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Researchers tracking free-roaming polar bears in northern Canada witnessed something remarkably uncommon this autumn: a mother bear taking in a cub that was not genetically her own.

The five-year-old maternal bear and her nearly year-old cubs were observed and filmed during the annual polar bear journey along the Western Hudson Bay near Churchill, Manitoba, an area known for its concentration of Arctic bears.

"It's unusual," stated a scientist with a polar bear conservation group. "The reasons are unclear why it occurs...but we know it is a rare event at all."

This represents only the 13th known case of fostering out of nearly five thousand bears researched for almost fifty years in the area.

The mother bear was initially tagged as she emerged from her maternity den this spring. At the time, she had one cub, which researchers marked for research.

In the autumn, she was seen again, but this time with a pair of cubs - the original cub and a second that was untagged. Researchers are uncertain what happened to the new cub's biological mother, but they are attempting to locate her using genetic samples.

"The bears need all the help they can get in the current era with a warming climate," noted a wildlife biologist. "When mother bears have the opportunity to take in an additional youngster and care for it and successfully wean it, it's a good thing for the bears."

Polar bears in the natural world only have a 50% chance of living into maturity, but having a maternal figure providing care improves their chances.

The young bears appear to be in good condition, according to experts, and they will probably remain with their mother until they are about 30 months of age.

Following this, the family is anticipated to head out to sea ice, where the youngsters will be taught from their mother bear how to catch prey and live on their own.

"It is heartening to know that the bears are caring for each other," concluded the scientist.