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Fornicating flies attract hungry bats

Coitus interruptus The sex sounds flies make can lead to a violent death - luring predator bats to their love nests while their defences are down, a new study reveals.

Researchers who studied a small community of flies and bats in a cowshed near Marburg, Germany, found that the insects' chances of becoming dinner were dramatically higher while copulating.

They were six times more likely to be eaten than single, sitting flies, says bat biologist and co-author Stefan Greif of Germany's Max Planck Institute for Ornithology.

"Sex kills," he says - at least in the case of the house flies that constitute the main menu item of a colony of Natterer's bats roosting in the ceiling of the cowshed.

Studies over 13 nights in 2001, 2005, 2006 and 2009, revealed not a single bat attack on a fly simply walking across the ceiling, compared to 59 attacks on those engaged in mating.

"On average, across four observation years, 26 percent of the flies that engaged in copulation were attacked by the bats," according to the study published in the journal Current Biology.

The average strike rate was 59 per cent, and in only two cases did one of the targeted pair manage to escape.

"Therefore, keying in on copulating flies typically afforded the bats with a double meal," write the study's authors.

The bats were not tempted by dead, noiseless pairs of flies mounted in a mating position on the shed ceiling, leading the researchers to believe it was the high-frequency sound, not the sight of the couples that attracted the predators.

This theory was boosted by the bats attacking speakers mounted in the shed and emitting recorded fly sex sounds.

Using sonar to zoom in on their prey, the bats had difficulty picking up sitting or walking flies, whose feint sounds are masked by background echoes, say the researchers.

But copulating flies emitted a burst of buzzing, click-like signals, likely from the male's wing-fluttering.