Shape variation of mandible and head in Lucanus cervus (Coleoptera: Lucanidae): a comparison of morphometric approaches

The exaggerated male traits under sexual selection are widespread throughout the animal kingdom, including ornaments to attract mates and weapons to deter rivals. Many species of stag beetles show a considerable interspecific and intraspecific variation in mandible size and shape. In the present study we used traditional and geometric morphometric approaches, on live Lucanus cervus males, to study the intraspecific variation in mandible and head size and shape and to get evolutionary insights about the exaggerated weaponry of the European stag beetle. We found that the shape of these structures strongly depends on their size (centroid size) and body size (elytron length), outlining two different morphologies: small males with wide and rounded mandibles and less developed head edges and larger ones with slender mandibles equipped with fully developed teeth and wider head edges. These shape differences may be related to alternative mating tactics and behaviours in smaller and larger males. Head and mandibles show a great shape variation, indicating that they can be considered as a functional unit, evolved under influence of the same selective pressures. By comparing traditional to geometric morphometric approach, we concluded that reducing the complex mandible structure to a linear distance is an accurate representation of size. Contrariwise, male head size cannot be reduced to linear distances, as usually done for the genus Lucanus, hence a geometric morphometric approach is needed to describe at best the magnitude of its changes.