Daniel Lewanzik, Holger R. Goerlitz
Insect-eating bats orientate and hunt insects in darkness by echolocation. They emit extremely loud calls and listen for the faint echoes returning from airborne prey. Luckily, bats usually use ultrasound frequencies that humans cannot hear; otherwise sounds of such high intensities – comparable to a jet engine at close distance – would harm our hearing. Many nocturnal moths, in contrast, have ultrasound-sensitive ears and can hear bat echolocation calls from far away and take evasive action in due time. Thus, the diet of many bats contains only a minor proortion of such ‘eared moths’. Contrastingly, barbastelle bats with their characteristic bumpy face almost exclusively feed on eared moths and it has remained a mystery how they trick them… until now.
We were convinced that the key to the barbastelles´ success lies in their call intensity dynamics during prey attack. To test this, we equipped tethered moths with miniature microphones and offered them to free-ranging barbastelle bats in a forest and to captive ones in a flight room. Simultaneously, we recorded the calls of the flying bat with a state-of-the-art four-microphone array, allowing us to reconstruct the bat’s three-dimensional flight path and thus to measure its distance to the moth. These positional data together with the calls recorded at the miniature moth microphone enabled us to back-calculate call intensities as emitted by foraging barbastelles until the final capture.
We demonstrate that the echolocation calls of barbastelle bats are more than 10 times fainter than those of other bats that search for insects in the open air. In addition, we showed for the first time that barbastelle bats, upon detecting a moth, emit fainter and fainter calls while closing in on it. Consequently, call intensity heard by the moth only increases very slowly, delaying the time and shortening the distance at which the moths are able to hear the attacker. Once a moth hears the calls, it is very likely too late to escape. In combination, these two strategies allowed barbastelle bats to access a food resource that is largely unavailable to competing species, thereby probably altering competition between predators.