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."

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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.

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^{Image credit: Steve Parker/Bat Conservation Trust}