Mammoths on a weight-loss diet.

Mastodons and mammoths went extinct in North America about 11,000 years ago, and scientists still don't know why. Some experts think human hunters killed off these relatives of the elephant; others suspect the animals succumbed to environmental stress caused by a changing climate. Chemical studies of the bones left behind by these beasts may help solve the prehistoric puzzle, says Paul L. Koch of the geophysical laboratory at the Carnegie Institution of Washington (D.C.).

Koch analyzed the ratio of nitrogen-15 to nitrogen-14 in the fossils of 19 mastodons and six mammoths from the Great Lakes region. Aside from revealing what an ancient animal ate, nitrogen isotope ratios can also tell whether the animal suffered from malnutrition, Koch says. Animals facing a food shortage have higher nitrogen-15 values because they metabolize their own tissues; in a sense, they are eating themselves.

Koch found that mammoths had significantly higher nitrogen-15 ratios than the mastodons. These preliminary data, he says, suggest that mammoths in the Great Lakes region may have had a harder time finding food than did the mastodons, which are believed to have eaten a more varied diet. Koch plans to run tests on the remains of more mastodons, mammoths and other animals from various geographic regions to help unravel the cause of these extinctions.