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Researchers tracking free-roaming polar bears in northern Canada witnessed something exceedingly rare this autumn: 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 yearly polar bear migration along the Western Hudson Bay near the Manitoba town of Churchill, an locale renowned for its concentration of Arctic bears.

"It's unusual," stated a researcher with an international bear organization. "We don't really know why it occurs...but we know it doesn't happen often at all."

It's only the 13th known case of adoption out of nearly five thousand bears studied for nearly five decades in that region.

The mother bear was initially tagged as she emerged from her birthing 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 a pair of cubs - the original cub and another that was untagged. Scientists are uncertain what happened to the second cub's biological mother, but they are trying to locate her using genetic samples.

"These animals need all the help they can get these days with a warming climate," commented a wildlife biologist. "When mother bears have the chance to pick up another cub and raise it and see it to independence, it's a positive development for the bears."

Polar bears in the natural world only have a 50% chance of living into adulthood, but having a mother providing care improves their odds.

The young bears appear to be healthy, researchers said, and they will probably stay with their adoptive mother until they are about 30 months of age.

Following this, the family is anticipated to head out to the frozen ocean, where the youngsters will learn from their mother bear how to hunt seals and survive on their own.

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