Shh! How Do Body Scales Protect Larger Moths from Echolocation?
Featured Image Caption: This study surveyed 58 species of moths across 8 families, including these four. From upper left to bottom left, clockwise: Purple Thorn (Selenia tetralunaria), Small Elephant Hawkmoth (Deilephila porcellus), Puss Moth (Cerura vinula), and Peach Blossom Moth (Thyatira batis). (The Purple Thorn and Peach Blossom Moth are by lastovka, the Small Elephant Hawkmoth is by mazziep, and the Puss Moth is by Nikolai Vladimirov. All photos are licensed under CC BY-NC 4.0 and have been cropped from originals.)
Featured Article: Simon, R., Dreissen, A., Leroy, H., Berg, M. P., & Halfwerk, W. (2023). Acoustic camouflage increases with body size and changes with bat echolocation frequency range in a community of nocturnally active Lepidoptera. Journal of Animal Ecology, 92(12), 2363–2372. https://doi.org/10.1111/1365-2656.14016
Avoiding predation is incredibly important, and animals have evolved a variety of strategies not only for escaping predators, but for not being detected in the first place. Beyond being speedy or hiding well, animals have a myriad of lesser-known techniques! Recently, scientists explored how body scales keep moths hidden from bat echolocation, and how the effectiveness of the scales differs for larger or smaller moths.
A different kind of camouflage
Many animals use camouflage to blend in with their surroundings. For example, some hares and weasels have white winter coats to match the snow, and some octopi and squid can even change colors to match their environment. The brown coloration of animals like squirrels, deer, and many birds helps them hide in forests or within undergrowth.
However, some predators primarily use other senses besides vision to hunt. Bats, for example, use echolocation—they send out pulses of high frequency sound, and listen for the echoes produced when these pulses bounce off of items in their environment. As a result, their prey may turn to a different sort of camouflage, called acoustic camouflage, to stay hidden. This generally means “muffling” the echo in some way, making it difficult for the bat to figure out where the prey actually is.
Moths are covered in tiny scales on their wings and bodies, which have functions including keeping the moth warm and stopping them from sticking to substances like spider silk. However, these scales may also function as a method for acoustic camouflage: they absorb bat echolocation calls rather than reflecting them, making it hard for the bat to pinpoint the moth’s location.
A group of scientists from the Netherlands and Germany wanted to know whether the scales on larger moths were more effective than the scales on smaller moths at providing acoustic camouflage. They specifically focused on the body scales, which are hair-like rather than the wings’ flatter scales. Moth species vary widely in size, and being larger has benefits including being linked to a greater ability to reproduce. However, larger moths are easier for bats to detect than smaller moths, simply because they are a larger target for echolocation pulses to hit. Additionally, bats tend to prioritize chasing larger prey over smaller prey. As a result, larger moths may compensate for this increased risk by evolving better defenses against predators compared to their smaller counterparts.
Mimicking and measuring echoes
The scientists collected 111 moths from different sites in the Netherlands, representing 58 species across eight families. To measure how much of an echo each moth produced when it was hit with a high frequency signal—designed to represent a bat’s echolocation—they set up a “sonar head” which would emit pulses at a mounted moth and measure the sound which bounced back.
Different bats use different frequencies for their echolocation calls, so the researchers experimented with a range of frequencies ranging from 19 to 144 kHz. (For reference, humans generally can’t hear sounds higher than 20 kHz.) They repeated the process before and after removing the scales from each moth’s body, in order to test whether having scales decreases the amount of echo detected by the sonar head.
How do scales differ between species?
The scientists found that the scales of almost all of the tested moth species absorbed the high frequency pulses, which indicates a reduced ability for bats to detect them with echolocation. Notably, the scales for larger moths provided a greater amount of acoustic camouflage compared to that of smaller moths. As an illustration, it was estimated that if a moth were five times greater than a second moth, it would be 100% more at risk of detection if it did not have scales. However, because of the scales, this moth actually only has a 25% increased detection risk. This suggests that due to the increased risk of predation that larger moths face, their scales have evolved to be more effective in keeping the moth undetected by echolocation.
The group also discovered that the scales were more effective at providing camouflage at certain frequency ranges. The best absolute protection against detection was found in the middle range of frequencies, which makes sense as the majority of bat species in the study region use echolocation signals in this range. However, moth species varied in which frequency range they were best camouflaged against, likely to account for the frequency used by their specific common predators.
So despite being hidden from both human sight and hearing, the unique way bats hunt leads to novel ways for moths and other insects to avoid predation—and scientists are still exploring further intricacies of how moths keep themselves hidden.
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