The evolution of sexual size dimorphism in Dendroctonus bark beetles

Sexual size dimorphism (SSD) is a widespread phenomenon in insects. However, depending on the species (and sometimes the population), the average female can be larger, smaller, or the same size as the average male. What evolutionary forces lead to these differences is often unknown although natural and sexual selection likely both contribute. Here, we investigate SSD in Dendroctonus, a genus comprised of many species well known for their economic and ecological impact. We compiled data on male/female size differences across multiple species and provide a general description of SSD within a phylogenetic framework. We investigated size-assortative mating as a mechanism leading to differences between certain species and conducted common garden experiments assessing if SSD changes in response to genetic divergence in overall body size. Our results suggest distinct differences in SSD between some Dendroctonus species, although we do not find evidence of size-assortative mating in species that differ in SSD. Additionally, size differences between sexes appear remarkably consistent, even in populations that have diverged substantially in overall size.