One of the unspoken rules of slasher movies is that sex means death. Teen hookups are like a siren song to B-movie killers, and as soon as a pretty, young supporting character loses her virginity, she loses her head. A new study shows that sex might also be a death sentence for some flies, as it attracts the attention not of a knife-wielding psycho, but bats.
Where there’s cows, there’s manure. Where there’s manure, there’s flies. On many farms in Germany, there’s also Natterer’s bats (Myotis nattereri), which flock to cow sheds for the convenient feast. Scientists couldn’t figure out how the bats got their meal, though. Bats usually hunt by echolocation, sending out a series of high frequency calls and finding prey based on the echoes that bounce back. It’s great for finding bugs in mid-flight, but less so when they’re sitting still or just walking around. It’s especially hindered in the cow sheds, where the roughly textured wall and ceiling surfaces almost completely mask the flies’ weak echoes. Over four years, a team of researchers led by Björn M. Siemers from the Max Planck Institute for Ornithology filmed thousands of flies in a shed in Weitershausen, Germany. The flies wisely refrained from flying at night, and when they walked over the walls and ceiling, not a single one of them was ever hunted by the 45 bats that lived there.
As soon as the flies started having sex, though, the bats embraced their inner Michael Myers and attacked the love-making couples. The researchers found that a quarter of mating flies were preyed on and over half of those attacks were successful, with the bat devouring both partners in all but two instances.
What is it about sex that got the bats’ attention? When the researchers stuck dead flies together in a morbid post-mortem sex simulation, they were ignored, so it isn’t the more conspicuous echo that a mating pair creates. Instead, the researchers suggest that its the sweet sounds of fly love that dooms them.
During sex, the female fly spreads her wings while the male flutters on top of her. This creates a series of clicks between 9 kHz and 154 kHz. We hear the lower end of that as a buzzing sound, but the higher pitches are loud and clear to the bats. When the researchers played a recording of the sexy soundtrack, the bats attacked the speakers, but not when they played other noises at the same frequency band and amplitude, suggesting that those specific sex sounds are important prey cues that ring like a dinner bell for the bats.
If all that noise means a fly winds up in a bat’s stomach post-coitus, one would like to think that there would be strong natural selection against it. The researchers say that the fact that it continues despite predation pressure suggests that might be a courtship element and a signal of the males’ quality as mates and fathers. The noise is perhaps an integral step to getting it on, important enough to run the risk of being noticed by a killer lurking in the shadows.
Reference: Björn M. Siemers, Eva Kriner, Ingrid Kaipf, Matthias Simon and Stefan Greif (2012). Bats eavesdrop on the sound of copulating flies. Current Biology, 22 (14), 563-564 DOI: 10.1016/j.cub.2012.06.030
Image: “Myotis nattereri” by Guido Gerding. Used under a Creative Commons License.