Predator and prey are at evolutionary odds with each other: prey ever so keen not to be caught and eaten, predator always hopeful of a meal. Think cat and mouse. Bat and moth. Many species of bats are well known as flying carnivorous mammals that use echolocation to hear prey on the wing and catch dozens if not hundreds of night-flying insects to sate their gustatory needs. One might assume that aside from taking evasive action if they hear a bat chasing them, that many of the 160,000 known species of moths around the world are simply a passive flying feast for the bats. New research shows that this is not necessarily the case and that the hairs, or scales, on some species of moth may put them into acoustic stealth mode, making the insects almost invisible to the echolators.
"Moth fur is thin and lightweight," explains Thomas Neil of the University of Bristol in the UK, who has demonstrated that some moths have evolved an acoustic stealth mode. "The fur acts as a broadband and multidirectional ultrasound absorber that is on par with the performance of current porous sound-absorbing foams." Of course, the discovery of deeper understanding of such meta materials is always an inspiration for materials scientists looking for the next powerful biomimetic target for research. Ultrathin sound absorbers and other noise-control devices might be possible modeled on the moth's acoustic camouflage
Some species of moths have excellent hearing tuned to the ultrasonic frequencies of bat echolocation calls but many have no hearing at all. Neil spoke at the Acoustical Society of America's 176th Meeting, held in conjunction with the Canadian Acoustical Association's 2018 Acoustics Week, at the Victoria Conference Centre in Victoria, Canada during 5th to 9th November 2019. There he explained how the fur on a moth's thorax and wing joints provide acoustic stealth by reducing the echoes of these body parts from the bat calls.
"Thoracic fur provides substantial acoustic stealth at all ecologically relevant ultrasonic frequencies," explains Neil. "The thorax fur of moths acts as a lightweight porous sound absorber, facilitating acoustic camouflage and offering a significant survival advantage against bats." Without the fur, the moths are detected at a 38 percent higher rate by the bats, Neil's team has found. They used acoustic tomography to quantify echo strength in the spatial and frequency domains of two deaf moth species that are subject to bat predation and two butterfly species that are not. Moth thorax fur absorbed up to 85 percent of the impinging sound energy whereas this figure was only ever as high as 20 percent in the butterflies studied showing that they do not benefit significantly from acoustic camouflage.
"We have shown that the furry thorax's of moths reduces the strength of echoes returning to bats by absorbing the sound energy," Neil told Materials Today. "The Lasiocampidae are a particularly furry group of moths and do not have ears, meaning the performance of the thorax fur to absorb ultrasound would be key to their survival chances against bats."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.