Each moth escapes its own way

Species-specific escape strategies of moths make hunting difficult for bats

July 02, 2019

Moths perform evasive flights with diverse tactics in order to avoid being eaten by bats. Researchers of the Max Planck Institute for Ornithology in Seewiesen found in their new study not only that the diversity of escape tactics is species-specific, but also that they differ between individuals within a species. This unpredictability of evasive flights experienced by bat predators is likely to protect the whole moth community.

PhD-student Theresa Hügel catching noctuid hearing moths.

The experimental setup shows how the moth is fixed to the membranes of two loudspeakers that transduced their flight strength into an electric signal.

For the survival of animals, successful foraging is equally important as predator avoidance. Therefore, in the relationship between predator and prey, both behaviors directly compete with each other. While the predator is searching for food, the prey animals try to avoid being eaten by any means.

This predator-prey relationship leads to different adaptations on both sides to optimize hunting and escaping, for example in bats and moths. Since bat-moth interactions take place in the dark, both animals rely exclusively on acoustic information. Therefore, simple ears evolved in many noctuid moth species, allowing them to perceive the echolocation calls of insect-hunting bats. As soon as a moth perceives an approaching bat, it tries to avoid being eaten by performing quick escape maneuvers like zig-zagging flights, loops, tight turns, passive dives or power dives. These are the same behaviors as seen in many other prey species, such as doubling-back in rabbits fleeing predators.

In this new study, these erratic escape movements of hearing moths were examined in detail by Theresa Hügel and Holger Goerlitz of the Max Planck Institute for Ornithology. In an acoustic chamber, they fixed moths carefully to small metal rods in a way that they were still able to fly. The rod was connected via a horizontal bar to the membranes of two loudspeakers. The flight movement of the insect thus moved the membranes, generating voltage fluctuations, which the researchers recorded. In this way, they were able to measure the flight forces and the escape behavior of the flying moth. The researchers tested a total of 172 individuals from eight different moth species, simulating the same predation threat by presenting all the same bat-like sound.

The researchers found that each moth species reacted differently to the acoustic stimulus. In addition, in some moth species also the individuals reacted differently to the stimulus. „The species-specific escape strategies together with different individual reactions likely provide the whole moth community with increased protection against bat predators,“ says Theresa Hügel, first-author of the study. „The unpredictable flight reactions of the prey increases the uncertainty for the bats, preventing them from adapting to them.“

„Now we want to study where the species-specific differences come from: differences in flight mechanics or differences in neuronal coding“, says Holger Goerlitz, head of the Research Group Acoustic and Functional Ecology. And last, but not least: whether there are adaptations in the bats’ echolocation to the moth’s ability to hear is a next exciting question in the study of the sensory battle between predators and their prey.

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