How hawkmoths jam the sonar signals from bats

In the dark caves and jungles of the world, there's an epic 65-million-year evolutionary battle going on between bats and moths.

Moths are obviously the underdogs. But scientists are learning that moths have found ingenious ways to counter the weapons of the bat.

The latest discovery is that the hawkmoths may be jamming the sonar signals bats use to track and catch moths in flight. How do they do it?

According to an article in Nature, behavioral ecologist Jesse Barber of Boise State University in Idaho and phylogeneticist Akito Kawahara of the University of Florida in Gainesville went to Borneo to study how hawkmoths defend themselves.

"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."

This is the second species of moth known to man to find a way to jam the echolocation system employed by bats.

In a 2009 article in the Scientific American, Aaron Corcoran, a biology PhD student at Wake Forest University and the lead author of the paper about how tiger moths jam bat sonar, described how scientists studied and tested behavior of the Bertholdia trigona against the big brown bat Eptesicus fuscus, using high-speed infrared cameras and an ultrasonic microphone to record the action over nine consecutive nights.

"Normally, a bat attack starts with relatively intermittent sounds. They then increase in frequency—up to 200 cries per second—as the bat gets closer to the moth "so it knows where the moth is at that critical moment," Corcoran explains. But his research showed that just as bats were increasing their click frequency, moths "turn on sound production full blast," clicking at a rate of up to 4,500 times a second. This furious clicking by the moths reversed the bats' pattern—the frequency of bat sonar decreased, rather than increased, as it approached its prey, suggesting that it lost its target."

But sonar jamming is not the only weapon in the moth's self-defense system. Researchers have known for 50 years that moths have an early warning system: They can hear the ultrasonic hunting calls of their nocturnal predator.

In a 2006 paper in Current Biology, Dr. James Windmill who was at the University of Bristol, England, at the time, wrote that not only was the simply designed moth ear detecting the echolocation pulses of the bat, but it was actually fine tuning its receptivity when under attack.

He noted that the hearing of Noctuid moths is most sensitive to frequencies at 20–40 kH, which put it at the lower range of the ultrasound frequencies used by bats. And as bats move in closer, both the loudness and the frequency of the bat's sonar calls increase. And the moth responded.

"The moth's ear mechanically tunes up and anticipates the high frequencies used by hunting bats," wrote Dr. Windmill.

Check out a rather cool video depicting what an underwing moth does once he hears that there's a bat in hot pursuit.

An acoustical engineer now at the University of Strathclyde in Scotland, Windmill has continued to study moths. His latest research, published in May in Biology Letters, reports that greater wax moths can hear higher frequencies than any other animal on earth, up to 300 kilohertz. 

The highest known frequency of bat echolocation calls is 212 kHz, but some scientists have suggested that bats are evolving higher frequencies or switching frequencies in order to confuse moths.

“A lot of previous work has suggested that some bats have evolved calls that are out of the hearing range of the moths they are hunting. But this moth can hear the calls of any bat,” Windmill told Nature.

And so the epic evolutionary war goes on outside the audible range of some very interested human spectators.