 | Transcript Audiovisual Program photos copyright: Merlin D. Tuttle Produced by: Bat Conservation International Inc. P.O. Box 162603 Austin, Texas 78716 |
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| Transcript Audiovisual Program photos copyright: Merlin D. Tuttle Produced by: Bat Conservation International Inc. P.O. Box 162603 Austin, Texas 78716 |
Updated 4/25/01
| SIDE 1: Introduction | |
| IMAGE DESCRIPTION | SCRIPT |
| Night sounds: crickets chirping, katydids, owl hooting, etc. | |
| 1. Title Slide: Agave palmeri at sunset | Most of us are familiar with these typical sounds of the night: crickets chirping, katydids calling and the occasional owl hooting. But, unfortunately our ears are not capable of picking up some of the most fascinating and pervasive night sounds, the ultrasonic echolocation calls of bats. |
| 2. Mexican free-tailed bat (Tadarida brasiliensis) emergence at sunset | Bats have been navigating the night skies for more than 50 million years, but it was not until the late 1700s that people began to understand and appreciate just how sophisticated the orientation system was. Late in the eighteenth century, Lazarro Spallanzani, an Italian scientist, first documented that... |
| 3. California leaf-nosed bat (Macrotus californicus) portrait | bats were using a kind of "sixth sense" to navigate in complete darkness. He found that blocking a bat's ears would interfere with its ability to orient in the absence of light. He concluded that bats could "see" with their ears but was unable to prove it, and his claims were met with disbelief until the research by Donald R. Griffin in the 1930s. At Harvard University, Griffin was the first to use special microphones to document that bats produce sounds far above the human range of hearing. Physical properties of sound make high-frequency emissions ideal for navigation. The pulses made by bats have short wavelengths... |
| 4. California leaf-nosed bat (Macrotus californicus) in flight to cricket on a cactus | and give strong echoes from small targets such as insects. The term "echolocation" was coined to refer to the process of sending out pulses of sound and listening to the echoes to locate or avoid objects. |
| 5. Mini-3 Bat Detector and Pettersson D-200 Ultrasound Bat Detector* | Today, several different types of bat detectors are available to translate inaudible, high-frequency bat sounds into humanly audible clicks, chirps and putts. The calls on this tape will enable you to eavesdrop on the seldom-heard world of bat communication and navigation. |
| 6. Big brown bat (Eptesicus fuscus) echolocating in flight | We will begin with the echolocation calls of a big brown bat (Eptesicus fuscus), one of North America's most abundant species. |
| Eptesicus fuscus echolocation calls in flight (Short broad-band format recorded by B. Betts) | |
| 7. Fringe-lipped bat (Trachops cirrhosus) portrait | Most bats, with the exception of flying foxes, use echolocation for orientation. They send out signals, usually through their mouths, but sometimes through their nostrils, and listen for the faint echoes that return. By comparing transmitted sounds with returning echoes, a bat measures the distance to a target and evaluates its size and texture. The complex structures of bat ears and facial features often play a part in focusing, directing or receiving ultrasonic pulses. |
| 8. Silver-haired bat (Lasionycteris noctivagans) echolocating in flight | This is a silver-haired bat (Lasionycteris noctivagans). Most bat echolocation signals are above the human range of hearing. Humans can hear frequencies up to around 20 kilohertz, but bats typically emit sounds between 20 and 100 kilohertz. |
| Lasionycteris noctivagans echolocation calls in flight (Short broad-band format recorded by B. Betts) | |
| 9. Woman sitting on rock listening to echolocation sounds on a Mini-3 Bat Detector* | Even simple bat detectors will translate high frequencies into audible sounds, allowing listeners to monitor bat activity in the night skies. Such detectors can be manually tuned to receive high frequency bat calls, which are then mixed to produce a humanly audible output. These detectors only pick up a limited portion of the call around the frequency to which the unit is tuned. Although calls can be heard in "real time" as the bat produces them, their structure cannot be fully appreciated. |
| 10. Young man showing Anabat Detector* | Sophisticated bat detectors can be used to more closely examine bat calls. By employing a time-expansion feature, these detectors can slow down the duration of the echolocation call, and also lower the frequency, allowing us to listen to both the structure and detail of calls. By using this technique we can begin to understand how the frequencies of calls change over time. The time-expansion calls on this tape have been slowed down by a factor of 10, and the recordings are repeated several times so the complexity of each call can be better appreciated. |
| 11. Little brown myotis (Myotis lucifugus) echolocating in flight This is a recording of a little brown bat (Myotis lucifugus). | This is a recording of a little brown bat (Myotis lucifugus). |
| Myotis lucifugus echolocation calls in flight (Short time-expansion format recorded by G. Jones) | |
| 12. Graphic depiction of a FM echolocation signal | Bat echolocation calls consist of combinations of several audio components. The two most common are "frequency modulated," or FM components, and "constant frequency," or CF components. Frequency modulated sounds sweep through a range of frequencies in a very short time, usually in just 1 to 5 milliseconds, or 1 to 5 thousandths of a second. A typical FM echolocation call sweeps from 100 kilohertz down to 50 kilohertz in a span of only 2 milliseconds. |
| 13. Big brown bat (Eptesicus fuscus) in flight to moth | While hunting insects, FM bats produce about 10 calls per second, processing the returning echo of each call before sending the next one. FM signals are known to give bats especially detailed information about target distance and texture. Bats calculate how far away a target is by measuring the time delay between sound emission and the return of an echo. In just one second, sound travels more than 1,000 feet (300 m.) through the air, which means that bats are able to make these calculations in tiny fractions of a second. Most bat echolocation calls are very loud, so intense that if we could hear them, they would sound like a smoke alarm. In fact, FM bats close their middle ears when sending calls to avoid deafening themselves. |
| 14. Southwestern myotis (Myotis auriculus) echolocating in flight over pond | This southwestern myotis (Myotis auriculus) provides a good example of a bat that uses FM echolocation signals. |
| Myotis auriculus echolocation calls (Time-expansion format recorded by G. Jones) | |
| 15. Dobson's horseshoe bat (Rhinolophus yunanensis) perched portrait | "Constant frequency," or CF echolocation calls, remain at a single frequency throughout their duration. Such calls are best illustrated by horseshoe bats, which live only in the Old World: Africa, Europe, Australia and Asia. |
| 16. Graphic depiction of a CF echolocation signal | CF sounds used by horseshoe bats are relatively long, about 50 milliseconds. These calls help bats to obtain detailed information about fluttering targets. The length of a CF call lets the bat saturate the air with sound for long periods of time. When the long CF signals strike an insect, its wing-beats cause little "glints" or shifts in frequency and amplitude in the returning echoes. For example, if you imagine a mosquito beating its wings at about 200 times per second, a CF signal from a horseshoe bat will return to the bat with about 10 little glints, making the insect easier to identify. Because big insects flap their wings slower than small ones, the bat can identify different kinds based on the number of glints in returning echoes. CF bats are also able to compensate for Doppler shift effects. These are the apparent changes in frequency that result from an object moving toward or away from the bat's signals. Doppler shifts will cause an echolocation call to return to the bat at a higher frequency than that at which it was sent. |
| 17. Hildebrandt's horseshoe bat (Rhinolophus hildebrandti) in flight with insect | To overcome this, horseshoe bats lower the frequency that they emit based on their flight speed so that the echo always returns roughly at the frequency to which their ears and brains are most sharply tuned. This also means that the out-going pulse is beyond the range of the bat's maximum hearing sensitivity, preventing it from deafening itself. Thus, CF bats are able to overlap sending out an echolocation call while processing a returning echo. Now let's listen to the signals of a constant frequency bat. |
| 18. Lesser horseshoe bat (Rhinolophus hipposideros) portrait | This is the sound of a lesser horseshoe bat (Rhinolophus hipposideros) from England. |
| Rhinolophus hipposideros echolocation calls (Time-expansion format recorded by G. Jones) | |
| It emits CF signals with little FM sweeps at the beginning and end. The constant frequency portion of the call is at about 115 kilohertz. Just by listening to the sound slowed down 10 times you can appreciate how precisely the frequency is maintained over its duration. | |
| 19. Desert red bat (Lasiurus blossevillii) portrait | Some bats add CF components to the FM portions of their signals. They are what we call FM-CF bats. The CF portion of the call helps the bat detect targets, while the FM portion translates information about a target's texture. Rather than spreading all the energy in their calls over a wide range of frequencies, FM-CF bats use the CF sounds to focus on a particular frequency, usually the lowest in the call. This allows them to send their calls over longer distances. As these bats "home in" on a target, their calls become more frequency modulated in order to obtain more information about the target. |
| 20. Red bat (Lasiurus borealis) echolocating in flight | A good example of an FM-CF bat is the western red bat (Lasiurus blossevillii) recorded while hunting in the Arizona desert. |
| Lasiurus blossevillii echolocation calls (Time-expansion format recorded by G. Jones) | |
| More than 800 species of bats worldwide use echolocation for navigating and hunting. They often use different approaches to echolocation, and some species can be identified by their unique call signatures. However, species identification solely on the basis of echolocation is seldom one hundred percent accurate. The same species may use different dialects in different places and may even vary its echolocation strategies according to different foraging conditions or kinds of insects hunted. To illustrate the diversity of bat echolocation calls, we will listen to several North American species. | |
| 21. Lesser long-nosed bat (Leptonycteris curasoae) portrait | The lesser long-nosed bat (Leptonycteris curasoae) is a member of the New World leaf-nosed bat family from North and South America. These bats send their echolocation calls through their noses, using their nose-leaves to direct the sound. The lesser long-nosed bat feeds primarily on nectar and pollen. Its FM calls often contain a number of harmonic components. These are "overtones," or multiples of their call frequencies, that enhance a bat's ability to determine target range. |
| 22. Lesser long-nosed bat (Leptonycteris curasoae) approaching agave flowers | This recording was made as a lesser long-nosed bat flew among agave plants in southeastern Arizona. |
| Leptonycteris curasoae echolocation call (Time-expansion format recorded by G. Jones) | |
| 23. Pallid bat (Antrozous pallidus) portrait | Pallid bats (Antrozous pallidus) often use such low intensity echolocation calls that they are said to "whisper." This is a common strategy for bats that pick their prey directly off the ground or from foliage. By "whispering" such bats can approach their prey without being detected. In addition to echolocation, these bats also rely on vision and listen to the sounds of footsteps or wing-beats to find their prey. |
| 24. Pallid bat (Antrozous pallidus) in flight to centipede on ground | The pallid bat's echolocation calls sound like this. |
| Antrozous pallidus echolocation call (Time-expansion format recorded by J.D. Altringham) | |
| 25. Townsend's big-eared bat (Plecotus townsendii†) portrait | The Townsend's big-eared bat (Plecotus townsendii†) sends two different types of signals depending upon the type of habitat in which it is feeding. In cluttered situations, such as in dense forests, this bat uses a low-intensity, short-duration call made up of several harmonics. In more open areas, the bat switches to calls of longer duration and lower frequency, sometimes as low as 12 kilohertz that are audible to humans. |
| 26. Townsend's big-eared bat (Plecotus townsendii†) echolocating in flight | This bat's low-frequency, long-duration calls are illustrated by the following recording, made as it flew over the Arizona desert. |
| Plecotus townsendii† echolocation call (Time-expansion format recorded by G. Jones) | |
| 27. Big brown bat (Eptesicus fuscus) portrait | In contrast to the faint whispering calls of big-eared and pallid bats, the big brown bat (Eptesicus fuscus) uses high-intensity echolocation calls that are more like "shouting." For example, a typical smoke detector produces a signal of about 100 decibels at a distance of four inches (10 cm.). At the same distance, big brown bat signals register at more than 110 decibels! In many cases, call intensity is related to a bat's feeding habitat. Bats that feed in open areas produce the loudest calls. |
| 28. Big brown bat (Eptesicus fuscus) in flight with insect | The following echolocation calls from a big brown bat were recorded as the animal hunted in a relatively open desert habitat. Bats can tailor their echolocation calls to many different conditions. For instance, some species will add "honks," high-intensity, low-frequency, components, to the tail end of FM calls. These are most often used when bats are flying in groups where they need to avoid collisions. |
| Eptesicus fuscus echolocation call (Broad-band format recorded by B. Betts). | |
| 29. Mexican free-tailed bat (Tadarida brasiliensis) portrait | Echolocation calls of the Mexican free-tailed bat (Tadarida brasiliensis) normally sweep from 60 to 40 kilohertz, but in traffic, this bat adds a lower frequency "honk" that sweeps from 40 to 25 kilohertz. |
| 30. Mexican free-tailed bat (Tadarida brasiliensis) evening emergence | The following recordings were taken as a colony of Mexican free-tailed bats left their roost under a highway bridge in Tucson, Arizona. |
| Tadarida brasiliensis echolocation calls while leaving roost (Time-expansion format recorded by J.D. Altringham) | |
| Bats also use a wide range of vocalizations to communicate. Many of these contain ultrasonic components that are above our range of hearing. | |
| 31. Mexican free-tailed bat (Tadarida brasiliensis) echolocating in flight | The following broad-band recordings were made at a large colony of Mexican free-tailed bats. These recordings are slowed down and repeated. You will hear two sequences, each repeated twice. The diversity of sounds illustrates the tremendous range of social communication. |
| Tadarida brasiliensis communication calls (Time-expansion format recorded by G. Jones and J.D. Altringham) | |
| The "feeding buzz" is a distinctive feature of bat echolocation. Most hunting bats emit one echolocation pulse per wing beat. For a small bat, this is typically about 10 per second. When the bat detects an insect, it increases its pulse repetition rate up to roughly 200 per second, creating the "feeding buzz" we hear on a bat detector. | |
| 32. Silver-haired bat (Lasionycteris noctivagans) in flight with insect | The following recording was made of a silver-haired bat (Lasionycteris noctivagans) as it caught an insect. |
| Lasionycteris noctivagans echolocation calls with feeding buzz (Broad-band format recorded by B. Betts) | |
| 33. Man with Mini-3 Bat Detector listening for echolocation signals* | Even simple bat detectors can be used to evaluate bat activity in an area or to estimate rates of insect capture. They also make excellent interpretive tools for nighttime nature walks. |
| Nyctalus noctula echolocation calls with feeding buzz (Heterodyne format recorded by J.D. Altringham) | |
| 34. Credit slide: Mexican free-tailed bats (Tadarida brasiliensis) emergence | Echolocation is just one of many fascinating aspects of bat behavior. We hope this brief glimpse into their amazing sophistication and diversity will help you better appreciate these often misunderstood masters of our night skies. |
SIDE 2: Compendium of Bat Echolocation Calls
The following echolocation calls, unless otherwise noted, were recorded in Southeastern Arizona using bat detectors with time-expansion features and are replayed several times for more convenient listening.
The Old World lesser horseshoe bat (Rhinolophus hipposideros), an example of a constant frequency echolocator (recorded in England).
A New World leaf-nosed bat, the lesser long-nosed bat (Leptonycteris curasoae). This nectar-feeding species produces its multiple-harmonic "whispering" echolocation calls through its nose.
Plain-nosed bats generally send their echolocation signals through their mouths.
The desert red bat (Lasiurus blossevillii)
The southwestern myotis (Myotis auriculus)
Townsend's big-eared bat (Plecotus townsendii†)
The big brown bat (Eptesicus fuscus) (broad band calls recorded in Oregon)
An example of a silver-haired bat (Lasionycteris noctivagans) feeding buzz (broad-band calls, recorded in Oregon)
Mexican free-tailed bats (Tadarida brasiliensis) echolocation calls with "honks" added
Mexican free-tailed bat communication calls at a roost
Finally, a heterodyne recording of a British noctule bat (Nyctalus noctula), a cousin to the North American big brown bat, illustrating feeding buzzes.
(Audio only. This portion does not have accompanying images.)
† Plecotus townsendii is now known as Corynorhinus townsendii.
Bats: Rainforest Allies Script
Mini Slide Set: Rare and Endangered Bats List of Images
Mini Slide Set: Common North American Bats List of Images
Mini Slide Set: Bats in Eastern North American Forests List of Images
Mini Slide Set: Bats in Western North American Forests List of Images
Mini Slide Set: Bats: Partners in Pollination List of Images
Los Murcielagos de America Latina
(English language script for review only)
Bat Chat: An Introduction to Echolocation
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