Deaf butterflies use acoustic camouflage to escape bats

When we think of camouflage, we imagine a visual image – something that blends with the environment. This is because when most creatures are hiding, they want to be hidden. But if you were hiding from a bat, for example, it would not make much sense: bats do not "see" with their eyes, but rather with their distinct echolocation ability (think of a biological sonar). So, to hide from a bat, you will need a different mechanism.

That's what some people discovered a long time ago.

Mites are a mainstay of the bat menu and, of course, they would like to avoid being eaten. In response, some butterflies developed ears that detect the ultrasonic calls of bats, but others remained deaf – and seemingly powerless. But this is not entirely true: a new study has revealed that these insects develop a type of "stealth coating" used as acoustic camouflage to avoid hungry bats.

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Thomas Neil, of Bristol University, UK, explains how the fur of the thoracic joints and wing wings of a butterfly provides acoustic stealth by reducing the echoes of these parts of the body caused by calls from bats.

"Thoracic fur provides substantial acoustic stealth at all ecologically relevant ultrasound frequencies," said Neil, a researcher at the University of Bristol. "The moth fur of nocturnal moths acts as a light porous absorber, facilitating acoustic camouflage and providing a significant survival advantage against bats." Removing chest fur from the moth increased the risk of detection by 38%.

Neil used acoustic tomography to quantify the echo strength of two species of moths that are subject to predation by bats and two species of moths that are not. He was able to show that acoustic camouflage appears in both species of moths, but is absent in butterflies.

"We found that butterfly fur was both thicker and denser than that of butterflies and that these parameters appeared to be related to the absorption performance of their respective furs," Neil said. "The chest fur of the moths was able to absorb up to 85% of the incident sound energy. The maximum absorption found in butterflies was only 20%. "

A rotating 3D image of a butterfly ladder. This type of structure is responsible for acoustic camouflage. Credits: Thomas Neil.

It is not known when this mechanism would have appeared. The hairs on the thorax are essentially elongated scales (as on the wing), appeared about 200 million years ago, well before the appearance of bats (there are 65 million years old, "said Neil in an email. It is very difficult to say whether the emergence of bats has made butterflies more hairy.

But what seems obvious is that bats and butterflies are in a sort of arms race – as the butterflies develop their camouflage structure, the bats try to overcome it – but that's not the case. is not so easy.

"While some bats have changed the frequency of their calls to try to hide butterflies that have developed hearing, changing frequency to try to overcome the acoustic camouflage of butterflies would not work," Neil said. at ZME Science.

"This is because the absorption is broadband, the effect being constant on the frequencies we measured (20-160 kHz, the range used by most bats). One of the things that bats could do would be simply to issue stronger echolocation calls to try to get more powerful echoes back, but we have not done any field trials yet to see if that is the case. "

Further research will attempt to establish the frequency of this event and to determine if there is a difference between deaf and deaf moths. There is no reason for the stealth coating and the ability to hear to be mutually exclusive; Although deaf butterflies are under more evolutionary pressure to develop this type of ability, it would still be beneficial for them to be able to camouflage themselves acoustically, adds Neil.

"We only tested two the mites in this study of the Saturniidae family (Antherina Suraka and Callosamia prometheus). The study is a kind of proof of concept, we showed that the fur of the thorax could absorb ultrasound, but its degree of presence among the many night butterfly species is currently unknown. "

"We are currently working on quantifying the" madness "of the mites He concludes in different families to see if there is any link between the different forms of defense against bats.

Neil will describe his work at the 176th meeting of the Acoustical Society of America.

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