Patterns of mass gain and sexual dimorphism in adult dragonflies (Insecta: Odonata)

Abstract

We present data on mass gain and sexual dimorphism from 61 collections of 54 species in eight families of Odonata. Males and females emerge at similar mass in most species (14 of 17 measured). However, females are significantly heavier than males when mature in 37 species. Males are heavier in only four species (though none significantly so). Females increase in thoracic mass as well as abdominal mass (presumably for egg maturation). Males allocate most additional mass to the thorax. Females increase in mass by an average of 125% between emergence and sexual maturity. The average increase for males is only 84%, and in some species males lose mass. Sexual dimorphism in mass at maturity is most striking in the nonterritorial Coenagrionidae, where females average > 60% heavier than males. In species known to be territorial, male and female masses are more similar; female mass does not exceed that of males by more than 32%. We propose that mass gain as an adult is beneficial to females regardless of mating system because it increases fecundity. In territorial species, heavier males are more likely to acquire and hold territories and thereby gain access to mates. In nonterritorial species, heavy males do not have the same advantage in access to mates. Because acquiring resources for mass gain has costs in time and mortality, and there are metabolic costs of maintaining additional mass, males of species with nonterritorial mating systems gain less mass. As a result, the mass of males and females in species with territorial mating systems are more similar than in species with nonterritorial mating systems.