The night skies are a dangerous place for moths: they make tasty, nutritious snacks for hungry bats. But first the agile mammals must detect their prey, guided in for the capture by the high-pitched sounds they emit, which reflect off objects back to their ears when hunting in the dark. This skill, called echolocation, is especially dangerous for moths without ears that cannot detect bat calls. However, earless moths may have come up with some surprising defenses against attack. Moths use colored hair-like scales arranged in patterns on their body for visual camouflage, and it turns out these same scales can also deafen a bat searching for its next meal.
Thomas Neil and co-authors at the University of Bristol, UK, compared the scales of two species of bat-deaf moths and two species of butterflies – which fly during the day and probably cannot hear bat echolocations – using a scanning electron microscope. They noticed that the arrangement of the moths’ layer of hair-like scales on the thorax was similar to the fibrous materials used for noise insulation; the scales are stacked in parallel like long hairs on a pelt, with porous spaces between them to trap air. To test whether this fuzzy layer of scales absorbed bat echolocation calls, the team built a model bat using a speaker for the mouth and a microphone that recorded the echoes for the ears. They then placed the moths and butterflies in different positions around the simulated bat to measure what the bat would hear when approaching its prey from different directions. Next, they removed the hair-like scales from the moths and butterflies and repeated the measurements to test the effect on the echolocation reflections, before finally using the microphone recordings to recreate the soundscapes that the bat should hear.
It turned out that the descaled moths made better targets for bat echolocation calls than the fully fuzzed insects. The bats had to be about 10–20% closer to a moth when its scales were intact to detect it, which gives the insects a better chance of flying by unnoticed. Unlike for the moths, removing the hair-like scales from the butterflies had little effect on the strength of the sonar signals they generated, likely because they have no need to conceal themselves from bats while active during the day. In addition, when the scientists compared their scale layers, they found that the butterflies’ layer was too thin to deaden bat calls, but that of the moths was just thicker than necessary, absorbing up to 67% of the sound. More surprisingly, they found that the moths’ sound-deadening scales outperformed most commercial fiber-based noise-proofing materials.
Even though some moths cannot hear, they still manage to elude the sneaky sonar of bats by donning a thick coat of hair-like scales to absorb and deaden their echoes. Locked in an evolutionary arms race with their lethal foe, moths have produced a bio-material that muffles sound better even than many of the engineered sound absorbers we use today. Maybe one day engineers will be able to take the lessons learned from the moths’ unconventional fuzzy muffler to design better noise proofing for us all.