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The grizzly bears in Yellowstone National Park are still trying to adapt to life without garbage, despite being weaned from the unnatural food source nearly two decades ago, say two independent researchers.

The entire food chain, from earth worms to elk, has been affected by the changes forced upon the bears, Marilynn and Steve French told a group of Forest Service Northern Region biologists at a midweek conference.They are founders of the Yellowstone Grizzly Foundation, an independent research group. French is a doctor. His wife is an intensive-care-unit nurse.

He said it has been 20 years since the National Park Service disallowed the feeding of grizzly bears from garbage dumps and other sources of human food. "They are still learning how to survive on what nature provides," he said.

"Elk calves have become a valuable part of the bears' spring diet," Marilynn French said, noting they have seen one grizzly kill as many as five calves in 15 minutes. The average is about two kills a day, she said.

The Frenches said bears have learned to search for newborn elk calves bedded in tall grass. The bears begin hunting two weeks earlier in the calving season than they did when the couple began researching nearly 20 years ago. When calves are several weeks older, they are harder to catch, and the bears' success rate drops.

The elk, in turn, are learning how to defend themselves. They explained how a calf's mother or another adult elk runs between a bear and an endangered calf, breaking the bear's eye contact and sometimes allowing the calf to escape.

An adult may also run beside a calf being chased, pacing the younger animal and guiding its escape. In some instances, they have filmed mother elk with newborn calves moving closer to a bear as the animal stalks the calf in tall grass, finally forcing the bear to chase the adult away and distracting it from the calf.

"We have also seen elk strike out at grizzly bears with their front hooves," Marilynn French said.

The couple said Yellowstone's bison tend to defend their ranks better against preying grizzly bears. Bison keep their young in the middle of a herd as a grizzly moves around the herd's perimeter looking for a likely victim.

"As the bear approaches, a couple of large bulls will come out and escort the bear around the herd, keeping themselves between the bear and the other bison," Steve French said.

Bears also go after the increasing number of cutthroat trout available in streams during the spawning season, the researchers reported. They reported instances of bears catching 100-200 fish a day.

The Frenches also said they have documented the only known cases of grizzly bears feeding on earthworms when the worms are driven to the tops of grassy fields by a rain.

"We don't know yet what the nutritional value of earthworms may be, but we suspect it might involve the mineral content," he said.