Bat Detector: Listen to the Secret Sounds of Bats
AbstractDo you have bats in your neighborhood? Have you heard them "whoosh" by you, but not been able to see them? In this science fair project, you will be able to detect flying bats by listening in on the ultrasonic signals they produce to locate objects in their environment. The bat detector is a useful and fun tool for studying the biology of this nocturnal flying mammal.
David Whyte, PhD, Science Buddies
ObjectiveThe objective of this mammalian biology science fair project is to study the behavior of bats in the wild, based on the sounds they make. You will use a device that is designed to detect the ultrasonic pulses made by bats.
Bats are the only mammals that have wings and the ability to fly. Their wings are made of a membrane, the patagium, that is an extension of the skin from the back and belly area. Their wingspans vary from 5 feet in the large flying fox bat to 6 inches in the hog-nosed bat.
|Sub-orders||Megachiroptera and Microchiroptera|
Table 1. This table shows the classification for bat species.
Bats belong in the order Chiroptera. Chiroptera is a huge order, containing nearly one-quarter of all mammal species. It is exceeded only by rodents in the number of species that comprise it. Bats are divided into two sub-orders, the Megachiroptera and the Microchiroptera. Megachiroptera tend to be bigger than the Microchiroptera, have a fox-like face, and lack features for echolocation, which is a way of locating distant or invisible objects, like prey, by sound waves that are reflected back to the emitter, such as a bat, from the objects.
Figure 1. Townsend's big-eared bat (Corynorhinus townsendii) is a microchiropteran bat that is found in western North America. It typically preys on moths over open pastures and forest canopies. (Wikipedia, 2008.)
All Microchiroptera, on the other hand, use echolocation to find prey and to navigate while flying at night. Echolocation calls are vocalizations that are produced in the larynx (voice box) and then sent out through the mouth or the nostrils. Like other types of sound, bat signals are transmitted through the air as vibrations, or periodic fluctuations in air pressure. The pitch of the sound is determined by its frequency. Frequency is measured in hertz (Hz), or cycles per second. The unit kilohertz (kHz) is equal to 1000 Hz. Echolocating bats emit an ultrasonic pulse and analyze the returning echo to determine the distance to the object, its speed, as well as what type of object it is. Ultrasonic signals, by definition, have frequencies that are higher than the upper limit for human hearing, which is about 20 kHz.
Most bats alternate between emitting sound and listening for returning sound. Bats can change the design of their calls to fit the task—navigating, searching for flying insects, or snatching insects off of plants.
The ability to echolocate has allowed many bats to exploit flying nocturnal insects as a food source, as well as to live in dark caves. Most likely as a result of increased reliance on echolocation, microchiropteran bats have reduced vision capabilities, having lost some of the complexity found in the eyes and brains of megachiropteran bats, but it not true that bats are blind!
Interestingly, some moths have evolved the ability to hear the echolocating sounds made by bats. When one of these moths hears calls of an approaching bat, it might make evasive maneuvers. Some insects actually emit sounds in response to bat calls. This apparently confuses the bat, although it does not directly jam the signal.
Because the frequencies of bat sounds are too high for humans to hear, scientists use bat detectors to transpose bat sounds to a lower frequency. For this science fair project, you will use a frequency division bat detector. These instruments lower the pitch (frequency) by a factor of 16 so the calls fall within the range of human hearing, which is between 20 and 20,000 Hz. For example, a call that was originally 64,000 Hz is played back at 4,000 Hz (4 kHz) with the frequency division bat detector. You can hear an example of a recording made using this type of bat detector at the USGS link in the Bibliography, below. There are other types of detectors besides the frequency division type, but they tend to be more expensive.
Before you start this science fair project, you should have a good idea of where you will go to study bats in your area. They are active starting a few minutes after sundown, which is a good time to go because you might be able to see them in the dim light. Wooded areas and freshwater sources are good places to spot them. Do your research before you start so that you can predict what species of bats you are most likely to encounter. Take an adult with you to help, and don't forget your bug spray!
Terms and Concepts
- Hertz (Hz)
- Kilohertz (kHz)
- Frequency division bat detector
- How do bats use echolocation to navigate and find prey?
- Do any other animals use echolocation?
- What do bats do during the winter when there are no insects to eat?
- What positive contributions do bats make to our environment? (Hint: what diseases can be transmitted by mosquitoes?)
- Are bats rodents?
- What do the bats eat?
- Baldwin, M. (2006, June 25). Questions and Answers: Bats. Retrieved June 11, 2008.
- Winters, P. (2008 June). Amazing Bat Facts. Retrieved June 11, 2008.
- Simmons, N.B. (1997). Microchiroptera: Echolocating Bats. Retrieved June 11, 2008.
- Wund, M and Myers, P. (2005). Order Chiroptera: bats. Retrieved June 16, 2008.
- United States Geological Survey. (2008). Bat Vocalizations - Feeding Buzz. Retrieved June 16, 2008.
- Bat World. (2008). Bat Myths. Retrieved June 16, 2008.
- Harris, T. (2008). How Bats Work. Retrieved June 16, 2008.
- Bat Conservation Trust. (2006, May). Bat Detector Fact Sheet 3: Sound Analysis Software. Retrieved November 26, 2008.
Materials and Equipment
- Purchase a frequency division bat detector. Two online sources are Edmund Scientific catalog # 3085040 and Convergence Tech, Inc.
- A digital recording device to record bat sounds; a laptop with a microphone will also work
- Optional: use a video recorder to document your observation sessions.
- Lab notebook
- Graph paper
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In this science fair project, you will be observing bats in their native environment. The goal is to learn as much as you can about the bats: Where can they be found? What time of day are they active? How do they use echolocation to find prey? How do they impact the local environment? These sorts of observations are the beginning steps for any wildlife biology project. Once you have observed the bats, try to make some predictions about them. For example, if you found lots of bats near a local water source, can you predict another spot that you would expect to find them? In interpreting your observations, consider how habitat destruction and the use of insecticides might affect the bat population. The following procedure offers a few suggestions for the types of experiments you can do. After a few days of observation, you will be the local expert on bat biology!
To start this science fair project, you should first do some research on the bats that are found in your region so that you can identify the ones that you observe. Check with your science teacher for information about your local bats before you purchase the bat detector. Make sure that you have bats near your home that you can study, and that they are active at the time of year you are doing your project (bats are not active in cold weather!).
- Purchase your bat detector, either as an assembled detector, or as a kit that your can build yourself (the kit may require soldering).
- Before you begin the experiment, ask your teacher where you can find information about the species of bats in your area. You can also use online resources and local natural history museums to learn about your local species of bats.
Go with an adult to a location where bats are active. Bats are usually found where there are flying insects. Wooded areas and bodies of freshwater are excellent places to hear bats. Going out 15-45 minutes after sunset allows you to still see them against the early night sky. If you arrive just before sunset, you'll be able to see and identify them before it's too dark.
- Try to identify the species of bats you observe.
- Caution: Like other wildlife, if handled or molested, bats will defend themselves by biting. And like all mammals, bats can be carriers of the rabies virus. You should not pick up a bat that is on the ground, or touch one that is roosting.
- Turn the bat detector on when you see the bats. Listen to the sounds the bats make as "translated" by your bat detector. How close do the bats need to be before you can detect them?
- Record the sounds made by your bat detector so that you can study them later. A digital audio recorder is preferred because it allows you to store your audio files on your computer.
- Log your observations in your lab notebook. Note the times that you observed the bats and take careful notes of their behavior. Also record the time of sunset for your area, which you can find online, such as at weather websites.
Your hypothesis and experiment can be related to any of the following you choose, but remember, in order to be confident about your data, you'll need to repeat your experiment three times (ideally, on three different days). Then you'll be able to make accurate conclusions.
- How do insect levels and bat activity correlate? Make a qualitative judgment about the insect activity levels. Do they seem high, average, or low? Make sure you use the same criteria each time to make this judgment. Then make a quantitative measurement of the bat activity level, using calls in a 5-minute interval as your metric.
- How many different types of calls do the bats make? Which calls are the most common? Note the different types of calls made by the bats as they navigate and as they approach their prey. What types of changes would you predict the bat would make in its call as it gets closer to its prey? Are the vocalizations different when the bat is closing in on prey?
- Does bat activity change over the course of the evening? At what time did you detect the most bats? Use the number of bat calls in a 5-minute interval as your quantitative measurement. Make a graph with time on the x-axis and the amount of bat activity on the y-axis.
- Can you identify the species of bats in your area? Make a chart showing the types of bats you observed and their relative abundance.
- Locate a bat call library online and try to match your recordings with bat calls in the library. This will work best if you already have a good idea of the species of bats you are observing, based on your research. More information about bat call analysis software can be found at http://www.bats.org.uk/publications_download.php/367/SoundAnalysisSoftware.pdf. Click the link and save the pdf on your computer in order to view it.
Ask an Expert
- Try to film the bats using a night-vision video camera as you record the bats' calls. Is there a relationship between the bats' speed or movement and the calls they make?
If you like this project, you might enjoy exploring these related careers: