Scientists in the UK have discovered that the fur of some moths can absorb up to 85% of incoming ultrasound. According to the researchers, this fur acts as a “stealth coating” providing the moths with a passive, acoustic camouflage that helps hide them from the ultrasonic clicks of insect-hunting bats. The team says that the fur could inspire the development of biomimetic materials for ultrathin sound absorbers and other noise-control devices.
Bats love to eat moths, which they hunt down using a biological sonar technique, known as echolocation. In the arms race between predator and prey, some moths have evolved ears so they can hear the ultrasonic calls of bats and take evasive action. But it turns out that others have evolved a more passive defence in the form of acoustic camouflage.
In work presented earlier this week at the Acoustical Society of America’s 176th Meeting in Canada, Thomas Neil and colleagues at the University of Bristol used acoustic tomography techniques to compare the sound absorbing properties of two species of deaf moth – the Madagascar bullseye moth and the promethea silkmoth, which are preyed on by bats – to two species of butterflies, which are not bat prey.
At bat echolocation frequencies, the team found that the thorax fur of the moths acted as an acoustic camouflage, absorbing up to 85% of the incoming ultrasound. In contrast, most butterflies absorbed just 20% of incoming ultrasound.
“We tested the absorption from 20-160 kHz, this covers the frequencies that most bats use to hunt their prey, with most calling from 20-60 kHz, whilst there are a few that go to higher frequencies,” Neil explains. “We found the absorption to be remarkably consistent across the range tested, with no apparent frequency dependence in the effect on absorption.”
When fur was removed from a moth’s thorax the researchers discovered that the chance of the moth being detected increased by up to 38%. Neil told Physics World that this is due to the change in the “detection volume”.
“We are able to calculate the change in detection distance by which a bat would be able to detect a moth with it’s fur intact and a moth with it’s fur removed,” Neil explains. “We do this for numerous angles which allows us to calculate a detection volume, which is a volume of 3D space within which the bat would be able to detect the moth.”
Porpoise’s forehead acts like an acoustic metamaterial
When the team examined the insects, they found that the fur on the moths was both thicker and denser than that of the butterflies. Neil says that the structure and layout of the moth fur is similar to that found in natural fibres like hemp and kenaf that are used in sound insulation, but on a much smaller scale. These natural fibres act as porous sound absorbers dissipating sound energy as the sound waves enter air filled cavities in the materials.
“We did some modelling to see if the moth fur behaved as a porous sound absorber and found good agreement between the predicted and measured absorption,” Neil says. “The moth fur is porous enough to allow the sound to penetrate the material without reflecting it back to the bat.”
Significant survival advantage
According to the researchers, by acting as a lightweight porous sound absorber the thorax fur of the moths provides acoustic stealth at all ecologically relevant ultrasonic frequencies, facilitating acoustic camouflage and offering a significant survival advantage.
They also found that the performance of the moth fur as a broadband and multidirectional ultrasound absorber is on par with current porous sound-absorbing foams, and say that it could inspire the development of ultrathin sound absorbers and other noise-control devices.