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Scientists monitoring wild polar bears in northern Canada witnessed something exceedingly rare this fall: a mother bear taking in a cub that was not genetically her own.

The half-decade-old mother and her nearly year-old cubs were observed and filmed during the annual polar bear migration along the Western Hudson Bay near Churchill, Manitoba, an locale renowned for its polar bear population.

"It's unusual," remarked a researcher with an international bear organization. "The reasons are unclear why it happens...but we know it is a rare event at all."

This represents only the 13th known case of fostering out of 4,600 bears researched for nearly five decades in that region.

The mother bear was initially tagged as she came out from her maternity den this springtime. At the time, she had one cub, which researchers marked for study.

In the autumn, she was spotted once more, but this time with two cubs - the initially tagged cub and a second that was untagged. Researchers are uncertain what became of the second cub's birth mother, but they are attempting to identify her using DNA analysis.

"These animals need every advantage they can get these days with a warming climate," noted a polar bear scientist. "When mother bears have the chance to pick up another cub and care for it and see it to independence, it's a good thing for the bears."

Polar bears in the wild only have a fifty-fifty survival rate of surviving into adulthood, but having a maternal figure caring for them boosts their odds.

The cubs are reported as healthy, researchers said, and they will probably remain with their adoptive mother until they are about two-and-a-half years old.

Following this, the bear family is expected to head out to sea ice, where the cubs will be taught from their mother bear how to hunt seals and live on their own.

"It is heartening to know that the bears are looking out for each other," added the researcher.