Flies may not moan and groan during sex like humans, that you should know, but that doesn't mean that they're stone cold silent. Believe it or not, during sex, female flies make click-like sounds with their wings, attracting attention that could potentially cause their deaths by predatory bats. Researchers have found these mating moans can be heard by bats hungry for a meal.

This is bad news for the flies — but a bonus for wild Natterer's bats who end up swallowing a double-double of copulating insects.

In a study detailed this week in the journal Current Biology, Stefan Greif and his team from the Max Planck Institute for Ornithology in Germany discovered that bats don't seem to notice when flies do normal activity. But when they flap their wings, everything changed.

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.

In a cowshed in Marburg, Germany, the researchers recorded the movements of 9,000 houseflies over four years. Their results showed that the flies rarely flew at night and any low-frequency noise made by merely sitting or moving along a surface was drowned out by a background echo.

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

When houseflies mate, the female spreads her wings while the male flutters on top of her, a fluttering that probably is responsible for his broadband click-like sounds, hence making them “visible” to the bats. In the study, about 26 percent of fly pairs having sex were attacked by bats — and more than half of them were eaten.

In order to be certain that their mating calls were the reason for their demise, the researchers mounted dead, noiseless fly pairs in a typical sex position on the aforementioned shed ceiling. And the bats never attacked them — it was only when the researchers played the sex calls of flies that the bats attempted to attack. But they ended up going after the loudspeakers instead!

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