The Acoustic Arms Race: Deaf Moths Versus Hunting Bats.

The evolutionary arms race between predator and prey has led to some incredible adaptations in the animal kingdom, some of which give the advantage to the hunter looking for its next meal, whilst others give the advantage to the prey hoping to escape and live another day. Recently, scientists from the University of Bristol have discovered that several moth species have developed sound-absorbing scales on their thorax - swinging the balance of life and death their way against echolocating bats.

Echolocation is the production and subsequent detection of soundwaves with the purpose of determining where objects are in space. Bats use echolocation to navigate and locate food in the darkness by listening to the echoes that return to them; and from this information they can figure out where an object is, how big it is, and its shape.

Whilst humans have only been able to use echolocation (through sonar and radar systems) for just over 100 years, the ability to echolocate in bats evolved approximately 65 million years ago. Since then, bats have tuned the ability to such a degree that they can detect objects as thin as human hair in pitch-black conditions, far superior to most modern submarines or aeroplanes.

Humans cannot hear the ultrasonic soundwaves produced by echolocating bats but the majority of their prey can. When they hear these sounds, insects like moths, beetles and crickets will instinctively change their flight pattern to avoid being picked off mid-air; however, as bats are auditory rather than visual hunters, any adaptation that reduces the chances of an insect being detected by echolocation would be a vital survival advantage.

Whilst some species of moth have evolved ultrasound-sensitive ears that serve for the detection and evasion of echolocating bats, others have evolved sound-absorbing hair-like scales which act as 'stealth acoustic camouflage'. Instead of the soundwaves bouncing off the moth and returning to the hunting bat, soundwaves are partially absorbed and partially deflected away from the predator.

Recently, a team of scientists from the University of Bristol's School of Biological Sciences analysed these sound-absorbing thoracic scales of two species (Antherina suraka and Callosamia promethea) through scanning electron microscopy and discovered that their structure is remarkably similar to that of natural fibrous materials commonly used in sound insulation.

Upon further testing with practical soundwaves being generated (imitating an echolocating bat), they found that the scales absorb as much as 85% of incoming sound energy and can reduce the effective distance of moth detection by nearly 25%.

The scientists believe that the piliform structure of the scales (the hair-like appearance as seen below) are stacked in such a way that air bubbles are trapped between them. This network of air pockets vibrates when in contact with soundwaves, which causes them to lose energy - essentially muffling the image that echolocates back to the bat.

"We were amazed to see that these extraordinary insects were able to achieve the same levels of sound absorption as commercially available technical sound absorbers, whilst at the same time being much thinner and lighter" said lead author, Dr Thomas Neil.

The next step in this research is to determine whether or not there are similar scales with sound absorption properties on the moth's wing as well as considering if biomimicry can lead to new and improved sound insulation technology.

Regarding the next step in this arms race, as with any evolutionary adaptation, the bat that hunt these moth species may develop a new type of ultrasound echolocation that counteract their prey's stealth acoustic camouflage.

Reference: Thomas R. Neil, Zhiyuan Shen, Daniel Robert, Bruce W. Drinkwater, Marc W. Holderied. Thoracic scales of moths as a stealth coating against bat biosonar. Journal of The Royal Society Interface, 2020; 17 (163): 20190692 DOI: 10.1098/rsif.2019.0692