The mother of three polar bear cubs that died at the Toronto Zoo did "everything right," to keep her offspring alive, says the zoo's curator of mammals.
"She completely kicked in and had maternal instincts in looking after the cubs," said Maria Franke.
The first male cub died shortly after birth last Thursday and the two female cubs died during the night on Sunday.
Ms. Franke said the cause of death is still unknown, but tissues samples have been sent out to be analyzed.
In October, 2011, the mother, an 11-year-old bear named Aurora, gave birth to three cubs but rejected the litter, killing two of her offspring.
This time, said Ms. Franke, "it was completely night and day."
Aurora "denned up" and also stopped eating in the last couple of weeks before birth, Ms. Franke said, normal behaviour for a pregnant bear.
"There was no trauma to the cubs, so we don't know [what happened]. I'm sure this does happen in the wild," Ms. Franke added.
Rob Laidlaw, the director of Zoocheck Canada, a national animal protection charity, says he wasn't surprised that the three cubs didn't survive.
Mr. Laidlaw, who wrote a report in 2005 entitled A Case against Polar Bears in Captivity, said that the stress Aurora might have faced in captivity could have contributed to the cubs' death.
"I think one thing that is overlooked is the fact that polar bears are very problematic in captivity and they are prone to all kinds of abhorrent behaviour," he said. "In my view, and the view of my colleagues, simply the stress of captivity may have been a factor as well."
But Ms. Franke said that Aurora had shown no signs of stress.
She added that the wild can also be an extremely stressful place for polar bears.
"We're constantly putting more pressure on the wild population, and there's actually been cannibalism observed in polar bears in the wild because they're starving," she said. "That's a serious sign of a population in stress."
Ms. Franke said the Toronto Zoo continues to provide researchers with pertinent information that can help them better understand the wild polar bear population, including collecting fecal matter to help with DNA coding, as well as conducting "scent smells" to better understand mating.
Polar bears are an endangered species and biologists estimate there are only approximately 20,000 to 25,000 in the wild.
About 60 per cent of them live in Canada.
With files from The Canadian Press