Continued source level reduction during attack in the low-amplitude bat Barbastella barbastellus prevents moth evasive flight

Abstract

Ears evolved independently in many insect taxa due to the selection pressure of echolocating bats. Eared moths perform evasive flight manoeuvres upon hearing approaching bats, thereby substantially increasing survival probability. Accordingly, eared moths constitute only a minor portion of many bats’ diets. In contrast, the barbastelle bat Barbastella barbastellus almost exclusively feeds on eared moths by emitting low-amplitude stealth echolocation calls that are undetectable by distant moths. While closing in on the prey, however, the calls might become audible. We thus hypothesised that barbastelle bats lower the source level of their calls even further while closing in, such that the level at the moth's ear remains below the moth's hearing threshold and the moth fails to elicit its evasive manoeuvre. We tested these hypotheses by offering tethered moths to barbastelle bats in the wild and in captivity. We reconstructed the bats’ three-dimensional flight paths based on time-of-arrival differences of the echolocation calls at four microphones, measured the received sound levels at the moth's position with an additional miniature microphone, and calculated call source levels. We show that barbastelle bats continuously reduced source level upon detecting a moth, thereby delaying the time and shortening the distance when the moth might detect the bat. The received level at the targeted moth never exceeded the moth's neuronal hearing threshold by more than 10 dB, which is likely sufficiently low to prevent moth evasive flight. Barbastelle bats emit low-amplitude calls to counter moth hearing. Continued amplitude reduction during an attack extends the time before the prey becomes aware of the predator, if at all. In combination, these two strategies allowed barbastelle bats to access a food resource which is largely unavailable to competing species, thereby probably altering competition between sympatric predators. A plain language summary is available for this article.