Sexual dimorphism occurs across many different groups of beetles and manifests itself in many different ways from antennal length to overall body size to extreme pronotal horns. A recent article describes a different expression of sexual dimorphism, which the authors refer to as EMM or enlarged male mandibles. The EMM discussed in this particular article focused on the beetle family Chrysomelidae, particularly those within the subfamily Crytocephalinae. In addition to discussing the EMM, the authors described three new species – two from New Caledonia and one from Borneo. All three of the species have incredibly impressive and very different expressions of EMM. Below is the newly described species Scaphodius drehu with male and female comparisons along with the male’s obvious cartoon doppleganger.
So what are these incredibly modified mandibles used for? Why do some groups of beetles have them while others don’t? It doesn’t appear that they are used as a clasping device to hold onto females during mating because the head is a poorly placed area for such a function in many chrysomelids – many species are rotund and so the head of the male is far away from the female during copulation. Instead the authors hypothesize that the mandibles are used in agonistic behavior – prying off other males that are already mating with females. Interestingly, the authors surveyed the presence and absence of EMM across the Chrysomelidae and found that they are generally restricted to certain groups, indicating that this character may be phylogenetically informative. Most of the species that have EMM have barrel-shaped bodies with high centers of gravity that are unstable, further supporting the hypothesis that the mandibles are used to dislodge opponents.
So, basically if you are an adorable rotund chrysomelid beetle, you’re more likely to have EMM than other chrysomelids that are not as adorably round. Answering this question within a phylogenetic context would be interesting – are sexually dimorphic male mandibles correlated with body shape when accounting for shared ancestral history? This can really only be addressed with a well-resolved phylogeny and to do that we need more research on Chrysomelidae taxonomy and systematics.
Check out this open access article – the pictures of the male mandibles for the newly described species are truly remarkable.
Reid CAM, Beatson M (2013) Chrysomelid males with enlarged mandibles: three new species and a review of occurrence in the family (Coleoptera: Chrysomelidae). Zootaxa, 3619 (1): 079–100.