We used the fat dormouse (Glis glis), a species from a monotypic genus of family Gliridae, as a model to promote the understanding of patterns of cryptic diversification along the ancient Hyrcanian Forests, one of the old growth relicts of the temperate deciduous forests worldwide. Mitochondrial Cytb data was used to investigate the phylogenetic status of two geographically-different populations of G. glis along the Hyrcanian Forests among all the worldwide known lineages of the species. Regarding phylogenetically informed partitioning of occurrence data, we then used two analytically different ENMs (i.e. environmental-space and geographic-space) to address whether niche divergence conforms G. glis diversification over the study area. Phylogenetic reconstruction showed significant heterogeneity between other fat dormouse lineages and those belonging to the Hyrcanian Forests as well as within the two hypothesized cryptic groups in the study area. Quantifying niche differences using the two ENM frameworks additionally confirmed divergence between the two cryptic lineages by indicating niche conservatism. The integration of phylogeny and ENM in this study confirms the development of distinct cryptic species and suggests that the Hyrcanian Forests, a well-known Pleistocene refugium, might contain multiple cryptic refugia for small forest-dwelling species during paleontological oscillations.
