An Introduction to Bats and Echolocation (and Tools to Use in the Classroom)
Bats Can Be Identified Through Their Unique Echolocation Patterns, They Even Have Dialects
Chirp, Chirp, Chrip Yall'
As an undergraduate my introduction to field research was through bats. I studied under Dr. John Leffler, a student of EO Wilson, and spent countless hours recording bat sounds and trying to match their sounds to visual patterns that were species specific. If my life had gone differently I might still be studying bats and hanging from ropes welding bat gates. Even though I moved on from bats, to birds, and then outdoor education, I still have a mad passion for them and bat programs are one of my favorites. Thus, I'll do a series of posts on bats in the coming months in tribute to these amazing flying mammals. Today let's start with echolocation.
What is Echolocation?
Very simply, echolocation is using sound waves to navigate. There are many types of animals that use echolocation including toothed whales and dolphins and even some birds. In bats echolocation is used for night time navigation (though their eyes are just as good as a human's). When a bat echolocates it uses pulses of sound that it generates from its throat/larynx (sometimes the nose) which are forced out as sound waves into the environment. The sound waves then bounce off of objects and then the "bounced" sounds are returned to the bat's ear. The time delay in the sound's release and return allow the bat to measure distance and depth of objects or prey.
The ears of bats are specially made to receive sound, and having two ears helps them triangulate the location of objects. When echolocation signals travel the sounds arrive at different times and different speeds to each ear. This delayed arrival, along with interference patterns of the sound waves, allows a bat to to triangulate objects, predict the size of an object, and track an insects path or location. If you look at the ears of a bat they have an elongated tragus, which acts like the receiver that sticks out on a satellite dish that receives waves. On your ear the tragus is the small hard (cartilaginous) protrusion that attaches at the middle of the ear and points towards the ear canal (see the diagram below).
Describing Voice Patterns and Echolocation
As you know humans have very clear voice patterns and modulations. There are as many dialects and variations in speech as there are humans. For a long time researchers have been curious about bat vocalization patterns (echolocation) and if they could use these patterns to identify specific species of bat or bats from two different regions. This could be a very useful technique, especially if you didn't have to trap bats to find out what species were in an area or if you wanted to know where endangered species of bats were roosting.
If you were to record a bat's echolocation chirps with a bat detector you would see that their chirps fall in the range of 12,000-100,000 Hz, which is pretty much beyond what humans can hear (humans usually hear between 20-20,000 Hz). Most insectivorous bats have a frequency range of 20-60 Hz, which is the best range for feeding on insects. This range is more difficult for insects to detect than lower frequencies.
Scientists can take the recording of a bat's echolocation calls and visualize the patterns using special software, much like a DJ or sound engineer would use. The visualization of the echolocation calls allow the researchers to quantify and mathematically describe the calls of each species during different activities. These activities include flying or navigating, hunting, and "talking" with other bats.
Here are some of the features researchers look for in bat calls:
- Frequency of the call, not just how often the bat calls but the Hz at which the call is made.
- Frequency modulation or consistency. Does the call go up and down or stay constant?
- Intensity of the call. Does the call start off loud and go soft or start off soft and get louder?
- Buzzes or zaps. When an insectivorous bat hones in on food you may hear an increase in the speed of the "chirps" from the bat and then a feeding "buzz" or "zap" when it grabs the prey in its mouth.
- Harmonies in the call. Some bats can create multiple frequencies in one call, but usually there's one dominant frequency.
- Call length or how long the call lasts.
- Time between pulses of sound.
Bats in a Crowded Room
When I was conducting bat research one of the things I found most interesting was that bats have to change or modulate their echolocation frequencies when they are in large groups. It's like talking in a crowded room, you have to raise you voice to be heard. For bats, they can raise their voice or chirp at lower or higher frequencies. What I found truly eye opening was watching a large emergence of bats with night vision goggles. There were so many bats echolocating that they couldn't hear their own sounds coming back to them. I watched several run smack into tree branches and tree trunks! This is why bats often disperse so quickly into the night, because they are avoiding the interference of other bat's echolocation patterns as well as competition.
What Have Researchers Found Out About Bats Using Echolocation?
The use of bat detectors gives scientists qualitative data more than quantitative data. Echolocation patterns allow researchers to describe the call patterns of different species but not how many of each species are present. It's nearly impossible to count how many calls each individual bat makes. However, this type of information has been useful in creating parameters that allow researchers to identify many types of bats by their call signatures. The only upshot of all this is that many insectivorous bats have very similar call patterns and Hz ranges of their calls. Statistical analysis can narrow down a bat to one or two species, but often researchers still need to go into the field to capture the bats to identify them. Another problem I ran into while doing bat research was that some bats don't use very loud echolocation. The endangered Virginia big-eared bat is also known as the "whisper bat" because it uses such a quiet echolocation call. this created huge problems in getting any quality recordings.
Researchers have found that there are unique dialects for bats of the same species in different regions of the world. It's like the difference between a brown bat in the Tennessee saying, "chirp-chirp-chirp-yall" and a brown bat in Canada saying "chrip-chirp-chirp-eh." There is ongoing research on dialects in bats and killer whales. You can find more in professional papers by Kanwal and Deecke et. al.
Although it has its limitations, the use of echolocation patterns is still way less invasive than trapping alone, and equipment can be left in the field overnight and analyzed at the convenience of the researcher with little impact on the bats.
Bat Echolocation Detection Devices
Do you want to try your hand to echolocation detection? There's a wide spectrum of tools that you can find on the market for recording bat echolocation signals. They range from the "dot-it-yourself" kit to the professional grade research tools. Here are a few options:
- Professional Grade: Anabat Systems from Titley Scientific ($900-$2,000) detects, records, and analyzes.
- Professional Grade: The Petterson D1000X ($2,000)
- Mid-Range Professional: The Petterson D100 Heterodyne Bat Detector ($350)
- Mid-Range Professional: Zoom H2 Professional ($259)
- Nature Center Use: Baton Bat Detector ($159)
- Nature Center Use: Bat Detector from Acorn Naturalist ($105)
- Back yard Enthusiast: Bat Detector by Convergence ($85)
If you're interested in a comparison of bat detectors here's a blog with reviews of over 20 different types. This page, from the Bat Conservation Trust, gives a good explanation of different types of bat detectors and how to choose one that fits your needs.
Resources for Teaching About Echolocation
If you're interested in teaching about echolocation here are some great resources for you:
- Here's a catchy video and song about echolocation for younger students by JumpStart Bat.
- A short video (3 min) on bat echolocation and detection of surface textures by Bat Sense.
- A lesson plan by the Acoustical Society of America on echolocation and sonar.
- Lesson plans from BatsLive for K-8th grade.
- A great hands-on activity for grades 5-8 about calculating sound wave speed from Scientific American Frontiers and PBS.
- Here's a lesson plan from Teach Engineering, for grades 3-5th, with a hands-on graphing activity about hearing sound waves.