Ultrasound - Heard by Human Ears:
As centuries of dark nights passed, people began noticing that bats possessed the ability to fly swiftly and safely….. never crashing into trees, cave dwellings, buildings or other structures and they started to wonder how this was possible. At first, people thought it was because bats had unusually sharp vision and an amazing ability to see after dark. It wasn't until the late 1700s that Italian scientist Lazzaro Spallanzani set out to determine just how this phenomenon was possibe. Spallanzani's main interest was studying night vision in animals. Having conducted a variety of scientific studies, specifically with bats, Spallanzani came to the conclusion that bats did not rely on sight for navigation and feeding, they used their hearing to fly and feed. Unable to fully explain his conclusion, other scientists immediately rejected Spallanzani's claims and instead insisted that bats relied on touch for feeding and navigating. Research suggests that the "touch theory" held for more than a century in the scientific world, until the early 1900s, when it was decided that bats detected echoes of sounds produced by the flapping from their own wings. All past hypotheses were equally unexplainable, so it remained a mystery until the first ultrasound microphone was invented in 1938.
Donald Griffin, a biology student studying bats at Harvard University in 1938, learned about a piece of equipment invented by Harvard physics professor G.W. Pierce. This device was capable of picking up high-frequency sounds, also known as ultrasound (meaning beyond sound) that humans could not hear. One day, Griffin decided to take some of his caged bats to Pierce's office in order to find out if bats made high-frequency sounds. When Griffin placed his bats next to the machine, he began to hear high-pitched squeaking noises. When he turned the machine off, he could no longer hear any sounds coming from the bats. Griffin thought that the amazing sounds he heard from the mouths and noses of his bats must somehow be the key to their flying swiftly and safely in total darkness. At that point, Griffin was able to determine that certain bats do rely on sound to navigate and feed, just not from the noise produced from the flapping of their wings. Griffin named this phenomenon echolocation, and from that point on, scientists began to study and understand just how bats use high-frequency sound to "see."
You may have thought that bats have a remarkable ability to see in the dark just like other mammals, or perhaps you've heard the common phrase "blind as a bat." Well, the truth of the matter is that bats can see, but are some of the few mammals that need sound to "see" and navigate through darkness. In order to successfully travel through dark caves and pitch-black night skies, bats use echolocation. Echolocation is locating objects by picking up the reflections of emitted sounds. Bats that have the ability to echolocate are in the Microchiroptera suborder, of which there are more than 800 species. Microchiroptera bats are capable of using the echoes from their own calls to decipher shapes. Echolocation is the active use of sonar (sound navigation and ranging) along with special morphological (physical features) and physiological adaptations, which allow bats to "see" with sound. Scientists have come to the conclusion that the unique ability to echolocate is the reason why certain bats look as they do. Scientists are not exactly sure why some bats have such odd-shaped noses and faces, or how they function. Some bats echolocate through their noses, which is why many have thought that the complex arrangements of slits and flaps of nose skin help to focus sounds into narrow beams that help direct the sounds to the intended target.
Even though it wasn't until after the invention of the first ultrasound microphone in 1938 that echolocation studies began, scientists were able to determine from studying the earliest known bat fossil, dating back more than 50 million years, that bats did in fact have the intricate middle-ear structure that made echolocation possible even back then. Bats inhabit an auditory sensory world, not a visual one. In the dark night sky, an echolocating bat will emit bursts or various pulses, (as many as 250 beeps per second) of high-pitched ultrasound as it searches for food. The ultrasound travels through the air until it strikes an object. After the sound strikes the object, the energy in the sound wave is absorbed by the object and then bounces back as echoes received by the bat. An interesting analogy is that of a ball bouncing back after it has been thrown against a wall. The kind of echo that bounces back depends on how far away the object is and whether or not it's hard, soft, moving or still. After picking up the echoes with their large, uniquely shaped ears, bats are able to determine everything they need to know about an object.
Did You See That Noise?
Using echolocation, bats "illuminate" objects with sound. Each bat species echolocates in its own way and uses a unique type, frequency and noise level of communication. While we know that almost all of the various sounds produced by bats can't be heard by human adult ears, it is interesting to know that some young children, along with dogs, cats and dolphins, can hear ultrasounds. Most bats echolocate at a frequency of more than 20,000 cycles per second (20 kilohertz) and most human adults cannot hear sounds with frequencies above 20 kilohertz. Bats are capable of getting echoes back from their sonar pulses from almost everything they come in contact with, thanks to acoustic impedance. Impedance is another word for resistance, and can be explained as a measure of the substance through which the sound wave travels (the medium). This is possible because almost everything that bats encounter in their environment has much higher acoustic impedance than the air they fly through, making it really easy for ultrasounds to bounce back. Bats rely heavily on the echoes and reflections of their ultrasound from objects in their environment but, as mentioned earlier, not all objects reflect sound equally. It all depends on acoustic impedance. Remarkably, echolocating bats not only can locate a moth, tell how big it is, which direction it is heading and how fast it is moving, but they also can detect objects under 1/25'' wide. Bat echolocation can detect objects as thin as human hair. To take an in depth, interactive look at biosonar and acoustic impedance, please click on the Biosonar – Seeing with Sound in the Related Links Section.
Illuminating Objects with Sound:
Bats, dolphins, and whales are not the only mammals capable of using echolocation to "illuminate" objects with sound. Humans also can skillfully echolocate, using echoes from their environment to "see" with sound. Just like bat detectors, devices for the blind have been developed that use sonar, similar to bats' sonar. The devices work by sending out sounds that bounce off surrounding objects. Blind people can use the differences in the returning echoes to determine the size, distance and location of certain objects in their environment. Echolocation has also recently led to the development of a new cane that uses bounced sound to further enable blind people to get around safely. And while there are many individuals who successfully use echolocation, Daniel Kish, who runs an organization called World Access for the Blind, has successfully mastered the unique sense and skill of echolocation. Kish, who has been sightless since he was one year old, has successfully shared his echolocation skill with 5,000 plus students in more than 18 countries.
Scientists recently have made another important discovery in regard to echolocation. The key gene involved in the echolocation process is almost identical to the FOXP2 gene, which plays a vital role in human speech. Humans who have a mutation in the FOXP2 gene develop severe speech and language disorders, and bats with this same mutation cannot use echolocation. The FOXP2 gene is present in many species and in most cases seems fundamentally connected to communication. Linking echolocation to this gene is important because it proves that this sense is as vital and sophisticated in bats as speech is in humans, and perhaps just as important to their success. While human beings have learned to echolocate successfully, it is interesting to know that bats have been perfecting this unique sense more than ten times as long as humans have been developing speech. Now that's impressive!
Bat Language Revealed:
Mexican Free-tailed bat. Photo courtesy of Wikipedia.
Since bats have such a unique way of "seeing with sound" by using echolocation and producing ultrasounds that most adult human ears cannot hear, what are all the squeaks and eeks that people can hear? Bats are also known for communicating with one another by using audible chirps, clicks, buzzes and purrs. But, what is all the bat chat about? In the past, scientists have wondered what their noises meant and what could these flying mammals possibly be talking about. Barbara French (biologist and former science officer at Bat Conservation International (BCI), currently at Bat World Sanctuary), Amanda Lollar (founder and president of Bat World Sanctuary) and several other scientists set out to discover the level of communication of the bat chat. Together, they have identified more than 20 different calls in Mexican free-tailed bats. When asked what bats talk about, French said, "The normal stuff – food, friends, mates territories and complaints." French explained that the most talkative bats are those that live in large bat colonies. Caves can have millions of bats roosting in one place, at one time. For example, the largest known bat colony in the world is Bracken Bat Cave near San Antonio, Texas, which is home to more than 20 million Mexican free-tailed bats all at once.
Millions of bats above Bracken Bat Cave near San Antonio, Texas. Photograph Courtesy of Bat Conservation International.
French said it is a "grammar" which bats can use by putting together all of their clicks and buzzes in certain ways that makes sense to them. In a document called "Bat Talk: The Secret Language of Mexican free-tailed bats," French and Lollar document the social interaction of Mexican free-tailed bats in captivity. Both French and Lollar run captive bat colonies, and both have successfully rehabilitated bats for more than 15 years. The bats residing in their colonies are there because of various injuries that left them unable to fly and stay safe from predators in their natural environment. French and Lollar have been able to extensively study bat mating and vocalization activity, which has allowed them to identify more than 20 bat calls. French and Lollar were able to identify calls because they could actually hear the bats chattering in their colonies. It continues to be extremely difficult for scientists to decipher the sounds of more than 20 million bats chattering at once because, unfortunately, the sounds of so many bats drown each other out in caves like Bracken.
The calls were recorded by Teh-Sheng Ma of the Institute of Neuroscience, who used a time-expansion digitizer. The recorded calls suggest that bats have a complex social order and use vocalization to greet, argue, play and express anger and irritation with one another. Some of the bat calls identified and recorded by French and Lollar included the following:
- Warning calls
- Questioning calls
- Irritation buzz
- Food-Solicitation buzz
- Protest Squeal
- Dominance scold
- Herding buzz
- Escape cries
- Isolation calls
- Alarm call
- Anticipation click
- Territorial buzz
- Marking calls
- Preflight call
Surprisingly, scientists have also picked up clues that some of the bat calls are composed of very specific phrases – sounds that can be mixed and matched to convey different meanings. George Pollak, a neurophysiologist at the University of Texas at Austin, who has also worked with French to record bat signals, said, "What is becoming apparent here is that the way bats use [their communication signals] is very sophisticated and may have rules to guide it." Along with Pollak and French, researchers Kirsten Bohn, biologist, Whitman College and Mike Smotherman of the Department of Biology at Texas A&M, also have spent a great amount of time studying bat recordings, and… among other things… have determined that along with the already-identified calls bats use in general, male bats use syllables and phrases, and put them together in the form of "love songs" to attract female bats.
Apparently, using love songs to attract females isn't a technique used only by human males, but by male bats as well. Using equipment to detect the ultrasounds, Bohn explained that her team was able to hear male bats making sounds in a specific, arranged pattern to form a song, with actual organized sequences in each phrase. After listening to recordings from two different locations in Texas, Bohn said, "We compared the recordings made by bats in Austin to those at Kyle Field, and we discovered they were almost exactly the same – the bats in both places use the same sounds, organized the same way in their love phrases." This became interesting when Bohn and the team discovered that male bat songs have hierarchical organization, specific syllables combined to form three different phrases… a chirp, buzz or trill… and those phrases are strung together to form songs. Bohn says, "These results are surprising because mammals such as bats are not expected to have hierarchical structure and syntactical rules of organization. You see it frequently in birds, but that's about it. We've learned that the vocal production of bats is very specific and patterned, and now we have a model not only to study communication similarities in other animals, but also human speech."
Bohn thinks these findings are a big first step in the right direction and is excited because researchers can now study a mammal that they know for certain sings like a bird. The fact that bats communicate using complex vocalizations adds to what scientists are learning about these misunderstood mammals. Scientists are beginning to understand more fully that bats are extremely intelligent, social, long-lived mammals.