Monday, April 03, 2017

Bats Wiggle Their Way to Better Echolocation

While pursuing prey in complete darkness, horseshoe bats can zip through dense vegetation guided solely by sound. Their only protection from the raging headache—or worse—of a headlong collision is the sound waves entering their two pointy ears. New experimental research out of Virginia Tech shows that the horseshoe bat’s knack for rapidly navigating its environment is partly due to how it “wiggles” its nose and ears.

A sonar model of a horseshoe bat’s ears and noseleaf built in Rolf Müller’s lab. The model is similar to the one used in this research.
Image Credit: Virginia Tech.

In an upcoming paper in the American Physical Society’s journal Physical Review Letters, the team shows that the shape-shifting nose and outer ears of the horseshoe bat capture more information about the bat’s environment than would be possible if they were rigid. This additional information helps the bat pinpoint the source of an echo more precisely.

“Reproducing the dynamic properties of the bat biosonar system could enable the design of novel sensors that would give drones the same spectacular mobility skills in natural environments that bats have enjoyed for millions of years,” says Rolf Müller, a scientist from Virginia Tech who led the research team.

Many bats navigate and hunt via echolocation, emitting sounds either from their mouth or nose, depending on the species, and learning about their surroundings from the echoes. Dolphins use echolocation, as do porpoises and others in the toothed whale family, along with shrews and a few types of birds. Some people who are blind also use echolocation, and manmade sonar systems are used to image shipwrecks and map the ocean floor.

Horseshoe bats are so-named because of the horseshoe-shaped fold of skin that protrudes from their noses, called a noseleaf. The horseshoe bat emits pulses though its nose and the noseleaf acts as a kind of miniature megaphone that amplifies the noise. These bats are found in regions of Europe, Africa, Asia, Australia, and the Pacific Islands. They can be up to 14cm long and come in varying shades of brown. They are agile fliers with broad wings, capable of snatching insects in flight and hovering over surfaces to catch them.

Several years ago, Müller was taking pictures to document differently shaped bat ears and noticed that pictures of horseshoe bat ears always came out blurry. Looking deeper, he realized that this was because the shape of the ear wasn’t rigid as scientists assumed, but was constantly changing. This started him on a quest to understand how, why, and what we can learn from these wiggles.

“My group was the first to describe how certain bats change their ear and noseleaf shapes while emitting ultrasonic pulses/receiving their echoes. There was a 1960 paper indicating that these bats may change the shape of their ears, but the information was lost and the ear motions were assumed to be rigid ever since,” said Müller.

In follow-up work, the team showed that this movement actually changed the acoustic properties of the bat’s sonar system. In other words, the sound waves emitted by the nose are affected by the rapidly changing shape of the noseleaf, and the sound waves received by the ears are affected by the rapidly changing shape of the ear.

In this new research, the scientists analyzed these shape-shifting capabilities to see whether they improve a bat’s echolocation performance. It’s pretty hard to get data on this from live bats, so the team devised an approach using computer-based simulations and physical reproductions of ears and noseleaves. Both sets of models had noseleaves and ears that rotated quickly through five different shapes. The team mimicked the sounds emitted by bats and measured the way each of the models impacted the sound signal.

A video of the sonar head in motion.
Credit: Virginia Tech.

Using methods rooted in information theory, the team then analyzed the data to see whether the models with multiple shapes collected more information about the environment than the models with one shape—and they did. In fact, the experimental results suggest that horseshoe bats can more accurately determine the direction a sound is coming from and resolve that direction with finer detail (by a factor of 100 to 1,000, depending on the noise level) because of their shape-changing ability.

The noseleaf and ears of a horseshoe bat are not limited to taking on a few distinct shapes as in this simplified representation. In reality, they change shapes continuously while the bat is emitting a pulse or receiving an echo, so, as the researchers say, this study probably underestimates the value of their wiggling ability.

It is important to note, however, that the results were similar across all of the different models the team studied. According to the researchers, this suggests that the main advantage doesn’t come in refining the details, but in the way the system is structured. That bodes well for the possibility of recreating this kind of system for other applications.

“We believe (and have some evidence already) that the new dynamic principle for encoding of sensory information we have demonstrated could be useful for applications such as speech processing and navigation of drones through natural environments,” says Müller. “Bats can navigate even through the most cluttered natural habitats with ease. We see our work as an important step that will enable engineered systems to do the same some day in the future.”

Kendra Redmond

2 comments:

  1. "One set of models had noseleaves and ears that rotated quickly through five different shapes. The other set had noseleaves and ears that rotated quickly through five of the same shapes."
    Well, that technique will certainly tell you that you don't know what you are doing (or that the author of this report didn't read what he/she wrote).

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    1. Thanks for pointing that out! The piece has been edited for clarity.

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