Calgary Zoo officials are caring for a Siberian tiger cub born unexpectedly Monday. (Calgary Zoo)Veterinary teams worked through the night to save the one of two tiger cubs born unexpectedly Tuesday at the Calgary Zoo after its littermate died shortly after birth.

The female cub is in stable condition in intensive care at the zoo's animal health centre, officials said Wednesday.

Zoo officials revealed on Tuesday that Katja, one of the zoo's four adult Siberian tigers, had surprised them by giving birth to two cubs sometime between Monday night and Tuesday morning.

The tiger's keepers did not realize the 10-year-old zoo-born tiger was pregnant, zoo officials said in a release.

"She's got a very thick coat and showed no outward signs, either physically or behaviourly — no change in appetite, no looking for a place to hide — absolutely nothing to indicate [she was pregnant]," said Dr. Sandie Black, the zoo's chief veterinarian.

'We're very comfortable with how we've handled it.'— Simon Scott, zoo spokesman

Baikal, a nine-year-old tiger from New York's Bronx Zoo, was brought to Calgary in January in the hope that he would breed with Katja.

Black said the second cub was likely killed accidentally by its mother when she moved it. An autopsy showed the cub suffered damage to its skull.

"It's important to understand, this wasn't an aggressive act at all. Just an inexperienced mom," said Black.

"If you can think of that quintessential picture of a very large tiger with a very small cub in its mouth, she just didn't do that perfectly."

The head of an animal protection group said it's unacceptable for zoo officials to have missed Katja's pregnancy.

"You know, the Calgary Zoo was responsible for that animal. They say it is a very valuable endangered species. They should have kept better tabs on that animal and they should have known," said Zoocheck Canada spokesman Rob Laidlaw.

"And the death that occurred on their watch — they are the ones that are responsible. Nobody else is."

Tiger expert supports zoo

Black said it would be too invasive to constantly check the animals for pregnancy.

"We can't examine a tiger without anesthetizing them, and so we make those choices very carefully," she said.

A tiger expert with the Minnesota Zoo who has 27 years of experience supports the way the Calgary Zoo handled the situation.

"I can recall at least half a dozen or more cases during this period of time that I've been responsible for tigers and in every case everyone was incredulous, [they] had no idea and there were these cubs on the floor the next morning just like this one," said Ron Tilson, Minnesota Zoo conservation director and co-ordinator of the tiger species survival plan.

In the past few years, numerous animals have died at the Calgary Zoo, including a baby elephant, a hippo, a wild goat, four gorillas and 41 stingrays.

There have also been several safety breaches, including an incident in March when a gorilla was able to reach a perimeter fence, perching on it briefly before jumping back down.

The death of a capybara — crushed by a hydraulic gate in 2009 — prompted zoo officials to invite the Canadian Association of Zoos and Aquariums and the Washington, D.C.-based Association of Zoos and Aquariums to conduct an independent review.

In June, the zoo revealed a 36-point action plan to address systemic problems highlighted in the auditor's final report.

"I think some people may try to paint this as another bad news story here at the zoo. But we just can't see it that way," said zoo spokesman Simon Scott.

"We're very comfortable with how we've handled it. Albeit, we do sincerely wish that both cubs could have survived."