The Grrrr in the Growl
Lessons from a New Polar Bear Vocalization
Figure 1. Spectrogram of Charly's rumble; the sound is concentrated at the bottom, indicating mostly low-frequency energy (peak at 110 Hz), followed by a huff that contains frequencies out to about 10 kHz. Click the image to enlarge.
The Grrrr in the Growl: Lessons from a New Polar Bear Vocalization
By Ann E. Bowles and Megan Owen
Editor's note: Knowing at what ranges polar bears can hear and communicate with each other is important in terms of protecting the bears from industrial disturbances. A PBI-funded study is yielding some surprising results.
Sound is an essential source of information about the environment for all vertebrates. The study of sound is called acoustics, and the study of how animals use sound to interact with their environment is acoustic ecology. The scope of this discipline includes hearing, sound production, and environmental noise. Our research interest is the acoustic ecology of polar bears, focusing on how human-made noise might affect them. However, despite what you might think from the popular media, no one really knows much about how polar bears hear or communicate. So, it’s difficult to predict effects. With funding from PBI, we’ve spent the last year and a half taking the first “baby step” by studying polar bear hearing, thanks to the help of animal trainers at Polar Bear Plunge (San Diego Zoo) and Wild Arctic (SeaWorld San Diego).
To conduct our experiments, we had to quiet test spaces at both facilities so the bears could hear all of our test signals. For San Diego bears used to big-city conditions, the change was a surprise. One of them, 1,100-pound Charly at SeaWorld, surprised us in return.
Low Frequency Growls
To understand our reaction, you have to realize that nobody has really studied bear vocalizations from an acoustic point of view. It’s hard to get close enough to them in the wild to get good recordings. You also need to know about frequency, which corresponds roughly to human pitch perception. Low frequency sounds are a signal of power and size, so wild carnivores growl and rumble at low frequencies to maintain their territories, defend themselves, and threaten. Low-frequency sound is also important because it travels over long distances, so large, wide-ranging animals like whales and elephants often use it to communicate. However, there’s usually more environmental noise at low frequencies, including human-made machinery noise. So, all these animals have to balance a need to communicate against the possibility that environmental noise will obscure their calls.
In a big city, we deal with low frequency noise every day. To run our experiments, we had to isolate our test spaces from environmental noise. The best way to keep noise out is to turn off mechanical sources and seal everything with something heavy – we’ve developed extra muscle by covering openings with lead-lined acoustic curtains, loading up the edges of doors with lead strips and sandbags, and covering windows with an extra-thick sheet of acrylic.
A Surprising Rumble
The first time we quieted the test space at SeaWorld, we heard a deep, resonating, hair-raising rumble. Listen to rumble. It sounded like it was coming from a VERY big, strong animal, but none of the bears seemed to be doing anything. We’d never heard anything like it! Eventually, we were able to localize the sound – it was emanating from Charly’s head. He was making it while lying quietly, nose down, and without any visible effort. He looked like he was resting, except for this frightening sound, which we would never have been able to hear if we hadn’t sound proofed his living space.
We were very excited when we realized what was happening, but by the time we got our recording equipment, Charly had stopped rumbling. Jennifer Keating, our research assistant, set up all the equipment and quieted the space on three separate days, but he stayed silent. She finally got good recordings during a recording session for a crew from National Geographic. This new activity created enough surprise to start Charly rumbling again.
A visual representation of the sound, called a spectrogram, is shown in Figure 1. In this visual format, you can see that the rumble is made up of a series of individual bursts with energy concentrated at very low frequency, about 110 Hz. The new sound is a bit like a cat’s purr, except that the bear is also using his voice, which shows up as a bright line across the bottom of the spectrogram. At the end of the rumble, Charly makes a more typical bear sound, called a huff, with a wide frequency range and no use of the voice.
By Ann E. Bowles and Megan Owen
Editor's note: Knowing at what ranges polar bears can hear and communicate with each other is important in terms of protecting the bears from industrial disturbances. A PBI-funded study is yielding some surprising results.
Sound is an essential source of information about the environment for all vertebrates. The study of sound is called acoustics, and the study of how animals use sound to interact with their environment is acoustic ecology. The scope of this discipline includes hearing, sound production, and environmental noise. Our research interest is the acoustic ecology of polar bears, focusing on how human-made noise might affect them. However, despite what you might think from the popular media, no one really knows much about how polar bears hear or communicate. So, it’s difficult to predict effects. With funding from PBI, we’ve spent the last year and a half taking the first “baby step” by studying polar bear hearing, thanks to the help of animal trainers at Polar Bear Plunge (San Diego Zoo) and Wild Arctic (SeaWorld San Diego).
To conduct our experiments, we had to quiet test spaces at both facilities so the bears could hear all of our test signals. For San Diego bears used to big-city conditions, the change was a surprise. One of them, 1,100-pound Charly at SeaWorld, surprised us in return.
Low Frequency Growls
To understand our reaction, you have to realize that nobody has really studied bear vocalizations from an acoustic point of view. It’s hard to get close enough to them in the wild to get good recordings. You also need to know about frequency, which corresponds roughly to human pitch perception. Low frequency sounds are a signal of power and size, so wild carnivores growl and rumble at low frequencies to maintain their territories, defend themselves, and threaten. Low-frequency sound is also important because it travels over long distances, so large, wide-ranging animals like whales and elephants often use it to communicate. However, there’s usually more environmental noise at low frequencies, including human-made machinery noise. So, all these animals have to balance a need to communicate against the possibility that environmental noise will obscure their calls.
In a big city, we deal with low frequency noise every day. To run our experiments, we had to isolate our test spaces from environmental noise. The best way to keep noise out is to turn off mechanical sources and seal everything with something heavy – we’ve developed extra muscle by covering openings with lead-lined acoustic curtains, loading up the edges of doors with lead strips and sandbags, and covering windows with an extra-thick sheet of acrylic.
A Surprising Rumble
The first time we quieted the test space at SeaWorld, we heard a deep, resonating, hair-raising rumble. Listen to rumble. It sounded like it was coming from a VERY big, strong animal, but none of the bears seemed to be doing anything. We’d never heard anything like it! Eventually, we were able to localize the sound – it was emanating from Charly’s head. He was making it while lying quietly, nose down, and without any visible effort. He looked like he was resting, except for this frightening sound, which we would never have been able to hear if we hadn’t sound proofed his living space.
We were very excited when we realized what was happening, but by the time we got our recording equipment, Charly had stopped rumbling. Jennifer Keating, our research assistant, set up all the equipment and quieted the space on three separate days, but he stayed silent. She finally got good recordings during a recording session for a crew from National Geographic. This new activity created enough surprise to start Charly rumbling again.
A visual representation of the sound, called a spectrogram, is shown in Figure 1. In this visual format, you can see that the rumble is made up of a series of individual bursts with energy concentrated at very low frequency, about 110 Hz. The new sound is a bit like a cat’s purr, except that the bear is also using his voice, which shows up as a bright line across the bottom of the spectrogram. At the end of the rumble, Charly makes a more typical bear sound, called a huff, with a wide frequency range and no use of the voice.