Flies pay ultimate price for sex

By John Upton

If you were spending an amorous weekend camping in the woods and you suspected that a bloodthirsty cougar was prowling outside in the dark, would you stay perfectly still in your tent, perhaps clutching a knife? Or would you blithely get busy with your lover, potentially alerting the cat to your presence, knowing full well that copulation could lead to decapitation?

Versions of this unlikely scenario are played out constantly in the wild. But while it has long been hypothesized that mating by insects and other animals increases their risks of predation, firm evidence of such risks has been a little bit hard to come by.

When houseflies mate, they risk being eaten by bats / Flickr: DeeJayTee23

To test one such scenario, German researchers used video cameras to monitor common houseflies in a barn filled with cows and fly-eating Natterer’s bats. When the flies lay on the cowshed’s ceiling or ambled across it, they were virtually immune to predation. The researchers didn’t spot a single bat attack on a walking fly during four years of research. The bats couldn’t find their prey: Their echolocation equipment simply wasn’t sensitive enough.

But once the flies started to get busy they emitted clicks and other subtle buzzing noises that helped the bats zero in on their distracted prey. Approximately one out of four copulating couples were attacked by a bat, often providing the predator with a hearty meal of two flies, the researchers reported in today’s issue of Current Biology.

“I can only speculate what the original function of this buzzing sound is,” Max Planck Institute for Ornithology researcher Stefan Greif told me. “My guess is that it’s a byproduct of the movements transmitting the sperm. Maybe it’s also easing the female that it really is a male jumping on her, and not a predator.”

So why would the flies choose to mate if it increased their chances of being gobbled up? Well, reproduction is the name of the game in the wild, and flies have only a short timeframe in which they can do it. After hatching from eggs and developing as asexual maggots, the average housefly will live winged and fancy free for just two to three weeks.

Natterer’s bats use the sound of mating flies to help them find their prey / Courtesy: Current Biology

Houseflies are an r-selected species, meaning the populations breed as fast as they can and are willing to take risks doing so. Houseflies endure high levels of predation and other pressures that would take heavy tolls on populations of K-selected species, such as elephants and tortoises, which breed slowly and carefully.

Studies of amphipods, water striders and locusts – all of which are r-selected species – have produced similar results.

“Maybe the cost ‘out in the wild’ is lower than in the cowshed, where we get more predators,” Greif said. “But on the other side, as there are so many flies in the cowshed, overall reproductive success might be so high that it outweighs and counterbalances the evolutionary pressure put on by the bats.”