In the air wars between bats and moths, the bugs are fighting back—by shaking their privates.
It’s no award-winning dance move, but a new study shows that hawkmoths in Borneo (map) jiggle their junk to produce ultrasound. That jams Malaysian bats’ built-in sonar, rendering the hawkmoths temporarily “invisible.”A hawkmoth. Photograph by Joris van Alphen, Nature Picture Library
Found worldwide, hawkmoths get their name from their large sizes—some species have wingspans greater than 4 inches (10 centimeters)—and superb flying abilities. The insects fly up to 12 miles (19 kilometers) per hour and can quickly dart from side to side to avoid predators, especially bats.
Over the past 65 million years, bats and moths have squared off in an evolutionary arms race. For the bats’ part, they have built-in sonar that allows them to emit high-pitched cries, then listen as the sound waves bounce off any nearby insects. (Also see “‘Whispering’ Bat Evolved to Trick Prey.”)
Those millions of years of evolution have also given the moths some rather unusual abilities—which now includes genital vibrations.
Jesse Barber, a behavioral ecologist at Boise State University, had been studying hawkmoths and bat-moth interactions since he was a Ph.D. student. He also found preliminary evidence that hawkmoths were capable of producing ultrasound.
“In tiger moths, the only other group of moths known to produce anti-bat sounds, ultrasonic signals warn of bad taste, mimic sounds made by other bad-tasting species, and even jam bat sonar,” Barber said. (Related: “Moths Jam Bat Sonar, Throw the Predators Off Course.”)
He reasoned that since hawkmoths are closely related to tiger moths, and that bats appear to find them just as tasty, hawkmoths might be creating similar anti-bat ultrasound.
So he and Akito Kawahara, an evolutionary biologist at the University of Florida, traveled to Borneo and set up experiments with three hawkmoth species: Cechenena lineosa, Theretra boisduvalli, and Theretra nessus.
To keep the moths from flying away, the researchers tethered the bugs to an experimental rig with fishing line and a drinking straw.
Once the insects were attached to the rig, the researchers recorded the moths’ speed and sounds with high-speed cameras and special microphones that can record sounds in the ultrasonic range. The team also used high-tech speakers that could play recordings of hunting bats’ ultrasound. (Interactive: Hear tropical bat calls.)
“We placed the moth in front of a ultrasonic speaker and microphone. We then played pre-recorded bat sounds to the moth and watched and recorded their behavior,” said Kawahara, whose study was published July 3 in the journal Biology Letters.
Sure enough, when the hawkmoths heard the recordings of the bat sonar, they produced ultrasonic sounds of their own.
This is where the high-speed camera came in handy: By slowing down the video of the hawkmoths in the rig, the scientists were able to see exactly how the moths produced these sounds.
“We had some suspicion that they might be making sounds with their genitals, based on some enlarged scales that we found on the outside of the claspers [a part of the genitalia] in museum specimens,” Kawahara noted.
Indeed, the moths were actually rapidly rubbing their genitals to create ultrasound—and further investigation showed that both males and females used this behavior. (Read about a bug with a singing penis.)
The scientists believe that the hawkmoths use these ultrasonic responses as a form of self-defense. “We suspect that these sounds are used to jam bat echolocation and startle or warn bats,” Kawahara said.
The night sky may not be as quiet as we might think, Barber added.
“It seems likely that the night sky is filled with insects producing ultrasound in response to bat attack.”