By Doreen Walton
Science reporter, BBC News
A grizzly bear Ursus arctos, spotted from the air in Manitoba, Canada
Grizzly bears are being spotted in Manitoba, Canada, where biologists say only polar bears are usually found.
Scientists from City University of New York say sightings are increasing.
"Grizzlies would likely hibernate in polar bear maternity denning habitat," says CUNY's Linda Gormezano.
They would come out of hibernation at the same time and can kill polar cubs," warned Ms Gormezano, who has seen the grizzlies from the air.
The researchers first spotted a grizzly bear (Ursus arctos ) in August 2008.
"The first time we saw a grizzly we were flying over the middle of Wapusk, counting fox dens," explained CUNY's Professor Robert Rockwell, who is also a research fellow at the American Museum of Natural History.
Since then they have examined the records and found no evidence of grizzly bears before 1996. Between 1996 and 2008 they found nine confirmed sightings. In the summer of 2009, three more were spotted.
The data has been published in Canadian Field-Naturalist.
Pizzlies or grolars?
The scientists say that the barren landscape north of the Hudson Bay had been thought to be impassable for migrating grizzlies. But now some have reached Canada's Wapusk National Park to be rewarded with abundant caribou, moose, fish and berries.
"We don't yet know if they are wandering or staying - the proof will come from an observed den or cubs - these animals will eventually be residents of this national park," said Professor Rockwell.
A wild hybrid between a grizzly and polar bear was shot in Canada
"The Cree elders we talked to feel that now grizzly bears have found this food source, they will be staying.
"A big question is how to deal with these new residents... In Canada, both the polar and grizzly bear are federally listed as species of special concern," he added.
As grizzly bears move into areas usually populated only by polar bears, hybrids between the two become more likely.
Also known as pizzly, prizzly or grolar bears, these have been bred in zoos. Their presence in the wild was confirmed by DNA tests after one was shot in Banks Island, in the Canadian arctic.