Ontogenetic Shift in Efficacy of Antipredator Mechanisms in a Top Aquatic Predator, Anax junius (Odonata: Aeshnidae)

Abstract

The ability of prey to escape predation often lies in the occurrence and efficacy of their predator avoidance and antipredator behaviors, which are often coupled with specialized morphology. How the use and efficacy of these behaviors change throughout ontogeny may be indicative of the vulnerability and ecological roles these animals experience throughout their lives. We examined the antipredator behavior of a large dragonfly nymph, Anax junius, from a historically fishless pond where these animals have traditionally been classified as top predators. These dragonfly nymphs displayed a series of distinct aggressive antipredator behaviors when grasped that involved stabbing with lateral and posterior spines and seizing with labial hooks. Larger (older) nymphs displayed these aggressive behaviors significantly more than smaller (younger) animals in simulated predation trials. During encounters with live larval salamander predators (Ambystoma tigrinum), all large nymphs, but only 12.5% of small nymphs successfully escaped predation attempts by the amphibians through the use of antipredator behavior. Large nymphs were also significantly more active than smaller nymphs in the presence of salamander larvae. Despite often being considered top predators in fishless ponds, our study demonstrates that their true role is more complex, depending on ontogeny and body size, and that effective antipredator behavior is likely necessary for survival in these systems.