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Speaking of Bats: Bat language discovered by neurobiologist who studies auditory system

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The Kono people of Sierra Leone, Africa have a wonderful legend that explains why bats fly at night.

According to the Kono, long ago when the earth was new there was no darkness or cold. Life was blissful, warm and light until one day a bat was given the task of flying a mysterious basket to the moon. Unbeknownst to the bat, the basket contained darkness!

Dr. George Pollak, a member of the Institute for Neuroscience, is shown here in an Austin-area barn where he records the social communication of Mexican freetail bats.

video icon Watch Dr. Pollak discuss the language of bats by clicking on the photo above.

Hoisting the secret cargo on his back, the bat took flight, bound for the sky. Soon the load became onerous, and he found that he must stop, unburden himself of the heavy bundle and venture off to find food and get much-needed rest. While he was away, other animals came upon the basket, and, curious, they raised the lid to peek inside. Out leapt the darkness!

According to the Kono, since that time bats have rested by day and spent their evenings rushing through the night skies, trying to catch all of the bits and pieces of darkness, put them in the basket and fly them to the moon.

Unlike the Kono, who were interested in explaining the “why” of bat night flight, Dr. George Pollak, a neurobiologist in the College of Natural Sciences at The University of Texas at Austin, has devoted decades to studying the “how.” Even though his findings don’t include a perilous flight to the moon or a heavy basket of darkness, the facts he has unearthed in the past couple of years regarding bats’ sophisticated navigational tools are as fascinating as folklore.

In the course of looking at bats’ auditory systems, which are responsible for their ability to navigate with speed and dexterity through pitch-black night skies, Pollak discovered that they seem to have a surprisingly complex language.

“Bats are very social animals—they live in enormous colonies, in the dark and they do not see in the manner that we do,” Pollak said. “They have to be able to communicate with one another, however, and orient themselves in time and space.

“Males have to get together with females. Babies have to be able to find their moms. A male has to be able to tell another male to get out of his territory. Amazingly, this is not done with crude little squawks. These animals are composing extremely elaborate songs, made up of syllables that merge into phrases and are repeated in particular sequences, with rules and what seems to be some kind of syntax. And they are situation specific. Finding this method of social interaction was stunning.”

As interesting as the finding is, revealing it without describing a little bit of the journey that led up to it is like opening a joke with the punch line.

Bat sits in little denim pouch

Barbara French has created cloth pouches for each of the 70 bats that live in her barn.

In this case, the story begins about 40 years ago with a bored undergraduate sitting in his first psychology course, about to have an epiphany that would set the course of his professional life.

While reading his psychology textbook the young ennui-burdened Pollak came upon a chapter devoted to the biological control of behavior and read about experiments being conducted on animals. If a portion of an animal’s brain was damaged or stimulated, unsettling things would occur—the animal might begin to eat uncontrollably or kill viciously.

“It occurred to me for the very first time that all of these things that were roiling around in me—anger, fear, appetites, joy—are a matter of something biological, something that’s happening in my brain,” said Pollak. “I had never thought of it that way before, and I found this to be the most interesting thing I’d ever heard. That’s how I got turned on to study of the brain.”

Pollak’s description of the brain’s mysteries and the questions surrounding humans’ sensory experience of the world has an enigmatic, almost Zen quality. As Pollak points out, each of us “sees” the world through our sensory receptors—our eyes, ears, nose, touch—and those are what inform us about our environment. What we define as our reality, however, is nothing more than a construct the brain has created.

“There is so much in the world that goes on that you’re completely unaware of,” Pollak said. “Reality is something that your brain is creating. For example, there is no such thing as color. If you had different sorts of receptors, you’d see different kinds of colors in your world, or if you had no receptors at all, you’d see no color. Your world is what your brain is designing for you.”

Most people may not realize just how much is unknown about the auditory system, for example, and about how the brain takes sound and gives it meaning for us. This portion of the mystery is what Pollak has devoted most of his career to studying.

Bat swoops down on cactus plant

Bats “see” their world through their ears. They send out loud ultrasonic calls then use the echoes they hear bouncing back from objects around them to navigate through darkness.

Photo: © Merlin D. Tuttle, Bat Conservation International

Scientists know that sound comes in through the inner ear and is changed from pressure waves, or mechanical information, to electrical impulses, which is the language of the nervous system. The electrical impulses go through an intricate circuitry, making contacts with nerve cells. Each of the nerve cells extracts something different from this common sound signal that came in and sends messages into the brain where other comparisons are made. All of this processing is occurring when one hears a dog bark, a bartender ask for a drink order, a motorcycle rev its engine or a door slam shut. Even more amazingly, this happens when you are hearing all of these things at once, each occurring in a different location and with varying degrees of loudness.

“All of this processing and sorting is done in a low level of the auditory system—the inferior colliculus—and that is what I am looking at,” Pollak said. “I am trying to ascertain how the information is sorted before it goes to higher regions, what is processed, how the information is changed and why. It seems that at the cortical level you begin to make perceptions of the things that you hear, but no one really knows anything about that.”

So where do the bats come into all of this?

Bats have auditory systems that are structurally and functionally similar to those of most other mammals, with the same chemistry and wiring, and they have prototypically mammalian brains. If a researcher like Pollak advances understanding of the bat’s brain and auditory system, scientists’ understanding of all mammals’ brains will be enhanced. Also, the bat’s auditory system is quite large in comparison to the rest of its brain, which makes it easy for researchers to study.

Pollak examines the world of bats from a sensory physiologist’s perspective, and for him it is very helpful to use a mammal for whom he “knows what’s important,” as he phrases it. By this Pollak is referring to the fact that scientists already know that bats use their exceptional hearing to locate food, for example, or navigate through their environment without colliding into other objects. Their highly advanced auditory systems are the key to their survival, or, as Pollak puts it, “the way they make their living.”

If I could explain how your brain allows you to focus on my voice, make sense of what I'm saying, write it down in a notebook and nod your head in agreement, I'd be in Stockholm accepting a Nobel. -Dr. George PollakTo create three-dimensional pictures of the world, bats “see” with their ears. They emit loud ultrasonic calls, then use the echoes that they hear bouncing back to them to recreate images from their environment. This technique is called echolocation and is one of the wonderful tweaks that nature gave bats which has allowed them to proliferate at almost every point on the globe and exploit food sources that other animals’ don’t or can’t access.

With this biological sonar they can zip around through the night sky, migrate hundreds of miles to a specific cave, feed on insects, detect a fish breaking the surface of the water just from the sound of the ripples and hear a beetle’s footsteps.

In his lab Pollak tries to determine how the bat’s brain organizes this barrage of information and represents it so that the animal can make sense of features of the external world through neuronal activity. Pollak hopes to find out how the brain processes the plethora of signals it is receiving and why it chooses to respond to some signals and not to others.

To do this, he has observed and studied bats from any number of colonies, but not until recently did the hand of fate point him in the direction of the most unusual colony he had ever seen and the source of his most surprising discovery.

“I met a woman named Barbara French a few years ago,” said Pollak, “And she informed me that she had a colony of around 70 Mexican freetail bats, on her land on the outskirts of Austin, that she’s nursed back to health and kept. She’s built a barn for her colony, named all of the bats, sewn pouches for them to live in, knows their habits, feeds them every night, understands them, listens to them and has an intuitive feel for them that just cannot be taught. I bring in scientists from around the world, they meet her and are in awe.”

Computer rendering of bats' vocal communication shows an example of two 'chirps'

“When I go to meetings and show scientists the computer renderings of these bats’ vocal communication, it stuns them,” Dr. Pollak said.

video icon Listen to the courtship song starring Sid the male bat.

video icon Listen to the mother’s call to her infant.

video icon Listen to Joshua defending his territory from an intruding male.

Download QuickTime Player.

French, who is a member of Bat Conservation International and, in Pollak’s estimation, “a stellar biologist,” noticed that members of her bat colony were exhibiting very elaborate social communication, but she did not have the expertise to record and evaluate what she was hearing. Knowing that Pollak was deeply involved in bat research, she enlisted his help and he transported the appropriate recording equipment to her barn, where he captured sounds that have transfixed biologists around the world.

“Barbara could tell us exactly what was going on when a bat would deliver a particular song,” Pollak said. “The more we studied these songs and this social interaction, the clearer it became that the songs were all different, there were rules to them and these animals were conveying very specific messages to one another. This was an unbelievably rich repertoire of communication, and it was stunning how effectively the bats were using vocalizations for something other than echolocation.”

With microphones in French’s barn, Pollak and his graduate students have been able to capture “bat talk” around the clock.

Recordings have revealed, among other things, patterns of the male Mexican freetail bat’s “advertisement call,” which he uses to woo the ladies. This bat ballad is accompanied by a slathering on of potent bat perfume (urine and pheremones) and by the male flapping his wings to spread this seductive scent to a female bat who may be passing by.

“This smelly liquid they mark themselves with and the serenade when a female is present are the males’ way of flirting, if you will,” says Pollak. “And as is the case in most of the animal kingdom, the females get to choose whom they prefer.”

There's much that goes on in the world that you're completely unaware of. 'Reality' is something your brain creates. -Dr. George PollakThe advertisement call is the most elaborate song that has been discovered by Pollak and his team, consisting of syllables that combine to form a phrase that Pollak refers to as a chirp. The male repeats the chirp six to seven times, and the entire production ends with the flourish of a trill.

This song is distinct and separate from the mother’s directive call to her baby or the territorial song of a male protecting his harem from the encroachment of other males.

Although Pollak amasses additional data daily, he hastens to point out that more questions than answers remain. In competing with other suitors, does a male bat “decorate” his song and add trills to make himself more attractive to the females? Do certain mating songs win out and allow a particular male to father more babies? Do bats in different caves use different dialects? How much of this is learned?

To all of that, Pollak can only offer, “We don’t know—yet.”

“I will spend the rest of my career doing this,” said Pollak. “My students will spend the rest of their careers doing this. And their students will spend the rest of their careers doing this. After all, we’re talking about the brain, the most amazing piece of machinery that we know of in the universe, something that can contemplate the origins of life as well as detect the accent of a speaker across a crowded room, effortlessly. How do you explain this?”

Kay Randall

Photos: Marsha Miller

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  Updated 2014 October 13
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