Bats hunt moths and other insects by using echolocation, where they emit ultrasonic calls and analyze the rebounding sound. So to avoid getting eaten by hungry bats, hawkmoths blast the flying mammals with their own ultrasound…which comes from their genitals. Seriously.
Bats and insects have engaged in an evolutionary arms race lasting nearly 65 million years. Bats first evolved to detect and catch prey by using echolocation; insects then evolved to hear the echolocation of predatory bats. In fact, most insect ears are connected to neuronal circuits that automatically evoke defensive behaviors — low bat call intensities cause the insects to steer away from the sound source, while high call intensities elicit aerobatic evasive maneuvers, including loops, swirls and dives.
But the race didn't stop there. Some bats have developed so-called "stealth echolocation," or the ability to echolocate at frequencies outside of their prey's hearing range. Tiger moths and tiger beetles, on the other hand, evolved an even cooler counter-ability: They produce ultrasonic clicks of their own. Research on the tiger moth's ultrasound — which the insect produces using a vibrating membrane in its thorax — suggest the noise startles naïve bats, warns bats of the moths' bad taste and jams the mammals' biosonar.
Now, scientists have discovered that at least three species of hawkmoths also produce ultrasonic sound in response to bat calls. But unlike their distantly related cousins, these moths produce the ultrasound using their genitals. Nature explains:
When the researchers played bat ultrasound to the hawkmoths, they found that three species (Cechenena lineosa, Theretra boisduvalii and Theretra nessus) they had captured emitted ultrasound clicks in response. The males did so by rapidly grating stiff scales on the outer surface of their 'claspers' — structures normally used to grab females during mating — against part of the abdomen, the researchers report. Females also seem to pull part of their genitalia inwards so that genital scales rub against their abdomens.
The scientists also found that the moths produce their ultrasonic clicks when they're handled roughly (as you can see in the video below). They aren't yet sure what role the sound plays in terms of anti-bat defenses, but think the clicks may serve a similar purpose for the hawkmoths as it does for the tiger moths — to startle, warn or jam up their predators.
A male and then a female produces ultrasonic trills after being harassed by researchers. Courtesy of Jesse Barber & Akito Kawahara, via ScienceNews.
The fact that the moths produce these sounds with modified genital structures suggests that the sounds might be used in mating behaviors, the team notes in their study, published recently in the journal Biology Letters. Scientists have previously documented male Privet Hawkmoths (Psilogramma menephron) producing shrill sounds while flying near females. "An intriguing possibility is that the ultrasonic ears in hawkmoths might have first evolved for mates, not bats," the researchers write.
Top image of the Yam Hawk Moth (Theretra nessus) via goldentakin/Flickr.