Litter sex ratio is a key component of parental fitness due to its impact on lifetime reproductive success. Multiple causes may lay at the origin of sex ratio variation among species and populations, such as maternal condition, local resource competition, presence of helpers, habitat quality or inbreeding levels. Whereas variation in sex allocation between species is relatively well understood, it is still unclear how and why litter sex allocation differs within species. Here, we present an analysis of litter sex ratio variation in two populations of edible dormice (Glis glis) over nine years of study. Populations are situated in the Montnegre and Montseny massifs in Catalonia (NE Iberian Peninsula). The Montnegre population is nowadays an isolated population located at the southernmost range edge of the species in the Iberian Peninsula. Litter sex ratio was male-biased in Montnegre but balanced in Montseny, whereas both populations showed a balanced adult sex ratio. We suggest that this differential sex allocation investment in Montnegre, may be a strategy to overcome isolation effects in this massif, as males are the dispersing sex in this and other rodent species.