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Scientists monitoring free-roaming polar bears in northern Canada witnessed something remarkably uncommon this fall: a mother bear adopting a cub that was not genetically her own.

The half-decade-old mother and her nearly year-old cubs were spotted 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 concentration of Arctic bears.

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

It's only the 13th known case of fostering out of 4,600 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 a single offspring, which scientists marked for study.

In the autumn, she was seen again, but this time with two cubs - the initially tagged cub and another that was untagged. Scientists aren't sure what became of the new cub's birth mother, but they are trying to identify her using DNA analysis.

"These animals need every advantage they can get these days with climate change," noted a wildlife biologist. "When mother bears have the opportunity to take in another cub and raise it and see it to independence, it's a good thing for the bears."

Polar bears in the natural world only have a fifty-fifty survival rate of surviving into adulthood, but having a maternal figure 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 two-and-a-half years old.

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

"It's just nice to know that the animals are caring for each other," added the researcher.