TORONTO - The Toronto Zoo says three polar bears born at the facility to the same mother last Thursday have died.

The first male cub died shortly after birth and two female cubs died during the night on Sunday.

The zoo says the mother, an 11-year-old bear named Aurora, was nursing the two female cubs after the male had died and "all was going well."

Despite this, and constant monitoring of the den, the two remaining cubs died.

The zoo says it is still unclear what caused the deaths and post mortem tissue samples have been sent out for analysis.

Maria Franke, the curator of mammals, says since the cubs are born so small and undeveloped, "there are many possibilities of what could have happened."

Despite the deaths, the zoo notes that Aurora's maternal instinct was "finally kicking in and she was learning to be a mother."

It says the births represent a "positive direction" in the zoo's breeding program.

Polar bears are a critically endangered species and biologists estimate there are only approximately 20,000 to 25,000 polar bears in the wild.

About 60 per cent of them live in Canada.

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  • Polar Bears Begin Seal Hunting On Frozen Icepacks In Northern Canada

    A Polar Bear plays with a bush on the tundra while waiting for the Hudson Bay to freeze 14 November 2007 outside Churchill, Manitoba, Canada. Polar Bears return every year to Churchill, the Polar Bear capital of the world, to hunt for seals on the icepack until the Spring thaw. AFP Photo/Paul J. Richards

  • Polar Bears Begin Seal Hunting On Frozen Icepacks In Northern Canada

    A Polar Bear plays with a bush on the tundra while waiting for the Hudson Bay to freeze 14 November 2007 outside Churchill, Manitoba, Canada. Polar Bears return every year to Churchill, the Polar Bear capital of the world, to hunt for seals on the icepack until the Spring thaw. AFP Photo/Paul J. Richards

  • Polar Bears Begin Seal Hunting On Frozen Icepacks In Northern Canada

    A Polar Bear plays with a bush on the tundra while waiting for the Hudson Bay to freeze 14 November 2007 outside Churchill, Manitoba, Canada. Polar Bears return every year to Churchill, the Polar Bear capital of the world, to hunt for seals on the icepack until the Spring thaw. AFP Photo/Paul J. Richards

  • Polar Bears Begin Seal Hunting On Frozen Icepacks In Northern Canada

    A Polar Bear plays with a bush on the tundra while waiting for the Hudson Bay to freeze 14 November 2007 outside Churchill, Manitoba, Canada. Polar Bears return every year to Churchill, the Polar Bear capital of the world, to hunt for seals on the icepack until the Spring thaw. AFP Photo/Paul J. Richards

  • Taiga

    Polar bear cub Taiga shakes water off her head as she plays in a pool Friday, Nov. 25, 2011 at Aquarium Park in Quebec City, Canada. (AP Photo/The Canadian Press, Jacques Boissinot)

  • Taiga

    A polar bear cub named Taiga plays in a pool Friday, Nov. 25, 2011, at Aquarium Park in Quebec City, Canada. Taiga was transferred from the Saint-Felicien zoo on Nov. 16, 2011 and will be officially introduced to the public in a few day.(AP Photo/The Canadian Press, Jacques Boissinot)

  • Polar Bears Begin Seal Hunting On Frozen Icepacks In Northern Canada

    A Polar Bear looks up as the sound of the camera catches his ear on the edge of Hudson Bay aheasd of the full freeze-over 14 November 2007 outside Churchill, Mantioba, Canada. Polar Bears return every year to Churchill, the Polar Bear capital of the world, where they remain hunting for seals on the icepack until the Spring thaw. AFP Photo/Paul J. Richards

  • A Polar Bear walks on part of a frozen l

    A Polar Bear walks on part of a frozen lake migrating North 14 November 2007 as Hudson Bay freezes outside Churchill, Manitoba, Canada. Thousands of Polar Bears return to the Canadian icepack to hunt for seals every year at this time and remain on the frozen area until the spring thaw. AFP Photo/Paul J. Richards

  • Polar Bears Begin Seal Hunting On Frozen Icepacks In Northern Canada

    A mother Polar Bear and her cubs wait on the tundra for the Hudson Bay to freeze 14 November 2007 outside Churchill, Manitoba, Canada. Polar Bears return to Churchill, the Polar Bear capital of the world, to hunt for seals on the icepack every year at this time and remain on the icepack feeding on seals until the Spring thaw. AFP Photo/Paul J. Richards

  • A polar bear mother and her two cubs walk along the shore of Hudson Bay near Churchill, Man. in this Wednesday Nov. 7, 2007 photo. They're etched onto canadian coins, are part of Canada's national identity and lure tourists to the Arctic every year, but the majestic Canadian polar bear could pose a significant risk to northern communities if climate change continues to wreak havoc on its natural habitat. (AP Photo/The Canadian Press - Jonathan Hayward)

  • Polar Bears Begin Seal Hunting On Frozen Icepacks In Northern Canada

    A Polar Bear walks on the edge of Hudson Bay ahead of the full freeze-over 14 November 2007 outside Churchill, Mantioba, Canada. Polar Bears return every year to Churchill, the Polar Bear capital of the world, where they remain hunting for seals on the icepack until the Spring thaw. AFP Photo/Paul J. Richards

  • Polar Bears Begin Seal Hunting On Frozen Icepacks In Northern Canada

    A Polar Bear walks on the frozen tundra on the edge of Hudson Bay waiting for the Hudson Bay to freeze-over 14 November 2007 outside Churchill, Mantioba, Canada. Polar Bears return every year to Churchill, the Polar Bear capital of the world, where they remain hunting for seals on the icepack until the Spring thaw. AFP Photo/Paul J. Richards

  • A mother polar bear and her cubs sleep o

    A mother polar bear and her cubs sleep on the tundra on the edge of the Hudson Bay waiting for the bay to freeze over, 13 November 2007, outside Churchill, Mantioba, Canada. Polar bears return to Churchill, the polar bear capital of the world, to hunt for seals on the icepack every year at this time and remain on the icepack feeding on seals until the spring thaw. AFP PHOTO/Paul J. Richards

  • Polar Bears Begin Seal Hunting On Frozen Icepacks In Northern Canada

    A Polar Bear walks on the frozen tundra waiting for the Hudson Bay to freeze-over 14 November 2007 outside Churchill, Mantioba, Canada. Polar Bears return every year to Churchill, the Polar Bear capital of the world, to hunt for seals on the icepack where they remain until the Spring thaw. AFP Photo/Paul J. Richards

  • TO GO WITH STORY by Paul Richards, Canad

    TO GO WITH STORY by Paul Richards, Canada-climate-warming-wildlife-polarbears A mother Polar Bear rests on the frozen tundra with her cubs waiting for the Hudson Bay to freeze over 13 November 2007 outside Churchill, Mantioba, Canada. Polar bears in Canada's Hudson Bay area are battling for survival, as climate change reduces the time they can hunt for food, warn environmentalists and locals in Churchill, the self-proclaimed polar capital of the world. AFP Photo/Paul J. Richards