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Researchers monitoring free-roaming polar bears in northern Canada witnessed something exceedingly rare this fall: a female bear taking in a cub that was not biologically her own.

The half-decade-old maternal bear and her 10- to 11-month-old cubs were spotted and filmed during the annual polar bear migration along the Hudson Bay's western shore near the Manitoba town of Churchill, an locale renowned for its concentration of Arctic bears.

"This is uncommon," stated a scientist with an international bear organization. "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 adoption out of 4,600 bears researched for nearly five decades in that region.

The female bear was first captured as she emerged from her birthing den this spring. At the time, she had one cub, which scientists tagged for study.

In the autumn, she was spotted once more, but this time with a pair of cubs - the initially tagged cub and another that was untagged. Scientists are uncertain what became of the new cub's birth mother, but they are attempting to locate her using genetic samples.

"These animals need every advantage they can get these days with climate change," commented a wildlife biologist. "When mother bears have the chance to pick up an additional youngster and raise it and successfully wean it, it's a positive development for the bears."

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

The cubs appear to be healthy, researchers said, and they will likely remain with their adoptive mother until they are about 30 months of age.

Next, the bear family is expected to venture out to the frozen ocean, where the cubs will be taught from their mother how to catch prey and live on their own.

"It's just nice to know that the bears are looking out for each other," concluded the researcher.