Some species of moth can produce ultrasonic emissions that confuse echolocating bats, and they do it by rubbing their sex organs together.
This sonar jamming is the product of a 60 million-year evolutionary arms race, according to researchers. As bats evolved echolocation – the ability to emit high-pitched, ultrasonic noises and locate prey based on the returning echoes – moths evolved countermeasures that increased their odds of escape and survival. More than half of the world’s 140,000 species of moth have ears sensitive to ultrasound. Detecting a bat’s sonar triggers evasive midair maneuvers which often allow the moth to dodge a swooping bat.
Some moth species go beyond evasive aerobatics. Tiger moths and hawkmoths actually wield sonar-jamming countermeasures against bats.
Both types of moth use genital stridulation to produce ultrasound, which means that they rub their genitals together to produce sound. The genitals of several species of tiger moths and hawkmoths have evolved to be really good at producing high-pitched sounds when rubbed together. This is best documented in male moths, but females of some species seem to have the same ability. At the right intensity and frequency, the sound can interfere with a bat’s ability to hear the echoes of its own ultrasound, effectively jamming the bat’s sonar.
In tiger moths, biologists believe these signals were originally meant to startle bats long enough to allow an escape, or to warn bats that their intended prey might actually be poison; tiger moths’ bodily fluids are toxic to bats and birds. Until recently, researchers have not been completely sure that hawkmoths’ ultrasound signals were really sonar jamming, rather than an attempt to mimic the tiger moths’ poison warning or startle the bats into backing off.
Biologists Akito Y. Kawahara and Jesse R. Barber reasoned that they could tell sonar-jamming from other functions based on how the bats reacted to the moths’ signals. If the moths really were jamming bats’ sonar, their ultrasound signals would be effective right away, and they would keep working even after bats had been exposed to the trick several times. On the other hand, if the signals were meant to startle the bats, the bats would eventually get used to the sound and stop being startled. And if the signals were an attempt to mimic the tiger moths’ warning, then the bats would learn to ignore it once they discovered that hawkmoths were safe to eat. To test these possible explanations, they needed to watch bats hunt moths.
“To test the function of ultrasound, we pitted big brown bats (Eptesicus fuscus) against hawkmoths over multiple nights,” wrote Kawahara and Barber in a study published in Proceedings of the National Academy of Sciences on May 4. This aerial version of Fight Club took place in what researchers call an “indoor flight facility,” a 25 x 22 x 10-foot chamber lined with sonar-absorbing foam to prevent echoes from ricocheting around the room. Researchers tethered each moth to a fine monofilament line before sending it into the chamber with the bat. Each time, the bat had one minute to hunt, while the moth tried to stay alive.
Researchers tested several moth species against the bats. Contenders included an ultrasound-producing species of hawkmoth called Xylophanes falco and a couple of species of naturally silent hawkmoths. The silent species were significantly more likely to be caught than X. falco, armed with its ultrasound-emitting genitals. The research team also amputated the sound-producing genitals from some X. falco moths, effectively muting them, and put them into the hunting chamber. They fared little better than the naturally silent moths.
The moths’ ultrasound production clearly confused the bats, according to researchers, who reported that bats faced with sonar-jamming moths often tried to catch moths from empty air, apparently because the moths’ ultrasound signals had left the bats confused about the actual location of their prey. This kept working over several repetitions of the experiment with the same bats. For the researchers, this confirmed that moths were actually jamming bats’ sonar in order to escape, not just trying to startle them or mimic another, less tasty, moth.
According to fossil studies, the first ultrasound-producing moths appeared about 26 million years ago. The first hawkmoths capable of producing sounds that could jam bats’ echolocation appeared between 14 and 18 million years ago.
So that’s how long it takes to evolve radar-jamming genitalia.
Image: C. Robiller / Naturlichter.de, via Wikimedia Commons