Bat 'mutes sonar signal to sneak up on its prey'

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A rare British bat has developed remarkable stealth technology to sneak up on the moths which are its principal prey, new research has shown.

The barbastelle bat uses the technique to outwit moths' defences against the ultrasonic echolocation signals bats use to hunt in the dark.

Many moths have evolved hearing facilities, using auditory organs which are commonly referred to as "ears", but which are usually located in the middle of their bodies. These enable moths to pick up bat echolocation signals and take avoiding action, and research has shown that moths can pick up bat signals as far as 30m away.

However, in a classic example of what is sometimes referred to as the evolutionary arms race, the barbastelle has evolved a counter technique – it has developed echolocation signals up to 100 times less powerful than those of other bats. This means that it has sacrificed the ability to detect moths at long distance, but once it does detect them, it can get much closer before the moths hear the signals, meaning the bat has a good chance of success.

The extraordinary secrets of the whispering bat have been uncovered by a team of researchers from Bristol University, who refer to their discovery, published in the journal Current Biology, as "the first example of an echolocation counterstrategy to overcome prey hearing, at the cost of reduced detection distance".

An ugly bat with ears that meet in the middle of its forehead, the barbastelle is uncommon in Britain but it is a successful hunter which eats moths in large numbers.

However, the researchers did not know whether most of these moths were earless and thus unable to hear predators approaching, or whether the bats had found a way to catch moths that could hear them coming.

While previous studies could only determine the types of insects the bats had eaten – beetles, flies and moths, for example – researcher Matt Zeale developed a method using genetic markers to identify the species of those insects – and this established for the first time that the barbastelle almost exclusively preys on moths that have ears.

The researchers then measured how well moths can detect different bat species by recording the activity of the nerve in the moth's ear – using captive moths on the ground – while tracking the position of flying bats at the same time.

"Recording from the ear of a moth in the field was a real challenge but it yielded some amazing results," said another of the researchers, Dr Hannah ter Hofstede. "Whereas moths can detect other bats more than 30m away, the barbastelle gets as close as 3.5m without being detected."

The researchers then analysed the barbastelle's echolocation calls and found that they are up to 100 times quieter than those of other bats.

"We modelled detection distances for bats and moths and found that by whispering, the barbastelle can hear the echo from an unsuspecting moth before the moth becomes aware of the approaching bat," said a third member of the team, Dr Holger Goerlitz. "This advantage, however, comes at the cost of reduced detection range, similar to us trying to navigate in the dark using a lighter instead of a spotlight."

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