When veterinary nurse Brittany Semeniuk went to Manitoba, Canada, in November, she and her vet partner were looking to photograph polar bears. They visited a place called Mile 5 Sanctuary, outside the town of Churchill.

There Semniuk saw a chained-up sled dog—and a massive polar bear was approaching it.

"I had no idea what was going to happen," Semeniuk says. "I was worried for the dog."

But then, the bear started playing with the dog. It began nudging the canine with its nose and poking it with its paws; the dog even appeared to be in on the bear's playful game.

This seemingly benign behavior continued for about 15 minutes. After that, the bear walked away to mosey around looking for food, leaving