The barbastelle bat has found a new way to sneak up on unsuspecting moths – by gradually emitting fainter calls as they approach. In an evolutionary bid to outsmart the bat, some moths had developed ears to detect echolocating bats and avoid being caught.

But new research by ecologists at the Max Planck Institute for Ornithology in Germany has found that upon detecting a nocturnal moth, this intriguing bat species can reduce its call intensity even further while closing in on its prey.

The barbastelle bat – with its characteristic bumpy face – is a very successful hunter that manages to almost exclusively feed on eared moths.

Stealth fighter

Comparable with stealth fighter jets, barbastelle bats use a ‘stealth echolocation’ tactic – echolocation at intensities that are inaudible to distant moths – to ambush prey. Their calls are more than 10 times quieter than those of other bats which hunt insects in the same way.

Consequently, call intensity heard by the moth only increases very slowly, delaying the time and shortening the distance at which it becomes aware of the attacker. Once a moth hears the calls, it is most likely too late to escape.

Dr Daniel Lewanzik from the Max Planck Institute of Ornithology said: “Barbastelle bats call with surprisingly low intensity, usually a characteristic of species that hunt in cluttered habitats and need to avoid distracting echoes from branches and leaves.

“Low intensity calls come at a cost though. They do not reach far and as a result, insects can only be detected from a close distance.” 

To test why the barbastelle can catch eared moths when other bats cannot, the authors closely investigated echolocation behaviour during pursuit and final attack.

They tethered moths – Noctua pronuba – to a long fishing rod with a miniature microphone positioned a few centimetres above, offering them to free-ranging barbastelle bats in a forest and to captive ones in a flight room. This allowed the team to analyse the echolocation calls from a moth’s perspective.

Echolocation behaviour

Simultaneously, the researchers recorded the calls of approaching bats with a four-microphone array in order to reconstruct three-dimensional flight paths and thus measure their distance to the moths.

Barbastelle bats can detect moths at about 1.6 m distance. Once approaching their unsuspecting prey, the bats lower their already faint calls by 4 decibels (dB) or 40% for each halving of distance. During the final buzz when they are less than 1 m away, call intensity decreases by more than 6 dB or 50% per halving of distance.

Echolocation call levels received by the moths remain almost constant during the attack (instead of doubling per halving of distance) as a result of the bats’ stealth tactic, keeping them low enough to prevent the moth from escaping.

Dr Holger Goerlitz, also from the research institute, said: “Our results suggest that barbastelle bats are able to outwit the hearing defence of moths and close in without triggering any last-ditch manoeuvres, making them very successful moth hunters.

“In fact, the evolution of moth ears might benefit barbastelles as they can avoid competition with other, louder bats.”

This Author

Sabrina Weiss works for the British Ecological Society. The full study is published on March 14th in the journal Functional Ecology.