Flies may not scream out in ecstasy during sex, but they do create quite a buzz with their wings. And now researchers have found these mating moans can be heard by bats hungry for a meal.

The result: Wild Natterer's bats get a double-size meal of copulating flies; the mating flies, rather than offspring, get death.

In the study, detailed this week in the journal Current Biology, Stefan Greif from the Max Planck Institute for Ornithology in Germany and colleagues found bats didn't seem to notice the flies walking on a ceiling or just sitting. That suggests there's something about mating sounds that outs the flies to bat predators.

The researchers said this is one of very few studies to show that copulating animals are at a higher risk of being eaten by predators. Other examples include studies on amphipods, water striders and other aquatic insects, as well as land insects like the Australian plague locust.

In a cowshed in Marburg, Germany, the researchers video-recorded the movements of 9,000 houseflies over four years. Results showed the flies rarely fly at night, when they mainly sit or run on the ceiling; the faint, lower frequency, noise made by these movements, it seems, was drowned out by a strong background echo. [The Animal Sex Quiz]

"The faint insect echo [of non-mating flies] is fully overlapped and masked by massive echoes from the background," the researchers write.