Flies living among bats have a hard time mating, as it seems bats can use the sound they make while copulating to find them, and earn themselves a "two-for-one" dinner deal. The study, published in the July 24th issue of Current Biology, is the first to show that an increased risk of predation, due to higher exposure to predators, is a significant cost of sex.

"When mating, the flies utter a burst of broadband, click-like signals, likely from the male's wing-fluttering," said lead author Dr. Stefan Greif, from the Max Planck Institute for Ornithology. And it seems that this buzzing is sufficient for the bats to find them.

Before starting this study, researchers knew one thing, that somehow this nocturnal bats where eating lots of these diurnal flies, which was a bit of a mystery. As a first step in this study, researchers filmed a bat colony living in a cowshed that housed a virtually unlimited supply of flies. From these observations researchers found that flies hardly ever fly at night, and that bats almost never attack flies while they are sitting still or walking. But as soon as flies start having sex, their risk of being captured by hungry bats sky-rocketed. According to a press release "on average across four years and 1,100 observed acts of fly sex, about 5 percent of the flies caught in the act by the researchers were also detected and attacked by the bats. In about 60 percent of those cases, the bat attacker successfully gobbled up both flies for a double meal." Dr. Greif thinks other predators might also use this strategy to get themselves a two-for-one dinner deal. "Many animals are not only conspicuous in being vocal during sex, but they are also distracted in their attention," he says. So, boy and girls, the message is clear: Sex kills, at least in the world of flies.

Two key implications from this study concern "how flexible the bats can be in their hunting behavior, and that it is dangerous for flies to copulate when bats are around". Dr. Greif told The Munich Times. This deadly eavesdropping bat behavior represents a major evolutionary pressure on fly copulation, and over evolutionary time it will be interesting to learn how these flies manage to deal with this pressure, as Dr. Greif explains. "...it would be interesting to have a closer look at potential consequences of the before-mentioned evolutionary pressure. Are the flies developing any counter strategies? Or is the pressure not high enough?"