Scientists study hearing in polar bears as the Arctic acoustic environment changes. As global warming melts Arctic ice and pressure mounts for oil exploration, scientists worry that increasing noise might interfere with polar bear reproduction.
In the frozen tundra on the northern edge of the Arctic National Wildlife Refuge in Alaska, solitary polar bears live in a world that is both bitterly cold and eerily quiet. No leaves rustle in the breeze. No insects buzz through the sub-zero air. The only sound is the hiss of the wind-blown snow, and when the wind fades, it is one of the quietest places on Earth.
Several years ago, San Diego-based research biologist Anne Bowles wondered what would happen if retreating ice sheets and intensified oil exploration and drilling changed the nature of this silent "acoustic environment" of the polar bears. The impact of increasing noise on how polar bears hunt and their other little-understood behaviors was a concern, she said. But perhaps the most crucial question was whether an increase in noise would affect mating habits and disturb female polar bears in their dens. Alaska's Prudhoe Bay, where Bowles is focusing her work, is a prime area for female polar bears to construct their dens. They spend five or six months in the dens before emerging in the spring, typically with one or two cubs.
What kind of noise might be a problem for the bears? Will noise from human activity bother bears in the open, but not females in their dens? Should there be limits on noise allowed in the vicinity of the bears, and exactly what kind of noise would be a problem? "If you want to mitigate noise, you first have to know what the bear can hear," Bowles said. "That's step one. That is disturbance ecology 101."
So Bowles, of the Hubbs-Sea World Research Institute in San Diego, set out to test the hearing of polar bears. Given the hostile environment, and equally hostile bears, trying to conduct hearing tests on bears in the wild was clearly impractical, she said. Polar bears are perhaps the largest land carnivore, with males weighing as much as 1,400 pounds. Although females are much smaller, they still can top 500 pounds. And they are smart, dangerous and unpredictable, Bowles said.
Bowles began her hearing tests at the San Diego Zoo with two female polar bears who had been raised in captivity. For the tests, the bears were trained to hold their noses against a button, or station, on the inside of a cage. A tone of a certain frequency was played, and when the bear heard it, she would move her nose to another station and be rewarded with food, Bowles said.
Before the testing could begin, the cage was insulated against outside sound with thick, lead-lined blankets. Even with the heavy insulation, in the urban setting of San Diego it was impossible to make the cage quiet enough to test for extremely low frequencies, which may be a critical range of hearing for polar bears.
"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," Bowles wrote recently in an article for the Polar Bear International organization, which funds some of her work. "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."
She also noted that there is usually more environmental noise at low frequencies, including noise from "human-made machinery."
Bowles conducted 4,000 hearing tests, first on the San Diego Zoo polar bears, and then on two polar bears at SeaWorld San Diego. Hearing thresholds were measured for 19 different frequencies, and the results suggest that hearing in polar bears is indeed shifted toward the lower frequencies. How low remains uncertain, she said, because it was impossible to make the test facilities quiet enough to test below 14 kilohertz (humans typically can hear down to 20 hertz).
"As far down as you can measure, they can hear it," Bowles said. "They got down below what a cat can hear." Conversely, she noted that Polar bears don't hear well at high frequencies, with their sensitivity to the high tones being much less than that of dogs and cats.
The sensitivity to low frequency sound, apparently at the expense of high frequencies, might merely be a result of size, Bowles said. Polar bears are big, and so are the structures in their ears. In other large mammals, such as horses and cows, the upper limit to what frequencies the animals can hear that seems to be based on size. "It could just be an outcome of scaling," Bowles said.
But the lack of high frequency hearing might be adaptive, because polar bears mostly eat seals, not small rodents that have high-frequency squeaks. Bowles said she would like to do a comparison study of land-based brown and black bears that do eat small mammals to see if their hearing is attuned to higher frequencies than polar bears.
The concern with the polar bears, she said, is that as the Arctic ice melts, the bears will move onto land and encounter human civilization, which is noisy. "Just as for humans, there is noise you can deal with and noise you can't, and we don't know what that is for the bears. Our job is to find out what they can tolerate and what we are looking for is not 'win-win,' but 'okay-okay' for both bears and people."
There is enormous political pressure for the oil industry to get into these areas, she said, and with the oil activities come people and more use of the land for recreation. From her time in the region, Bowles said, the oil industry workers are treating the bears and other wildlife well. "You wouldn't believe the care these guys are giving to the wildlife," she said. "I wish we could get people in the cities to behave that way."
After three years of testing the bears in San Diego, Bowles and her co-investigator, Megan Owen, a researcher with the zoo's Conservation and Research for Endangered Species facility, are writing several scientific papers, including one for the Acoustical Society of America, about their results and looking toward what they hope is the next phase of their research.
In December or January, Bowles hopes to go to Prudhoe Bay and construct a polar bear den, then measure outside noise from the inside. "We need to create a den to bear specifications, and have it accessible enough that we can have vehicles nearby so we can measure the sound," she said. "We have to get information on what they can hear inside the den, how much noise gets into the den. If they can't hear it, it isn't an issue, but we have to make the measurements."
There won't be a real bear in the den, for as Bowles noted several times, polar bears are very dangerous. Even the San Diego bears who have been raised around people remain unpredictable carnivores. "Most of the time they are sweet and glad to see you, and glad to have the food your bring them," Bowles said of the bears. "Then there is the day when you are the food. They are very opportunistic predators."
For images to go with this story, please go to: http://www.aip.org/isns/reports/2008/028.html