A Bear Footnote to Isotope Ratios

Don Schell does isotopes. That means when he says you are what you eat, he's speaking factually, and in a special sense.

From his base at the Institute of Northern Engineering of the University of Alaska Fairbanks, Schell studies isotope ratios in the tissues of different kinds of northern animals. It's informative work, thanks to the property of plants to pick and choose among different isotopes of carbon.

Almost 99 per cent of all naturally occurring carbon has an atomic mass number of 12; almost all the rest is a heavier isotope, carbon 13, with a smidgin of radioactive carbon 14. As a plant takes in carbon dioxide during photosynthesis, it slightly alters the initial proportions of the carbon isotopes. In the marine environment, geographic differences in environmental conditions cause distinct geographic signatures in the isotope ratios of the plants. These isotopic signatures are conservative; that is, they can be traced through the food chain. The ratios are so reliable that Schell and his colleagues believe it's often possible to track what proportion of a marine animal's food came from which region. This seems to hold true even if that food is several steps removed from the phytoplankton that provided the original carbon.

Eventually the isotope studies led Schell into work identifying the home bases of polar bears. Direct observation and radio tracking over the years led U.S. Fish & Wildlife Service researchers to believe that bears congregating in winter on Alaska's Chukchi Sea coast spent the rest of the year in different places--even in different national waters. Some of the bears came down from the ice north of the Soviet Union, while others wandered in from the Beaufort Sea, perhaps from beyond the Canadian boundary. Knowing about this is important for managing polar bear populations, not to mention international politics. Schell was sure that isotope signatures would show up in the bears and would help determine where they came from. Using bits cut from along the claws, which take several months to grow, it might even be possible to get some indication of a bear's past movements.

Analysis of bear claws supplied by USFWS researchers seemed to support his claim very nicely. He was, he says, confident that the isotope ratios were telling him which polar bears hailed from the Beaufort Sea and which were from other populations. However, the technique needed to be verified by checking it in a bear whose movements were known.

Steve Amstrup of the USFWS knew the exact movements of one bear over several years. That bear had worn a radio collar monitored by satellite--until she was shot. Amstrup sent the hide to a taxidermist, then donated it to the University of Alaska Museum. Now he challenged Schell to prove that the isotopes would match what he knew about the bear's home range.

Even in the cause of science, it's not easy to convince a museum to let go of an exhibit. Schell finally talked the requisite officials into parting with two claws, one from a left forepaw and one from a right hind. Carefully, he did the analytical work--and found the samples did not match. Back to the museum he went, wheedling more claws from the bearhide. This time the ratios indicated that a couple of claws (not even neighboring ones) had isotope ratios proper for a Beaufort Sea bear, while others should have grown perhaps on a Wrangell Island bear.

The unhappy Schell spoke to bear biologists about claw loss and regrowth, uneven wear. anything that might help. Nothing did. Then he began to check the processes of tanning a bearhide, hoping that might have affected the carbon in the claws. He found that an expert polar bear taxidermist presently lived in Cordova. Schell tracked him down, and together they went over the chemistry of preparing polar bear hides.

"Listen," Schell finally said. "None of what you're telling me could affect what I'm looking for. Is there anything special you do with the claws?"

"Oh, the claws often fall out in tanning," the taxidermist said. "So we keep this box of polar bear claws, and the last thing we do with a mount is glue on some claws. They're all pretty much the same, after all."

It took some time for Schell to be able to laugh when he tells this story, and it's still not wise to make jokes about bear-foot boys around him.