Sound-absorbent wings and fur help some moths evade bats

Some moths aren’t so easy for bats to detect.

The cabbage tree emperor moth has wings with tiny scales that absorb sound waves sent out by bats searching for food. That absorption reduces the echoes that bounce back to bats, allowing Bunaea alcinoe to avoid being so noticeable to the nocturnal predators, researchers report online November 12 in the Proceedings of the National Academy of Sciences.

“They have this stealth coating on their body surfaces which absorbs the sound,” says study coauthor Marc Holderied, a bioacoustician at the University of Bristol in England. “We now understand the mechanism behind it.”

Bats sense their surroundings using echolocation, sending out sound waves that bounce off objects and return as echoes picked up by the bats’ supersensitive ears (SN: 9/30/17, p. 22). These moths, without ears that might alert them to an approaching predator, have instead developed scales of a size, shape and thickness suited to absorbing ultrasonic sound frequencies used by bats, the researchers found.
The team shot ultrasonic sound waves at a single, microscopic scale and observed it transferring sound wave energy into movement. The scientists then simulated the process with a 3-D computer model that showed the scale absorbing up to 50 percent of the energy from sound waves.

What’s more, it isn’t just wings that help such earless moths evade bats. Other moths in the same family as B. alcinoe also have sound-absorbing fur, the same researchers report online October 18 in the Journal of the Acoustical Society of America.
Holderied and his colleagues studied the fluffy thoraxes of the Madagascan bullseye moth and the promethea silk moth, and found that the fur also absorbs sound waves through a different process called porous absorption. In lab tests, the furry-bellied moths absorbed as much as 85 percent of the sound waves encountered. Researchers suspect that the equally fluffy cabbage tree emperor moth also has this ability.

Other moths that have ears can hear bats coming, and can quickly swerve out of the way of their predators, dipping and diving in dizzying directions (SN: 5/26/18, p. 11). Some moths also have long tails on their wings that researchers suspect can be twirled to disrupt bats’ sound waves (SN: 3/21/15, p. 17). Still other moths produce toxins to fend off foes.

Having sound-absorbent fur and scales “might require a lot less energy in terms of protection from the moth’s side,” says Akito Kawahara, an evolutionary biologist at the Florida Museum of Natural History in Gainesville who was not involved with the study. “It’s a very different kind of passive defense system.”

Holderied and his colleagues hope next to study how multiple scales, locked together, respond to ultrasonic sound waves. The findings could one day help in developing better soundproofing technology for sound engineers and acousticians.