Researchers in northern Canada have observed a rare case of polar bear adoption, capturing video footage of a wild female bear caring for a cub that was not her own.

"Cub adoption is relatively rare in polar bears. We've documented 13 cases in our study population over the last 45 years," said Evan Richardson, a scientist with Canada's ministry for environment and climate change.

The footage of a bear caring for an adopted cub was captured during the annual polar bear migration along the Western Hudson Bay in Churchill, Manitoba, widely known as the polar capital of the world.

Canadian researchers encountered the mother in the spring as she left her maternity den. She had only one cub, which was tagged — a common practice to aid study of the population.

They encountered the same mother again weeks ago but saw a second cub with no ear tag, Richardson told AFP.

"When we went back and looked at the data, we realized that she had adopted a second cub," he said.

Video footage collected by the researchers shows the cubs surveying a snow-covered landscape, with the mother pacing behind, and one sequence where one cub hurries to join the others.

Both cubs are 10 to 11 months old, and will likely stay with their mother until about 2.5 years of age.

"When we got confirmation that this was an adoption, I had a lot of mixed feelings, but mostly good," Alysa McCall, a staff scientist with Polar Bears International, said in a . "It's just another reason why this species is so incredible, why they're so fascinating and interesting, and it gives you a lot of hope when you realize that polar bears may be looking out for each other out there."

Researchers currently have no information as to what happened to the adopted cub's biological mother.

But having a maternal figure increases the chances of the cub's survival into adulthood, Richardson said.

"It's really a feel-good story to know that this female bear is looking after this cub and that it has a chance at survival," Richardson said.

"These female polar bears are such good moms, they're maternally primed to take care of offspring, and when there's a lone cub out on the tundra, bawling and crying, they just take them under their wing," he added.

There are currently about 26,000 polar bears worldwide, according to the nonprofit . The International Union for Conservation of Nature lists polar bears as a vulnerable species, saying sea ice loss from climate change is the biggest threat to their survival. 

This isn't the first time wild animals have been observed with babies that aren't their own. Earlier this year, scientists said video showed capuchin monkeys carrying at least 11 howler babies in Panama.

At first, the researchers thought it was a "heartwarming story of a weird capuchin adopting these infants," said Zoë Goldsborough, a behavioral ecologist at the Max Planck Institute of Animal Behavior in Germany. 

But Goldsborough said they eventually realized the abductions were a social tradition or "fad" among the island's young male capuchins, and in most or all cases, the baby howlers died.