New research from the ÌìÃÀÓ°ÊÓ´«Ã½ suggests that the Egyptian fruit bat is using similar techniques to those preferred by modern-day military and civil surveillance. The results could inspire new directions for driverless cars and drones.

The new open-access in PLOS Biology shows how the animals are able to navigate using a different system from other bats.

“Before people thought that this bat was not really good at echolocation, and just made these simple clicks,” said lead author , a researcher at the UW’s Applied Physics Laboratory. “But this bat species is actually very special — it may be using a similar technique that engineers have perfected for sensing remotely.”

While most other bats emit high-pitched squeals, the fruit bat simply clicks its tongue and produces signals that are more like dolphin clicks than other bats’ calls. Fruit bats can also see quite well, and the animals switch and combine sensory modes between bright and dark environments.

An showed that Egyptian fruit bats send clicks in different directions without moving their head or mouth, and suggested that the animals can perform echolocation, the form of navigation that uses sound, better than previously suspected.

“But no one knew how they do it, and that’s when I got excited, because there’s something going on that we don’t understand,” Lee said.

Lee and colleagues measured the animals in the “” at Johns Hopkins University by capturing high-speed video and ultrasonic audio of bats during flight to study the mechanism of their behavior and navigation.

In measuring echolocation signals from fruit bats with a three-dimensional array of microphones, Lee did not solve the mystery of the seemingly motionless tongue clicks, but she did notice something strange. The beam of different frequencies of sound waves emitted by the bats do not align at the center and form a bullseye, as one would expect from a simple sound source, but instead the beam of sound is off-center at higher frequencies.

Lee recognized the pattern as a common one in radar and sonar surveillance systems. Invented in the early 20th century and now used throughout civil and military applications, airplanes, ships and submarines emit pulses of radio waves in the air, or sound underwater, and then analyze the returning waves to detect objects or hazards. While a simple single-frequency sonar has a tradeoff between the angular coverage and image sharpness, a “frequency-scanning sonar” solves this problem by pointing different frequencies of sound at slightly different angles to get fine-grained acoustic images over a large area.

Lee wondered if the fruit bats could be using the same technique when echolocating. She created a computer model of what might happen when the tongue click from the front of the mouth travels out and passes between the bat’s lips. The elongated shape of the bat’s mouth creates varying distances between the sound source and the gaps between its teeth, and this creates positive or negative interference between sound waves of different frequencies. The result, Lee’s model shows, is that different frequencies point in different directions — just as a frequency-scanning sonar would act.

“For me, what’s exciting is the idea that you almost have a convergence between a system that was evolved, and the effects are very similar to what we have invented as humans,” Lee said. “This is not the classic case where we learn from nature — we found out that the bat may be doing the same thing as a system we invented many years ago.”

(Sound doesn’t have to pass between the bat’s teeth, just through its lips, as the researchers discovered from one toothless bat.)

After doing calculations with a rough approximation of the bat’s skull shape, Lee worked with co-author in the UW Department of Biology to get a CT scan of an actual fruit bat’s skull. Incorporating an anatomically correct skull shape in the model confirmed the results. Though Lee can’t say exactly what’s happening inside the bat’s mouth when a tongue click is produced, she believes her model suggests that could be how the bat creates its sonar beams.

This mechanism might be a simple evolutionary solution — the Egyptian fruit bat looks like closely related bat species that don’t echolocate, and also has large eyes. This means the shape of its head has not changed through evolution. From an engineering point of view this simplicity offers a similar benefit.

“You don’t have to do anything, you just have to get the distance right. It’s by design,” Lee said. “This may be a way to produce a very cheap sensor that has this kind of sensing capability.”

Other co-authors are Benjamin Falk, Chen Chiu and Cynthia Moss at Johns Hopkins University and Anand Krishnan at the Indian Institute of Science Education and Research, Pune. The research was funded by a UW-APL SEED postdoctoral fellowship, an Acoustical Society of America postdoctoral fellowship, the U.S. National Science Foundation, the U.S. Office of Naval Research, the U.S. Air Force Office of Scientific Research, the France-based Human Frontier Science Program, and the Government of India.

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For more information, contact Lee at wjlee@apl.washington.edu or 206-685-3904. Find more .

NSF grant IOS1460149; ONR grant N00014-12-1-0339; AFOSR grant FA9550-14-1-0398; HFSP grant RGP0040

Researchers at the ÌìÃÀÓ°ÊÓ´«Ã½ and Johns Hopkins University took a detailed look at the acoustics of the common luna moth, to see how long tails could throw off predators that use echolocation to pursue prey. Results published in the suggest a strategy for how even a fairly small tail could confuse bats on the hunt.

“The interesting thing about these tails is they are not just extensions — there is a twist toward the end,” said first author , a researcher at the UW Applied Physics Laboratory who did the study as a postdoctoral fellow at Johns Hopkins. “We think that twist could be a key for how the tails function acoustically.”

The shows that without any tail, the echo center is a bullseye right on the moth. But the twisted tail creates an echo from all directions that tends to shift the echo cloud past the tip of the moth’s body. With the tail’s reflection, about 53 percent of the time the echo center from experimental chirps fell past the tip of the moth’s abdomen.

“If the bat always aims for the highest-amplitude echoes, there’s a very small percentage of the time that the tail echoes would be dominant,” Lee said. “But maybe by displacing the echo center, that can do the trick.”

Striking patterns on some butterfly wings are well-studied visual decoys that have evolved to confuse birds and other daytime hunters. The new paper is part of emerging research that explores acoustic camouflage in moths and other nocturnal creatures.

A 2015 led by Boise State University found that the big brown bats are about 47 percent more successful at hunting Luna moths that have lost their tail, showing that the moth’s extended tail somehow helps it survive. Those authors believe that the tails serve a role in acoustic deflection, and show that moths have evolved extended tails independently on different continents, suggesting it offers a key advantage.

The new research, carried out in parallel with the 2015 study, explores the acoustics in more detail. To analyze how sound waves bounce off the moth, the researchers aimed short chirps similar to the ultrasonic pulses that bats use to navigate and capture insects. The pulses had frequencies that cover the hearing range of bats and are beyond the range of human hearing.

While the 3-millisecond experimental chirps provided only fuzzy echoes from the tethered flying moths, Lee applied common signal processing techniques to sharpen the resolution to 1 centimeter, about a third of an inch, which gives a clear image of a 10-centimeter luna moth.

The goal was to create a bat-centered view of the moth, although Lee cautioned it’s not necessarily an exact match.

“We don’t know what type of signal processing the bats are using,” she said.

By analyzing the returned echoes and comparing the signal strength with video footage of the flying moth, they found the tail doesn’t provide a strong false target to replace the moth’s body. This is not surprising, since the tail is much smaller than the abdomen or wings.

But the echo off the wings varies a lot depending on where in the moth’s wingbeat the chirp strikes. If the wings are perpendicular to the incoming chirp sound waves, it creates a big echo, but if they are parallel the wings offer a very small target. The moth’s twisted tail, on the other hand, provides a consistent acoustic response regardless of the angle, which could create confusion around the varying main echo.

“No matter which angle you hit the tail, you usually have some area,” Lee said.

And if a bat was confused about the exact position of its prey, it might go for the center of several echoes. The twisted tail significantly throws off such an estimate, meaning the bat is more likely to miss.

“A moth is a very complicated object in space,” Lee said. “It could be difficult for a bat to track each individual point of the echo cloud. It would be much easier for it to say, ‘There’s a ball of echoes coming back, I’m going to hit the center of it, and maybe I’ll catch something.'”

https://youtu.be/6D-h3jkoHvc

To further evaluate this theory, Lee would like to learn more about how the tail influences moths’ flight, and study how different tail structures in other species of moths affect their survival rate against predatory bats.

“This study provides part of the clue, but we don’t have the full answer yet,” Lee said.

The research advances understanding of predator-prey interactions, insect behavior and evolution. The study could also shed light on how to track sonar targets — or evade sonar detection — in other settings, Lee said.

The other author is , a neuroscientist at Johns Hopkins University. The research was funded by the National Science Foundation, the U.S. Air Force Office of Scientific Research, the international Human Frontier Science Program and the Acoustical Society of America.

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For more information, contact Lee at wjlee@apl.washington.edu or 206-685-3904. Note: Lee will be out of the office Aug. 19-27 with limited access to email.

Grant numbers: NSF: IOS-1010193, HFS: FA95501210109 AFOSR: FA9550-14-1-0398