Geographical variation in Eurasian red squirrel (Sciurus vulgaris L., 1758) mandibles and the issue of subspecies-level organization: a failure of history?

Sciurus vulgaris is a widespread, highly polytypic tree squirrel species, under which a large number of subspecies have been described. This study tests the robustness of the current subspecific classification by using geometric morphometrics to quantify morphological variation in mandible shape, along with canonical variates analysis to test hypotheses of morphological distinctiveness. Patterns of mandible allometry were examined in eight out of 16 currently recognized subspecies. The significance of the discriminate functions was tested statistically, and the iterative jackknife procedure was applied to evaluate stability of the subspecies-specific discriminant functions. Applying criteria that focus on shape diagnosability, rather than mean group differences, indicates that most regional subspecific groupings show intergradations and continuity in mandible shape and size and that allometric effects on mandible shape are negligible. Evidence of a distinct subspecies confined to the Iberian Peninsula (Sciurus vulgaris infuscatus) and a discrete group originating from an extinct, 19th century population in Dorset, UK were identified based on these mandibular data. All other regional subspecific groupings were not diagnosably different. These results suggest that most red squirrel subspecies might represent non-diagnosable morphological variants whose taxonomic validity seems doubtful. More generally, our results highlight the importance of applying objective, quantitative and reproducible criteria to the issue of subspecies delimitation.