Mandible morphology as a tool to investigate origin, adaptation and stress in invasive alien species: first insights into Callosciurus erythraeus (Rodentia: Sciuridae) in Europe

When an alien species is introduced in a new area, the new population may be genetically and phenotypically different from the parent population because of the population bottleneck, increased inbreeding rate and adaptation to the new environment. In this study we investigated the variation in shape and size of the mandible among and within three populations of the invasive Pallas's squirrel introduced in Italy, Belgium and France. Significant differences in both size and shape of the mandible were found among all population pairs, with France being the most distinct. French squirrels showed a larger and slenderer mandible with a broad angular process, a restricted condyle, and a backward-oriented coronoid process. The Italian and the Belgian population also differed significantly but to a lesser extent, the Italian squirrels having a lower coronoid process, a broader angular apophysis, and a restricted condyle. Size explained 15% of the total shape variation, but the slope of allometric trajectories did not reveal any significant difference among populations. A significantly high fluctuating and directional asymmetries were found respectively in the French and the Italian squirrels. Results are discussed in terms of different selective pressures in the invaded areas and possible effects of developmental instability.