Polar bears face a new threat besides melting ice — male grizzly bears are moving into their territories, competing for food and are even mating with their females.

Scientists have already discovered one case of a hybrid “grolar” bear and are circulating requests to hunters and polar tour operators to look out for more.

One possible explanation for closer interaction between the species is climate change, which has allowed grizzlies to move north into areas that were once too cold for them.

“The two species are likely to interact in a variety of ways, including competing for food and places to build dens, as well as possible mating,” said Professor Robert Rockwell, of the American Museum of Natural History in New York.

Grizzly bears have traditionally been found in Alaska, western Canada and northwestern American states, including Idaho and Montana. They are thought to number about 60,000.

However, Rockwell will soon publish research showing how grizzlies are moving into Wapusk National Park on the edge of Hudson Bay in eastern Canada, an area previously dominated by polar bears. “They were never seen here until recently but this year we have had several sightings already,” he said.

Grizzlies have also been found hundreds of miles further north on the remote Canadian Arctic islands.

In 2006 Jim Martell, a hunter, shot what he thought was a polar bear on Banks Island, but was threatened with prosecution for shooting a grizzly. It had the long claws and hump back typical of grizzlies but genetic tests revealed that it was a hybrid with a grizzly father and polar bear mother.

On nearby Victoria Island, an invading grizzly ate two polar bear cubs. Such encounters seem likely to become increasingly common. Professor John England, a geologist at Alberta University in Canada, recently photographed newly arrived grizzlies on Melville Island, 1,000 miles north of the Arctic Circle — the most northerly sighting ever recorded. They would have had to cross 60 miles of sea ice to get there.

The contradictory behaviour of the two species — sometimes mating and sometimes fighting — may reflect their uncertainty towards each other.

Polar and grizzly bears are actually closely related. Genetic studies suggest polar bears evolved from a population of grizzlies that became cut off in a remote part of Alaska during an ice age about 200,000 years ago — a short period of time in evolutionary terms.

Scientists suggest that the white coat of polar bears evolved because paler creatures would have had an advantage in hunting seals.

In genetic terms, however, such differences are superficial. In captivity polar bears and grizzlies can interbreed, with their offspring also being fertile — a sign that their DNA is similar.

German scientists are to publish a study of hybrid bears bred in zoos in the Czech Republic, Russia, Spain, Poland and Germany. Dr Alexandra Preuss, of Osnabrück Zoo in Germany, found the hybrids had the longer necks typical of polar bears and the shoulder humps found on grizzlies.

The soles of their feet were only partially covered in hair. Polar bears’ feet are totally covered in hair to insulate them from the ice; brown bears have hairless soles and clearly visible toes.

Some zoologists believe that the differing lifestyles of polar bears and grizzlies will prevent widespread interbreeding in the wild.

Ian Stirling, a research scientist with the Canadian Wildlife Service, said the two species had coexisted on the coast of Alaska for many years, often feeding together on whale carcases left by Inuit hunters in the autumn.

He said: “The areas they inhabit do overlap for part of the year but it is outside the breeding season. More often the species are just aggressive to each other.”

Rockwell said: “When female polar bears have their cubs the young will be in great danger of being eaten by the grizzlies. On the other hand, male polar bears can weigh up to 1,200lb — twice the size of a female grizzly. They could mate with the female or eat her or perhaps both. Things could get very different for both species.”

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