Grizzly bears in the nation's oldest national park eat more meat than their relatives anywhere else in North America, according to the preliminary findings of a new study.

Grizzlies have learned to rely on Yellowstone's vast herds of elk and bison as a primary food source and those animals have replaced the calories grizzlies lost when the park's dumps closed in 1971, says the study by Kerry Gunther and Mark Haroldson of the Interagency Grizzly Bear Study Team.The study, now in draft form, is undergoing peer review, a formalized evaluation by other scientists.

Observers have noted for years that the park's bears rely on meat they find or kill, but now there is data to back them up, researchers say.

And the meat seems to suit the bears. Females are now bearing larger litters of cubs and doing so at an earlier age, the study shows.

Adult male grizzlies in the park take 79 percent of their energy from other animals and adult females get 45 percent of their energy that way, according to Gunther, head of the Yellowstone bear management office.

The test results come from hair samples gathered in Yellowstone over the years and recently evaluated at an independent laboratory in Washington.

"Major things in the diet show up in the hair," Gunther said. The analysis shows high levels of protein in the diet, though it is not detailed enough to show how much comes from the herd animals and how much comes from rodents and insects, also favorites of bears.

In most other places, grizzlies get most of their food from plants. In Glacier National Park, for instance, grizzlies rely on berries, roots and other plants for 90 percent or more of their calories, National Park Service officials say.

The study, if proven valid in peer review, could further complicate decisionmaking in the park.

"Managers really ought to take all this into account when they make ungulate decisions," Gunther said.

There is an ongoing debate over grazing conditions and elk and bison numbers in the park, with some calling for large reductions in the number of animals. But grizzlies are protected as a threatened species and rely on elk and bison, the study says, and that could have implications in elk or bison management decisions.

Most of the meat grizzlies consume comes from elk and bison carcasses bears find, Gunther said, but some bears are becoming very effective hunters, especially for elk calves.

Grizzlies are also preying on weakened bull elk and bison and are scavenging leftovers from wolf kills.

Better fed bears are "fat and happy" and tend to avoid people, Gunther said.