Ecology of sexual dimorphism: Insights from the flower longhorn beetle Grammoptera subargentata

Longhorned beetles display remarkable variation in reproductive biology and sexual dimorphism, with males of some species possessing exaggerated antennae, mandibles, or legs while others are nearly monomorphic. Ecological factors may have shaped this diversity, as male-biased dimorphism is expected to evolve when resources required for mating or offspring development are scarce. As part of my ongoing thesis work on the evolution of sexually dimorphic traits in cerambycids, I examined the morphology and ecology of a common and subtly sexually dimorphic flower longhorn species, Grammoptera subargentata, in Alberta, Canada. Using censuses, field observations, and a mark-recapture approach, I monitored the distribution of adults relative to the distribution of their floral food plants and mating sites. I also measured and performed a MANOVA on antenna, leg, and elytron lengths to quantify sexual dimorphism (SD). Although most measured traits were sexually dimorphic, male and female ranges tended to overlap. Moreover, the abundance of suitable adult food plants and mating sites far outstripped the number of G. subargentata in my field site. Thus, competition over access to mating resources may be weak in this species, and this may explain why G. subargentata shows weaker SD than other cerambycid species (e.g., Monochamus scutellatus) that defend transient or rare mating and oviposition sites. This preliminary work provides insight into ecological traits associated with weak SD, and it will also contribute to my future comparative studies of SD, allometry, and mating systems in cerambycids.