Grizzly bears, which may grow to about 2.5m long and weigh over 400kg, occupy a conflicted corner of the American psyche — we revere them even as they give us frightening dreams.

Ask the tourists from around the world that flood into Yellowstone National Park what they most hope to see, and their answer is often the same: a grizzly bear.

"Grizzly bears are re-occupying large areas of their former range," says bear biologist Chris Servheen.

As grizzly bears expand their range into places where they haven't been seen in a century or more, they're increasingly being sighted by humans.

The western half of the U. S. was full of grizzlies when Europeans came, with a rough number of 50,000 or more living alongside Native Americans.

By the early 1970s, after centuries of cruel and continuous hunting by settlers, 600 to 800 grizzlies remained on a mere 2 percent of their former range in the Northern Rockies.

In 1975, grizzlies were listed under the Endangered Species Act.

Today, there are about 2,000 or more grizzly bears in the U. S.

Their recovery has been so successful that the U. S. Fish and Wildlife Service has twice attempted to de-list grizzlies, which would loosen legal protections and allow them to be hunted.

Both efforts were overturned due to lawsuits from conservation groups.