The Grrrr in the Growl
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Figure 2. Spectrogram of rumble after filtering by the hearing curve of a dog. Notice that most of the low-frequency energy is gone. Click the image to enlarge.
Implications
Two things struck us immediately. First, this was a vocalization we’d never seen described in the polar bear literature. The closest thing was a growl that researchers back in 1980 tried to use to scare bears away from human activities, but it didn’t have energy concentrated at low frequency, nor were we able to see any sign of the resonant bursts. The second thing was the frequency of the sound. If we treat the hearing of laboratory carnivores as a model for the polar bear, Charly was making a sound he couldn’t hear very well, which didn’t make much sense. To show you what we mean, we modified Charly’s rumble by passing it through a filter we based on the hearing curve of a dog. Conceptually, what we did is like turning down the bass speaker on a stereo. Afterwards, the deep, low rumble that aroused our primitive hair-raising response was hardly audible! (Figure 2, Charly’s rumble as heard by a dog).
Our discovery suggests that polar bears might be specialized to communicate at low frequencies. Without more observations, and some information about polar bear sensitivity to vibration, we can’t really know how Charly’s rumble might be used in the wild. Until we know more, we can’t speculate about how human-made noise might affect polar bears. All we can say for now is that the discovery emphasizes how much we have to learn about polar bear behavior!
Anne E. Bowles is a Senior Research Scientist with the Hubbs-SeaWorld Research Institute. Megan Owen is a Conservation Program Specialist with the San Diego Zoo’s CRES (Conservation and Research for Endangered Species). In addition to funding from PBI, the project is supported by the Hubbs-SeaWorld Society and CRES, with additional in-kind support from SeaWorld of California and the Zoological Society of San Diego.
Two things struck us immediately. First, this was a vocalization we’d never seen described in the polar bear literature. The closest thing was a growl that researchers back in 1980 tried to use to scare bears away from human activities, but it didn’t have energy concentrated at low frequency, nor were we able to see any sign of the resonant bursts. The second thing was the frequency of the sound. If we treat the hearing of laboratory carnivores as a model for the polar bear, Charly was making a sound he couldn’t hear very well, which didn’t make much sense. To show you what we mean, we modified Charly’s rumble by passing it through a filter we based on the hearing curve of a dog. Conceptually, what we did is like turning down the bass speaker on a stereo. Afterwards, the deep, low rumble that aroused our primitive hair-raising response was hardly audible! (Figure 2, Charly’s rumble as heard by a dog).
Our discovery suggests that polar bears might be specialized to communicate at low frequencies. Without more observations, and some information about polar bear sensitivity to vibration, we can’t really know how Charly’s rumble might be used in the wild. Until we know more, we can’t speculate about how human-made noise might affect polar bears. All we can say for now is that the discovery emphasizes how much we have to learn about polar bear behavior!
Anne E. Bowles is a Senior Research Scientist with the Hubbs-SeaWorld Research Institute. Megan Owen is a Conservation Program Specialist with the San Diego Zoo’s CRES (Conservation and Research for Endangered Species). In addition to funding from PBI, the project is supported by the Hubbs-SeaWorld Society and CRES, with additional in-kind support from SeaWorld of California and the Zoological Society of San Diego.
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