Craniometrics are not outdated: Interspecific morphological divergence in cryptic arvicoline rodents from Iran

We studied interspecific variability in external, cranial, and dental traits in seven species belonging to two closely related arvicoline (Arvicolinae) subgenera, the social voles (Sumeriomys) and the grey voles (Microtus). These voles were for long regarded as morphologically cryptic and the species complexity was fully appreciated only after chromosomal and molecular markers were utilized. Specimens we examined displayed clear external differences between the subgenera. Social voles had five plantar pads, relatively shorter tail (22-27% of head and body length), lighter dorsal pelage, whitish grey belly, and monochromatic or indistinctly bicolored tail. Grey voles showed six plantar pads, relatively longer tail (33-44%), dark brown back, grey belly, and sharply bicolored tail. Our results retrieved considerable heterogeneity in cranial and dental morphology. Major cranial differences between the subgenera associated with the interorbital region, the braincase and the tympanic bullae. Genetic (Kimura 2-parameter) distances, which presumably provided a priori correct estimate of the true phylogenetic divergences, explained 42% of morphometric distances between species. Below the level of a subgenus the phylogenetic signal was conserved in grey voles but it dissolved in social voles. The molecular and morphological rates of evolution were obviously decoupled in the latter possibly by selective pressures for a particular phenotype. Climatic variables however explained only 11% of interspecific heterogeneity in cranial shape. The most distinct in terms of morphology and the climatic properties of its habitat was Microtus irani which also occupies the very edge of Microtus distribution. High species richness of social and grey voles combined with heterogeneity of environmental conditions makes the Middle East an ideal region for studying diversity of developmental trajectories in voles at the levels of species and populations.