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DEBBIE ELLIOTT, host:

This is ALL THINGS CONSIDERED from NPR News. I'm Debbie Elliott.

This year, NPR and National Geographic are traveling the world reporting on the Earth's changing climate. This month, we visit the world's biggest ocean - the Pacific. Scientists are trying to pick up clues to the effects of climate change by tracking animals that migrate across the ocean. One of the most promising of these living probes, and the most tireless, is the northern elephant seal.

NPR's John Nielsen recently spent a day with several thousand molting elephant seals and the researchers studying them at Ano Nuevo State Park in northern California. These creatures, he says, are capable of more than meets the eye and the ear.

JOHN NIELSEN: They are huge. They are fat. They stink. They lay around in piles napping all day and they belch operatically at strangers.

(Soundbite of elephant seal belching)

NIELSEN: Even the experts like Dan Costa of the University of California at Santa Cruz, elephant seals look like good for nothing bums when they lay around molting in the spring.

(Soundbite of elephant seal belching)

NIELSEN: But on the other hand.

Dr. DAN COSTA (Ecology and Evolutionary Biology, University of California-Santa Cruz): These guys have earned the little vacation sitting here on the beach, not doing anything because when they're at sea, they're just working nonstop. We have females that live here in Ano Nuevo and have gone clear and across the international dateline and back within eight months. And they're diving to 2,000 feet routinely, nonstop for that eight-month period of time.

NIELSEN: Costa said these long-range swimming skills make it easier for elephant seals to deal with regional food shortages tied to natural climate patterns like El Nino. But he worries that the seals won't be able to swim away from broader changes linked to warming oceans.

For example, what if the fish and squid the seals like to eat move further down into the water?

Dr. COSTA: If food goes deeper, they're going to have to hold their breath longer. They're going to have to swim harder. So in many respect, you could argue that these deep-diving animals are the ones that would be more sensitive to environmental change.

NIELSEN: The indicator species...

Dr. COSTA: The indicator species.

NIELSEN: To use the phrase.

(Soundbite of elephant seal belching)

NIELSEN: It takes a lot of high-tech gears to follow seals whose hunting trips take them halfway across the Pacific. It also takes some low-tech wrangling skills.

At Ano Nuevo, the researchers move through the seals like border collies moving through a crowd of sheep, gradually zeroing in on a 500-pound female, which by the standards of elephant seals is practically anorexic.

Jason Hassrick moves in quickly and injects a load of sedatives into the rear end of the female. Two minutes later, she's out like a light, although her eyes remained wide open. So far, so good says Hassrick, who works in Dan Costa's lab.

Mr. JASON HASSRICK (Ph.D. Student, Ecology and Evolutionary Biology, University of California-Santa Cruz): And now the key is to - watch her breathing and see how her nostrils are shut right now. She's not breathing. She's doing an apnea. It's something to really watch.

NIELSEN: These researchers can't get down to work until the seal starts breathing again. When it does, they will stick a bunch of high-tech tracking gizmos onto the body of the beast.

In their toolbox, they've got devices that will record depth and water temperatures. The latest global positioning receivers and stomach probes that will tell researches what the seals have been eating.

(Soundbite of elephant seal belching)

NIELSEN: These instruments send information back via satellite. Researchers sitting at their computer screens can follow the seals in real time. The researchers have been doing this tracking for three years now. And in that time, they've found that year after year, groups of seals tend to follow the same path out across the northwestern part of the Pacific.

Those tracks take the seals through a lot of feeding grounds that could get rearranged by warming oceans. Hassrick wonders how the seals will adapt.

Mr. HASSRICK: Do they have enough body reserves to successfully reproduce? Do they have enough body reserves to survive to the next year? What are the effects of not finding food on their diving behavior? Do they stay out longer? Are the numbers of pups born each year different depending on the climatic variations?

NIELSEN: Costa says the information gathered by these gauges will help oceanographers to learn more about changing ocean temperatures and currents. He says the seals may also lead scientists to biological hotspots that deserve special protection.

Dr. COSTA: For example, you see blue fin tuna and blue whales and California sea lions in the same area. You'll see elephant seals and Laysan albatross and salmon sharks feeding in the same areas way out in the North Pacific. What we're now interested in is defining what - where these hotspots are, what defines them oceanographically, why are they attracting this diverse group of animals.

(Soundbite of elephant seal belching)

NIELSEN: When the drugged seal starts breathing again, the researches go right to work. In less than an hour, they've weighed it, tested its blood, measured its body fat, checked it for injuries and glued a bright orange tracking device that looks for all the world like a party hat to the top of its head. This seal is now part of an early warning system used to measure changes linked to global warming. Costa says he hopes it works.

Dr. COSTA: I'm not going to pretend that by studying elephant seals, I'm going to come up with the solutions of global climate change. But I can, at least, identify that what animals are likely to be more sensitive to environmental change, and here's the species that we need to be more concerned about.

NIELSEN: The elephant seal with the orange party hat glued to the top of its head swam out to sea a few days later, diving repeatedly and swimming north and east. As of Friday, it was more than 900 miles from the beach at Ano Nuevo.

(Soundbite of elephant seal belching)

NIELSEN: John Nielsen, NPR News, Washington.

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