SeaWorld's polar bear Charly has been trained to repond to certain sounds
Scientists in California are testing the hearing of polar bears to try to find out whether the noises associated with melting Arctic ice could affect their ability to survive.
In the wild, polar bears live in one of the quietest places on Earth. For much of the time, the Arctic is a bitterly cold, silent world.
But global warming is changing that. Ice, which is crucial to the bears' survival, is disappearing and people are moving in.
"We're expecting industrial activity, shipping, recreation, all of those human activities to increase in the Arctic," says Dr Ann Bowles, a senior research scientist at Hubbs-Seaworld Research Institute in San Diego.
"We're going to be bringing noise and activity much closer to these guys. What we're trying to do is help to protect the bears during this period of transition," she says.
When things are happening to (polar bears) they're also happening to all of the wildlife underneath them - all the Arctic food web.
Dr Ann Bowles, Hubbs-SeaWorld Research Institute
Polar bears are known to be extremely sensitive to sound. But the scientists are trying to establish the animals' precise range of hearing.
"What we're trying to do is understand how sensitive they are and how well they hear both the high end and low end of the frequency range," explains Dr Bowles. "Whether or not an animal can hear industrial noise depends upon the shape of that curve."
Increasing noise levels could affect polar bears in a number of ways. Their breeding patterns could be disrupted if they are unable to hear each other over long distances. They may be afraid of what they are hearing since human activity signals danger to them.
Conversely, the animals may mistakenly think unusual noises indicate food and they could be encouraged to migrate to inappropriate areas.
How to test a polar bear's hearing
At the Seaworld marine mammal park, scientists have been performing an experiment with a 12-year-old male polar bear named Charly. The animal has been trained to respond, by moving his head between metal plates on the wall of his enclosure, when he hears a computer-generated tone.
"The deal we've made with him is that when you do that, you get something that you like. Today it's different kinds of fruit, fish and pure fat, which he really likes," says Mike Price, Charly's trainer.
"He very much enjoys the game. When I'm setting up equipment he gets really excited. For him it's a very enriching day. This game works very well for him but is also helping science, so everyone's winning."
The data suggests that polar bears are much more sensitive to sound than humans. The implication is that noise generated by human activity, such as construction or shipping, could be audible to the bear over a wide area, while people are unable to detect anything.
Dr Bowles says: "Now we're going to have to start asking the wild bears the question; if you hear this what are you going to do?"
To eavesdrop on the bears in their natural habitat the scientists are tracking animals, wearing electronic collars, along the coast of Alaska and Canada.
Charly was rewarded for his part in the experiment
The US government recently listed the polar bear as a threatened species because its Arctic sea ice habitat was melting at a rate that alarmed many conservationists.
But the animal, often referred to as the poster child of climate change, symbolises a much broader problem for the Arctic environment.
"The polar bear is the apex, so we model them as being the top predator," explains Dr Bowles.
"When things are happening to them, they're also happening to all of the wildlife underneath them - the seals, the fish, the invertebrates, all the Arctic food web.
"This is about looking at the collapse of an entire ecosystem. And that's why the biologists are so concerned about this problem."
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