Barbastelle bats catch moths using stealth echolocation

20 August 2010, by Tamera Jones

Now scientists have discovered that the barbastelle bat has evolved a sneaky way to outwit these moths. Instead of emitting loud echolocation calls, the bats whisper so that moths can't hear them approaching.

This means the barbastelle (Barbastella barbastellus) feeds almost exclusively on moths with ears, while other bats are lucky if they manage to catch one.

Not much competition

The only snag for the barbastelle is that because the whisper is akin to using a candle instead of a spotlight to search in the dark, the bat's detection distance is relatively short, so it's less likely to detect as much prey as other bats. On the other hand because most other bats struggle to catch moths, there's not so much competition.

'Normally bats use ultra loud echolocation calls – which are as loud as jet engines if we could hear them – to detect their prey,' says Dr Holger Goerlitz from the University of Bristol and lead author of the study published in Current Biology. 'But this bat uses very low amplitude calls to overcome moths' defences.'

Although other bats use similarly low amplitude echolocation, those bats mainly feed on insects that sit on leaves. For these so-called gleaning bats, scientist aren't sure if these lower amplitude calls evolved to counteract moth hearing, or to stop the echoes from plants' leaves deafening the bats.

To test the idea that barbastelle bats use stealth echolocation only to counter moth hearing, Goerlitz and colleagues from the University of Bristol genetically analysed the bats' poo using the NERC Biomolecular Analysis Facilities at Sheffield and Edinburgh to find out exactly which species of moth they were eating.

This novel approach established for the first time that the bat preys almost exclusively on moths with ears. 'We were amazed at the high proportion – 85 per cent – of eared moths these bats are catching,' says Goerlitz.

Next, to find out why the barbastelle is so good at catching eared moths, the scientists tracked the flight paths of bats in the wild at the National Trust's Mottisfont Abbey in 3D using arrays of microphones, while at the same time recording how active the nerve in the ear of the moth Noctua pronuba was.

They found that while most moths can detect bats when they're as far as 30 metres away, the barbastelle got as close as 3.5 metres before Noctua pronuba picked it up. By then the moth's fate has been decided. Sometimes the barbastelle could get as close as 1.9 m without any activity in the moth's nerve that would tell it to make a hasty escape.

10 to 100 times lower

The team also analysed the amplitude of the barbastelle's echolocation calls and found that they're around 10 to 100 times lower than the calls from other bats that pursue and catch their prey while flying – so-called aerial-hawking bats.

'Most moths can hear bats before bats can locate them so have time to evade them. But the barbestelle bat is exceptional. It can locate moths before the moth realises it's being hunted,' says Goerlitz.

Goerlitz and his colleagues argue that this strategy doesn't have any other benefits for the bat, so it's highly likely that lower amplitude calls have evolved to outwit eared moths.

'It's rare to find examples of predators prevailing over prey in the co-evolutionary arms race, because there's a lower selection pressure on the predator than the prey,' explains Goerlitz.

Although the barbastelle bat is found throughout most of Europe, its natural woodland habitat is disappearing, so the bat is becoming increasingly rare in Britain and is only found in the southern half of the country.

The research was funded by the Biotechnology and Biological Sciences Research Council, NERC, Mammals Trust UK, the Countryside Council for Wales, Natural England and Bat Conservation International.

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