Officials at the Toronto Zoo say a second polar bear cub has died of injuries inflicted by its mother.

A 10-year-old polar bear named Aurora prematurely gave birth to three cubs - two males and a female — on Tuesday night and zoo staff stepped in when she rejected them.

The uninjured male cub remains in the zoo's intensive care unit under round-the-clock care and staff say it's hoped he will get stronger.

Polar bears typically give birth in late November and in December and the zoo says this birth is likely the earliest recorded birth of polar bears in captivity.

Female polar bears den at the start of winter and nurse their newborns for several months before emerging with their surviving cubs in the spring.

Aurora is still on exhibit in the Tundra Trek at the zoo with her twin sister Nikita and a male bear named Inukshuk.

The Toronto Zoo is involved in a collaborative research project involving multiple zoos to understand polar bear reproductive biology.