A pair of twin polar bear cubs born at an aquarium in China have celebrated their first 100 days on earth. Born March 5 at Tianjin Haichang Ocean World, the baby bears spent their first days in an incubator, and staff were unsure if they would survive.

"They were so tiny at birth. The elder sister was 497 grams and the younger one was 560 grams," keeper Sun Qinbao told the Daily Mail.

But now, the bear cubs are healthy and doing well. They celebrated their "100th-day birthday" by romping around with a ball.

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The mother bear became pregnant in August of 2011 and the birth of the two bears was unprecedented at the aquarium, according to keepers.

The polar bear cubs might look cute, but they play rough. According to Sun, they will also sometimes scratch and bite while being fed.

Polar bears are a threatened species, and there is some controversy about keeping them in captivity. As Canadian bear researcher Robert Laidlaw pointed out in a 2005 paper, "no polar bear enclosures constructed yet ... satisfy the full range of biological and behavioral polar bear needs."

However, polar bears in captivity can live longer than their wild counterparts. While individuals kept at zoos have lived as long as 43 years, wild polar bears rarely live beyond 25 years. The oldest wild bear on record died at age 32.

The average polar bear litter has two cubs, but one bear at the Moscow Zoo recently gave birth to three, according to the Telegraph.

Some researchers postulate that habitat loss due to climate change is to blame for a recent increase in incidents of cannibalism among polar bears.

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  • China Polar Bear Twins

    A pair of hand-reared polar bears have celebrated their 100 day milestone at Tianjin Haichang Ocean World in northern China. The twins were born on 5th March and to give them the best chance of survival they were taken from their mother and placed into incubators. Since then they have been taken care of by four keepers around the clock.

  • China Polar Bear Twins

    Keeper Sun Qinbao comments: "They were so tiny at birth. The elder sister was 497 grams, and the younger one was 560 grams. In the first few days their eyes were still not opened and they just ate and slept".

  • China Polar Bear Twins

    Their parents - 15-year-old father Bai Long and 12-year-old mother Huan Huan - came to the aquarium in September 2010 and Huan fell pregnant last August. Sun comments: "We were very excited and nervous, as it's first time polar bears got pregnant in our aquarium." Both of the young polar bears are now thriving.