Skull modularity of the European ground squirrel Spermophilus citellus (Linnaeus, 1766)

Abstract

The skull is a complex structure that has frequently been studied for the patterns of morphological integration and modularity. The ventral side of the skull can be divided into two functional modules, the neurocranium composed of the braincase, eyes and ears, and the viscerocranium composed of the jaw apparatus. The aim of this study was to test the ventral cranium of the European ground squirrel Spermophilus citellus(Linnaeus, 1766) for this partitioning as sciurid skull is believed to be highly integrated without clear divisions into subunits. Additionally, I compared the degree of modularity between juveniles and adults. Hypothesized modularity was tested on 159 (43 juveniles and 116 adults)skulls by applying geometric morphometric method based on Escoufier RV coefficient. In adults, the results yielded strong support to the hypothesis that the viscerocranium and neurocranium are separate modules. In juveniles, two-module organization of the skull was also confirmed, but the hypothesized modules were much more integrated with each other. Although allometry can be a strong integrating factor, it had very little influence on the hypothesized modularity of the S. citellus skull. A permutation test for the difference in the degree of modularity between juveniles and adults was marginally significant. The change in the strength of integration between the viscerocranium and neurocranium in the S. citellus skull during ontogeny, with the higher level of modularity in adults than in juveniles, is probably a consequence of the transition from suckling to gnawing of food and greater specialisation of the two functional modules.