Alternatively, perhaps cold climate favours winter aggregation and inhibits the dispersal of winter‐born offspring, which would Nature and mate with close relatives; dispersal and outbreeding would occur during the warm months. Thus the episodes of dispersal and inbreeding would be seasonal rather than density‐dependent and the supposition of discontinuous habitat is