Skull morphology of two cryptic bat species: Pipistrellus pipistrellus and P. pygmaeus - a 3D geometric morphometrics approach with landmark reconstruction

Differences in skull morphology between two cryptic species of bat, Pipistrellus pipistrellus (n = 14) and P pygmaeus (n = 15), originating from Great Britain, were investigated. Four different data sets were analysed: (1) 23 landmarks and (2) 26 landmarks on the dorsal and ventral sides of the cranium, respectively, (3) 49 landmarks on the upper jaw, and (4) 34 landmarks on the labial side of the mandible. For almost all data sets, when compared within sex groups, P pipistrellus were significantly larger than P pygmaeus; the biggest difference being observed in the mandible size. Interspecific differences in shape, analysed by Principal Component Analysis and Discriminant Function Analysis (DFA) of the Procrustes superimposed landmarks, were also mostly visible in the mandible, and were related to dietary differences between the species. For example, the longer and more upright canines of P pipistrellus allow them to pierce harder prey, the bigger molars case its processing, and the shortened body of the mandible and the more developed coronoid process presumably generate a stronger bite. The shape and size of the mandible proved to be a good characteristic for distinguishing both cryptic taxa. A procedure for estimating missing landmarks for 3D geometric morphometric purposes was created. Our procedure of finding the missing landmarks had no effect on the within-group loss of variation. DFA of data sets with reconstructed versus orginal (but reduced) landmarks yielded similar results (three versus two misclassified specimens in leave-one-out cross-validation).