Auditory Sensitivity and Defence Strategy in Insects

Predation pressure is a powerful agent of natural selection and is responsible for the evolution of various antipredator defence strategies in animals. Sensory thresholds for predator detection could play an important role in the evolution of defence strategies. I tested the hypothesis that the ability of animals to detect predators is correlated with defence strategy, with the prediction that animals with poor predator detection abilities rely more on preventative (primary) defence strategies than animals with low predator detection thresholds. Bats and their insect prey were used as a simple study system for these experiments due to the reliance on a single modality (hearing) for both predator and prey detection. Many insects have ears tuned to the ultrasonic echolocation calls of bats and can use this predator cue to initiate evasive action, but variation in auditory thresholds exists among species. In moths, a group in which the only known function of hearing is predator detection, a clear relationship was found between auditory thresholds for predatory ultrasound and a risky behaviour, nocturnal flight time. A more complicated situation exists when the sensory system serves more than one purpose, as with the ears of orthopteran insects used for both predator detection and mate localization. Some gleaning bats use calling song as a cue to locate these insects as prey, and both primary (reduced calling) and secondary (song cessation in response to ultrasound) defences have been identified in orthopterans. The auditory interneurons considered the “bat-detectors” in katydids and crickets most likely have context dependent functions in several groups, as a predator-detector in flight and mate-detector on the ground. The relationship between reliance on primary over secondary defence and auditory sensitivity in these insects appears to be influenced by the nature of the calling song of the species and their mating strategy.Ph