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Scientists tracking free-roaming polar bears in northern Canada observed something remarkably uncommon this fall: a female bear adopting 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 captured on camera during the yearly polar bear migration along the Hudson Bay's western shore near Churchill, Manitoba, an locale renowned for its polar bear population.

"This is uncommon," remarked 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 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 maternity den this spring. Initially, she had a single offspring, which researchers tagged for study.

In the autumn, she was spotted once more, but this time with a pair of cubs - the original cub and a second without a tag. Researchers are uncertain what happened to the second cub's birth mother, but they are trying to identify her using genetic samples.

"These animals need every advantage they can get these days with climate change," noted a wildlife biologist. "If females have the chance to take in another cub and raise it and successfully wean it, it's a positive development for the population."

Polar bears in the natural world only have a 50% chance of living into maturity, but having a mother caring for them improves their chances.

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

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

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