Sex can be a bit risky for humans, but for flies it carries an extra element of danger: death by bat.

German scientists have found that bats listen for the sounds of fly coitus to get a better drop on their prey, with the added bonus of often nabbing two flies for the price of one. They filmed flies and bats inside an experimental cowshed and reported their observations in the journal Current Biology on Monday.

If an insect sits on or walks along a surface, it's almost impossible for bats to find them by echolocation.

But when flies have sex, they utter a burst of broadband, click-like signals, likely from the male's wing-fluttering, coauthor Stefan Greif of the Max Planck Institute for Ornithology said in a statement Monday.

That noise allows the bats to zero in on the enraptured insects. Over four years and 1,100 fly sex accents observed by the scientists, 5 percent of the flies were attacked by bats mid-coitus. In 60 percent of those instances, the bat was able to chow down on both partners.

The researchers think that other bat species besides the Natterer's bats they study must also be listening for the sounds of prey species' lovemaking.

Many animals are not only conspicuous in being vocal during sex, but they are also distracted in their attention, Greif said. It turns out that sometimes, sex kills.

SOURCE: Siemers et al. Bats eavesdrop on the sound of copulating flies. Curr. Bio 22: R563-R564, 24 July 2012.