Weapon allometry and shape variation in the Helm’s stag beetle (Geodorcus helmsi)

Many animals develop and use exaggerated weapons in contests over access to mating opportunities. Stag beetles (family Lucanidae) show striking sexual dimorphism, where the males have heavily exaggerated mandibles that the females do not possess. In some genera, size and shape can also vary drastically between small and large males resulting in discrete alternative male morphs, described as male dimorphism. The genus Geodorcus is a poorly studied and threatened group of stag beetles that are endemic to New Zealand. Geodorcus helmsi is the most widespread species in this genus and presents an opportunity to test for the presence of both sexual and male dimorphism and investigate the scaling relationships (allometry) of mandibles in this group. We used linear and geometric morphometric analyses on wild populations and museum specimens from Stewart Island, New Zealand, to assess whether G. helmsi exhibits male dimorphism. We found that within a single population, there is no evidence for alternative male morphs, but a gradual shift between smaller and larger individuals. We also found that male mandibles exhibit steep positive allometry in contrast to the negative allometry found in the mandibles of females, as is often found for sexually selected weaponry. Moreover, we found evidence that left and right mandibles of G. helmsi are subtly asymmetric. Our results provide context for the competitive behaviour observed in this species, and give insights into potential trade-offs between smaller and larger individuals of the Helm’s stag beetle.