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Scientists tracking free-roaming polar bears in northern Canada observed something remarkably uncommon this fall: a mother bear taking in a cub that was not biologically her own.

The half-decade-old maternal bear and her nearly year-old cubs were observed and captured on camera during the annual 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.

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

This represents only the thirteenth documented instance of fostering out of 4,600 bears researched for nearly five decades in that region.

The mother bear was initially tagged as she came out from her birthing den this spring. Initially, she had a single offspring, which scientists marked for research.

In the fall, she was seen again, but this time with two cubs - the initially tagged cub and a second that was untagged. Scientists are uncertain what happened to the second cub's biological mother, but they are trying to locate her using DNA analysis.

"These animals need every advantage they can get in the current era with a warming climate," noted a wildlife biologist. "If females have the chance to take in an additional youngster and raise it and successfully wean it, it's a positive development for the population."

Polar bears in the wild only have a 50% chance of surviving into adulthood, but having a mother caring for them boosts their chances.

The young bears appear to be in good condition, researchers said, and they will probably stay with their mother until they are about two-and-a-half years old.

Next, the family is expected to venture 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 animals are looking out for each other," added the researcher.