Read our COVID-19 research and news.

Bats Catch Mating Flies in the Act

A house fly couple settles down on the ceiling of a manure-filled cowshed for a romantic night of courtship and copulation. Unbeknownst to the infatuated insects, their antics have attracted the acute ears of a lurking Natterer's bat. But this eavesdropper is no pervert—he's a predator set on a two-for-one dinner special. As a new study reveals, the hungry bat swoops in on the unsuspecting flies, guided by the sound of their precoital "clicks."

Previous studies of freshwater amphipods, water striders, and locusts have shown that mating can make animals more vulnerable to predators, but these studies did not determine why. A team from the Max Planck Institute for Ornithology in Germany, which was led by Björn Siemers, found that the bat-fly interactions in the cowshed provided clues for understanding what tips off a predator to a mating couple. The researchers observed a teenage horror film-like scene as Natterer's bats (Myotis nattereri)preyed on mating house flies (Musca domestica).

Bats find prey primarily through two methods: echolocation and passive acoustics. For most bats, echolocation is the go-to tracking tool. They send out a series of high frequency calls and listen for the echoes produced when the waves hit something. The researchers found that by using echolocation, bats could easily find and catch house flies midflight, yet they had difficulty hunting stationary house flies.

"The problem is that these flies sit on the ceiling at night, and when the bat tries to echolocate them, the substrate masks the weak echo of the flies," says Stefan Greif, a doctoral student who worked under Siemers. The cowshed ceiling is covered in small bumps similar in size to the flies. So when the bat bounces its signal off the surface, the bugs are invisible among the bumps.

That's when passive acoustic cues, or the sounds that prey make, come into play. The team noted that the male made a clicking sound with his wings before copulating that alerted the bats to the pair's location. These clicks were between 9 kHz and 154 kHz and came in 3-second bursts. So to humans, whose hearing maxes out around 20 kHz, the clicks sound like low-frequency buzzing. But to bats, which can hear sounds up to 150 kHz, the clicks are clear auditory alerts. Locked onto the sound, the bats would swoop in and snatch the fly pair with a "prey pocket" formed from the excess skin extending from their tail, in a process called gleaning. Bats attacked 26 percent of mating flies, grabbing a double meal almost 60% of the time, the researchers reveal online today in Current Biology.

Two-for-one special. Bats attacked 26% of mating flies, grabbing a double meal almost 60% of the time.
Credit: Stefan Greif

To confirm that the bats were responding to the clicks, the researchers played recordings of fly sounds through speakers. The team found that the bats attacked the speakers whenever they played the buzzing recordings, but ignored them when they played white noise or the sound of the flies walking.

Christian Voigt, a behavioral ecologist at the Leibniz Institute for Zoo and Wildlife Research in Berlin, calls the work "a breathtaking story." It's also a "striking example," he says, of the bats' spatial memory, which allows the animals to pinpoint where prey are after hearing their sounds.

The study also illustrates learning in Natterer's bats, says Greif. The hungry bats "cannot find the flies [through echolocation], but they have apparently learned that the buzzing sound that the flies do when they copulate actually means food." Rachel Page, a behavioral ecologist at the Smithsonian Tropical Research Institute in Panama, agrees. "It's a combination of inquisitiveness and rapid learning that allows the bats to home in on something like a fly copulating and then realize, 'Wow, now that's something that would make a good meal.' "

John Ratcliffe, a neuroethologist at the University of Southern Denmark, Odense, praises the study for "meticulously testing different hypotheses and coming up with a strong explanation as to 'what's going on.'"

Aerial assault.The team found that the bats attacked the speakers whenever they played the buzzing recordings.
Credit: Stefan Greif

So if buzzing can get them gobbled, why don't the flies keep the noise down? Research by Lisa Meffert who was at Rice University in Houston, Texas, suggests that buzzing is a part of their courtship behavior. A male house fly vibrates his wings while on top of the female. Although the female can choose to reject the male by kicking him off at any point, copulation always occurs after the males make their buzzing noise.

The authors of the paper speculate that the females may use the buzzing to assess some aspect of the males' fitness, so they may need to hear this sound before allowing mating to proceed. If so, the male's performance makes for potentially fatal foreplay.