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Researchers monitoring free-roaming 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 five-year-old maternal bear and her 10- to 11-month-old cubs were observed and filmed during the yearly polar bear journey along the Hudson Bay's western shore near the Manitoba town of Churchill, an locale renowned for its polar bear population.

"This is uncommon," stated a researcher with a polar bear conservation group. "The reasons are unclear why it happens...but we know it doesn't happen often at all."

It's only the 13th known case of adoption out of 4,600 bears studied for almost fifty years in that region.

The female bear was first captured as she came out from her birthing den this springtime. Initially, she had a single offspring, which scientists tagged for study.

In the fall, she was spotted once more, but this time with two cubs - the original cub and another without a tag. Scientists aren't sure what happened to the second cub's biological mother, but they are trying to locate her using genetic samples.

"The bears need every advantage they can get these days with climate change," noted a wildlife biologist. "If females have the opportunity to pick up an additional youngster and care for it and see it to independence, it's a good thing for the population."

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

The young bears are reported as healthy, researchers said, and they will probably stay with their mother until they are about 30 months of age.

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

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