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Scientists tracking wild polar bears in northern Canada witnessed something exceedingly rare this fall: a female 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 area known for its concentration of Arctic bears.

"This is uncommon," remarked a researcher with an international bear organization. "We don't really know why it happens...but we know it is a rare event at all."

This represents only the thirteenth documented instance of adoption out of nearly five thousand bears researched for nearly five decades 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 scientists marked for study.

In the autumn, she was spotted once more, but this time with a pair of cubs - the original cub and a second that was untagged. Researchers aren't sure what became of the new cub's biological mother, but they are attempting to locate her using genetic samples.

"These animals need all the help they can get in the current era with climate change," commented a polar bear scientist. "When mother bears have the chance to take in an additional youngster and care for it and successfully wean it, it's a good thing for the population."

Polar bears in the natural world only have a 50% chance of surviving into maturity, but having a maternal figure caring for them improves their chances.

The cubs are reported as in good condition, researchers said, and they will probably remain with their mother until they are about 30 months of age.

Next, the family is expected to head out to the frozen ocean, where the cubs will learn from their mother how to catch prey and survive on their own.

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