Geometric morphometric analysis of the plateau zokor (Eospalax baileyi) revealed significant effects of environmental factors on skull variations.

The plateau zokor (Eospalax baileyi) is employed as an ideal model for examining the relationships between phenotypic and ecological adaptations to the underground conditions in which the skull morphology evolves to adapt to tunnel environment. We evaluated the influence of environmental factors (altitude, temperature, and precipitation) and geographical distance on the variations in skull morphology of a native subterranean rodent plateau zokor population. Thin-plate spline showed that the trend of morphological changes along the CV1 axis was as follows: the two zygomatic arch and the two postorbital processes moved down, the two mastoid processes and the tooth row moved upward, and the tympanic bulla grew longer. The changes along the CV2 axis were as follows: the nasal bone and the tooth row became longer, the distance between the two anterior tips of zygomatic arch lengthened, the infraorbital foramen became smaller, the whole posterior part of the skull became shorter, the zygomatic bone and the two posterior tips of zygomatic arch moved down, and the foramen magnum became bigger. Thus we found significant differences in the skull shape among the seven populations studied. Along with the reduction in the altitude and increase in the mean annual temperature and mean annual precipitation, the nasal bone became shorter, the distance between the two anterior tips of the zygomatic arch became shorter, the whole posterior part of the skull lengthened, the infraorbital foramen became smaller, the two mastoid processes moved upward, and the occipital bone moved down on the dorsal surface of the skull. On the ventral surface of the skull, with an increase in the altitude, mean annual temperature, and mean annual precipitation, the tympanic bulla became shorter, the tooth row moved down, and the foramen magnum became smaller. The morphological changes in the skull were significantly positively correlated with environmental factors. Finally, there was a significant positive correlation between the Procrustes distance matrix of the skull and the geographic distance matrix, which indicates that the evolution of the plateau zokor follows the distance isolation model, but it needs to be further explored from genetic perspectives.