Bat Species Uses Stealth Technique to Capture Prey

A rare British bat has developed remarkable stealth technology to sneak up on moths.

THE GIST

Western barbastelle bats whisper their echolocation calls so moths can't hear them coming.

The pinging is 10 to 100 times lower in amplitude than other aerial-hunting bats' echolocation calls.

In a long-running war between bats and moths, at least one bat has gotten the upper wing.

Western barbastelle bats in Europe typically ping out their echolocation calls softly enough to locate a moth for dinner before the moth hears them coming, says Holger Goerlitz of the University of Bristol in England.

It's the first documented case of a bat species outwitting its prey by quiet stealth, he and his colleagues say online in a Current Biology paper released August 19. The battle between bats and moths has become a classic system for studying the evolution of predators and their prey.

In searching for moths, barbastelles echolocate at about the 94 decibel level, roughly the equivalent of a busy highway, Goerlitz reports. This bat version of whispering is 10 to 100 times lower in amplitude than other aerial-hunting bats' echolocation calls. Those rank more in the range of jet engines and the vuvuzelas blaring at the latest World Cup, Goerlitz says.

People can't hear frequencies high enough to detect any of this bat racket -- "quite lucky for us," Goerlitz says.

To measure the loudness of the barbastelle calls, researchers needed to know how far away from a microphone a flying bat was when it pinged. So they set up a microphone array where bats swooped through at night. The slight differences in times that the calls took to reach different microphones let researchers figure out the bat's position for each of more than 100 calls.

This array also let the researchers answer the critical question of whether the barbastelle's softer echolocation was soft enough for stealth attacks on eared moths. Researchers restrained European moths called large underwings along the bat flight alley and monitored the activity of their auditory nerves.

A European bat with louder echolocation, at 127 decibels sound pressure level, triggered the moth's auditory nerves from about 30 meters away. Yet the barbastelle's pings didn't register until the bat had closed in to 3.5 meters. That's close enough for the barbastelle to have already detected the moth from calls echoing off the insect.

"The use of low-intensity echolocation calls when foraging for airborne prey is to my mind the clearest example of an adaptation that can best, and perhaps only, be explained as a means of eluding detection by eared insects," says behavioral ecologist John Ratcliffe of the University of Southern Denmark in Odense, who has also studied bat hunting.

To see if barbastelles' muted echolocation actually allows them to feast on more sharp-eared moths, Goerlitz's colleagues developed genetic markers to identify prey species in bat droppings. The analysis found that 89 percent of what barbastelles catch are moths, and 85 percent of those moths have ears. In contrast, the Leisler's bat, which pings at a similar frequency to the barbastelles' but much more loudly, has been recorded catching at most 56 percent moths.

"Moths are nice food," Goerlitz says. "They are big and fat and have a lot of energy."

Gorelitz notes that there doesn't seem to be any other evolutionary advantage to keeping the noise down besides stealth in hunting. And there is a tradeoff: more sneak power but less ability to detect potential food from a longer distance.

Many moths can hear the echolocation calls of Leisler's bat some 30 meters away. This recording slows the calls down from a frequency of 28,000 Hz to about 2,800 Hz so people can hear it. Barbastelle calls, however, don't give away the bat until it's 3.5 meters away. (Recording slowed from about 33,000 Hz to about 3,300 Hz to make it audible.)