Exaggerated allometric structures in relation to demographic and ecological parameters in Lucanus cervus (Coleoptera: Lucanidae)

Enlarged weapons and ornamental traits under sexual selection often show a positive allometric relationship with the overall body size. The present study explores the allometry of mandibles and their supporting structure, the head, in males of the European stag beetle, Lucanus cervus. This species shows a remarkable dimorphism in mandible shape and size that are used by males in intraspecific combats. Stag beetles were captured, measured, weighed, and released in the framework of a capture-mark-recapture study. The relationship of mandible length (ML) and head width in respect to the overall body size was described by a segmented regression model. A linear relationship was detected between ML and head width. The scaling relationships for both ML and head width identified the same switchpoint, highlighting the advantages of using combined results of weapons and their supporting structures in such analysis. These results led to a more consistent distinction of males in two morphologies: minor and major. The survival probability of individuals was dependent on the morphological class and was higher for minor males than for major. Elytron length and body mass of the individuals did not show any significant variation during the season. Differences in predatory pressure were detected between morphs by the collection and analysis of body fragments due to the predatory activity of corvids. Morphological differences and shift in demographic and ecological parameters between the two classes suggested that selection continues to favor intrasexual dimorphism in this species throughout a trade-off mechanism between costs and benefits of carrying exaggerated traits.