The evolutionary arms race between predator and prey has given rise to many impressive defence mechanisms; as it turns out, the lowly moth is no exception.
These unassuming creatures may not have the charming appearance of their butterfly cousins, but their furry facade conceals some tricks that would impress the most advanced military division.
Over millions of years, moths have evolved a sneaky form of camouflage that’s reminiscent of stealth bomber technology – all in an effort to resist the clutches of their main predator, the bat.
Bats are unusual predators because they use echolocation – a series of high frequency calls bounced off their surroundings – instead of sight to track down their prey. The resulting sound- and hearing-based competition between moths and bats has long fascinated scientists.
But while some moths have evolved ears to detect a bat’s ultrasonic calls, many remain deaf to their approaching demise.
In moths that are hard of hearing, researchers have now discovered a “stealth coating” that acts sort of like acoustic camouflage.
Just like stealth technology is used to make an airplane invisible to radar, the fur on a moth’s thorax and wing joints was found to leave it largely impervious to bat echolocation.
“Thoracic fur provides substantial acoustic stealth at all ecologically relevant ultrasonic frequencies,” says lead author Thomas Neil, a researcher in bioacoustics at Bristol University.
“The thorax fur of moths acts as a lightweight porous sound absorber, facilitating acoustic camouflage and offering a significant survival advantage against bats.”
When bats echolocate, they call out into their environment and listen for the returning echoes from nearby objects.
By absorbing these ultrasonic calls, the fur on moths reduces the sound that can bounce off of them, making it more difficult for bat’s to locate their prey in the all-consuming darkness.
To figure out how effective this stealth coating truly is, the researchers compared the acoustic properties of two deaf moth species that are preyed on by bats, and two butterfly species that are not.
When the team removed the fur from the thorax of the butterflies and measured the sound bouncing off, they figured it was not used as part of camouflage. But removing the fur from a moth’s thorax increased its detection risk by as much as 38 percent.
“We found that the fur on moths was both thicker and denser than that of the butterflies, and these parameters seem to be linked with the absorptive performance of their respective furs,” Neil says.
“The thorax fur of the moths was able to absorb up to 85 percent of the impinging sound energy. The maximum absorption we found in butterflies was just 20 percent.”
The unique coating provides deaf moths with a significant survival advantage against bats. It also offers scientists inspiration when it comes to developing ultra-thin sound absorbers and other noise-control devices.
“Moth fur is thin and lightweight,” explains Neil, “and acts as a broadband and multidirectional ultrasound absorber that is on par with the performance of current porous sound-absorbing foams.”
This study has been published in the Journal of the Acoustical Society of America.