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Researchers tracking wild polar bears in northern Canada observed something exceedingly rare this autumn: a mother bear taking in a cub that was not biologically her own.

The half-decade-old mother and her 10- to 11-month-old cubs were observed and filmed during the yearly polar bear migration along the Western Hudson Bay near Churchill, Manitoba, an locale renowned for its concentration of Arctic bears.

"This is uncommon," remarked a researcher with an international bear organization. "The reasons are unclear why it occurs...but we know it doesn't happen often at all."

This represents only the 13th known case of fostering out of 4,600 bears studied for nearly five decades in that region.

The mother bear was initially tagged as she emerged from her maternity den this springtime. At the time, she had one cub, which scientists marked for research.

In the autumn, she was spotted once more, but this time with a pair of cubs - the initially tagged cub and a second without a tag. Scientists aren't sure what happened to the second cub's birth mother, but they are trying to locate her using genetic samples.

"These animals need every advantage they can get in the current era with a warming climate," commented a polar bear scientist. "When mother bears have the opportunity to pick up an additional youngster and raise it and see it to independence, it's a positive development for the bears."

Polar bears in the natural world only have a 50% chance of living into adulthood, but having a maternal figure providing care boosts their chances.

The cubs are reported as healthy, according to experts, and they will probably stay 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 youngsters will learn from their mother how to hunt seals and live on their own.

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