Safe and Ultrasound

Some moths have evolved special ears to detect the ultrasonic cries of bats.

A yam hawkmoth

Jesse Barber and Akito Kawahara

For moths and other nocturnal insects, the high-pitched clicks of hunting bats mean danger. Those flying mammals use echolocation to map their surroundings and find food: they emit ultrasonic sounds (sounds whose frequency is above the range of human hearing), which bounce off objects around them. In the nearly 65 million years that moths have been trying to avoid becoming a bat’s meal, some groups, including tiger moths and hawkmoths, have evolved special ears that alert them to the telltale ultrasonic cries of bats. Taking their defenses further, tiger moths (subfamily Arctiinae) can produce their own ultrasonic sounds to deter predatory bats. This has been shown to startle bats, warn them of the moths’ bad taste, fool them using acoustic mimicry, and jam their sonar. But how rare—or common—is this kind of ultrasonic defense among moths?

Behavioral ecologist Jesse R. Barber of Boise State University, Idaho, and evolutionary biologist Akito Y. Kawahara of the Florida Museum of Natural History, in Gainesville, decided to focus on hawkmoths (Sphingidae). To tune in to the auditory arms race, they toted sensitive electronic equipment into the high humidity of Malaysia’s tropical forests. Armed with ultraviolet lights, the duo gathered hawkmoths representing three species in the subtribe Choerocampina. They tethered each moth and placed an ultrasonic microphone next to it to document sound production. As the moths fluttered on their leashes, the researchers played both synthetic and actual recordings of bats’ sounds as they attacked insects. The team also recorded the moths’ reactions to touch.

The results were music to the researchers’ ears. Both males and females of all three hawkmoth species responded to touch and to bat calls by emitting ultrasonic clicks, which they produced using reproductive structures—suggesting that click production, as well as the moths’ ears, might have originally had a role in mating. Next up is to uncover the effect of the hawkmoths’ ultrasonic sounds on bats, to see if it works in the same ways as the defense in tiger moths. (Biology Letters)