Scientists in Canada have documented a rare case of female polar bear adopting a new cub, in an episode of “curious behaviour” that highlights the complex relationships among the apex Arctic predators.

Polar Bears International, a non-profit conservation group, said on Wednesday that when they first placed a GPS collar on a female polar bear in the spring, she had one young cub. But when she was spotted with two cubs of roughly the same age last month, they realized they were witnessing an exceedingly rare case of adoption.

“Female polar bears are really good moms and so they’re just primed for looking after and caring for their offspring,” said Evan Richardson, a research scientist with Environment and Climate Change Canada. “We think if there’s a little cub that’s bawling on the coast and has lost its mother, these females just can’t help themselves but to take them on and look after them. It’s a really curious behaviour and an interesting aspect of polar bear life history.”

The mother, known to scientists as bear X33991, and the cubs are part of the Western Hudson Bay polar bear subpopulation, which is among the most studied in the world, with scientists documenting their feeding habits and movements for nearly half a century. Of the 4,600 bears studied over the years, the adoption is only the 13th ever witnessed in the population.

“Polar bear adoptions are very rare and unusual and we don’t know why they happen,” said Alysa McCall, of Polar Bears International, calling it an “amazing” sight to witness.

The two cubs, which appear healthy and well-nourished are believed to be 10 or 11 months old and are expected to stay with their mother for another 1.5 years.

GPS data show the three bears are already on the sea ice. The cubs will rely on their mother to catch seals and often share in the food she provides or teaches them to eat. They will stick close by and gradually learn the skills needed to survive in a harsh but at times bountiful environment.

A winter spent on the ice hunting for prey – and months in the summer without any food – are a difficult existence for even the hardiest bears. Half of all cubs born won’t make it to adulthood.

But experts say having a mother now gives the adopted cub a far greater chance of surviving. Still of the 13 cases documented over 45 years, only three have survived.

Researchers plan to study genetic samples from the cub to determine if the mother is known to them, and could still be alive. In some previous adoption cases, biological mothers were still alive and a “switching of litters” had occurred, rather than the cub being orphaned.

“We might never know what happened to the mother,” said McCall. “When we confirmed this was an adoption, I had a lot of mixed feelings. But it’s just another reason why this species is so incredible and why they’re so fascinating and interesting. It gives you a lot of hope when you realize that maybe, polar bears are looking out for each other.”