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Patterns of cranial size variation in two sibling species Plecotus auritus and P. austriacus (Chiroptera: Vespertilionidae) in a contact zone

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Correspondence

Tomasz Postawa, Department of Vertebrate Zoology, Institute of Systematics and Evolution of Animals PAS, 31-016 Kraków, Poland. Tel: +48 12 422 19 01; Fax: +48 12 422 42 94

Email: tpostawa@gmail.com

Abstract

The coexistence in one area of two species with similar ecological requirements can lead to their morphological convergence or divergence. Convergence may be the result of adaptation to new conditions (species share a niche), whereas divergence may be the effect of competition for a resource (species compete for a niche). Compatibility with Bergmann's rule is possible in species with a significant latitudinal range. We tested whether potential differences between two long-eared bat species are consistent with character displacement or Bergmann's rule by investigating variability in cranial morphology of Plecotus auritus and P. austriacus, which commonly occur in Central and Eastern Europe. We used 111 complete specimens from the allopatric range of P. auritus (nine localities) and sympatric P. auritus and P. austriacus (44 localities) from Poland and Ukraine. A traditional morphometric method was used to evaluate variation in cranial size between the species in their ranges. Discriminant function analysis of cranial dimensions showed larger differences between sympatric populations of P. austriacus and P. auritus than between allopatric P. auritus and a sympatric population of P. austriacus. A subsequent analysis showed that most cranial variables (excluding elements of the skull responsible for prey capture and elements partly associated with echolocation) from the sympatric population of P. auritus are smaller than those homologues from allopatric populations. Larger individuals from the allopatric population originate from the northern part of the study area; however, we did not detect an association of cranial variability with latitude pattern. The variation in size of the cranium between individuals from allopatric and sympatric ranges of P. auritus can be explained by different preferences in each range for prey that vary in hardness. P. auritus consumed significantly more hard-bodied insects in allopatry than in sympatry. This example is one of few confirmed morphological cases of competitive character displacement.

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