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For the second year in a row, a brood of polar bear cubs born at the Toronto Zoo have all died within days, although zoo officials are tentatively calling the episode a success since Aurora, the cubs’ mother, did not try to kill them.

“Zoo Staff are disappointed by this latest development in the polar bear breeding program however it does demonstrate, positively, that Aurora’s maternal instinct was finally kicking in and she was learning to be a mother,” reads a Monday statement by the zoo.

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Last Thursday, Aurora — an 11-year-old polar bear who has lived at the zoo since 2001 — gave birth to one male and two female cubs. A male cub died soon after birth, and two others lived into the weekend as their mother attempted to nurse them to health, but on Sunday they too were found dead.

The last time Aurora gave birth, in October, 2011, she immediately killed one of her cubs and left two others to die. It was thought that Aurora rejected the cubs because of their acute prematurity: The trio were among the earliest recorded births of polar bears in captivity.

This time around, “the fact that Aurora was doing everything right this time is a huge step in the right direction,” said Maria Franke, the zoo’s curator of mammals, in a statement.

Despite the fact that they grow into one of the world’s most fearsome predators — and one of the only animals known to actively hunt humans — polar bears are known to be surprisingly vulnerable at birth.

“Given that polar bear cubs are born so small and undeveloped, there are many possibilities of what could have happened,” said Ms. Franke.