Seasonal changes in body size, sexual size dimorphism and sex ratio in relation to mating system in an adult odonate community

Abstract

Seasonal environments impose developmental time constraints on insects which can be reflected in body size and sex ratio. By tracking these two aspects in recently emerged adults of 10 species of an odonate community in a number of lakes, we investigated whether (a) body size in both sexes decreased as the flight season progressed and whether this led to seasonal changes in sexual size dimorphism (SSD); (b) SSD patterns were related to mating systems; (c) biases in sex ratio could be explained by mortality rates associated with the largest sex (e.g. in species with male-biased SSD, a female-biased sex ratio; in species with female-biased SSD, a male-biased sex ratio). Our results indicated that adults in most species, but not all, tend to reach a smaller body size as the season progressed. However, the opposite pattern was found in a few species. Predictions about the relation between SSD and mating systems were confirmed: a female-biased SSD in nonterritorial species and monomorphism for territorial species. However, predictions of biases in sex ratio according to SSD were not met in all species. Interestingly, changes in body size and SSD along the season were lake-specific in two species in which these patterns could be examined. These results, although partially supportive of environmental and sexual selection patterns acting on size and sex ratio as documented in other odonate species, indicate that we are still far from understanding seasonal constrains in these animals.