SEX can be a risky business if you're a fly.

Scientists have discovered some bats locate their prey by eavesdropping on the buzzing sounds of fornicating couples.

German researchers filmed the interactions between Natterer's bats, found mostly in Europe, and a common species of fly in a cowshed in central Germany. They found more than a quarter of mating flies were attacked by bats on average over four observation years, while bats made no attempt to attack the insects when they walked on the shed ceiling.

A bat biologist and the study's lead author, Bjorn Siemers, from the Max Planck Institute for Ornithology, said bats that pounced on copulating flies succeeded catching the insects almost 60 per cent of the time.

''Keying in on copulating flies typically afforded the bats with a double meal,'' Dr Siemers said.