Calgary Zoo officials say they don't know why a man scaled one of two fences around the Siberian tiger exhibit where he was attacked by one of the big cats and badly injured.
Zoo spokesman Grahame Newton says the incident, which happened in the dead of night, has left staff wondering what the man was thinking.
"It wouldn't be something I would try," Mr. Newton told reporters at a news conference Monday.
"I don't think any reasonable person would want to get that close to a potentially dangerous carnivore. They are behind those enclosures for good reason and most definitely I would have to wonder what was going through a person's mind to do something like that."
Mr. Newton said two men, both 27, broke into the zoo at around 1 a.m. by scaling the 2 1/2-metre-high main fence. That fence is topped with barbed wire.
They then made their way to the tiger exhibit.
Mr. Newton said one man climbed the outer safety fence around the enclosure. The tiger, a two-year-old male named Vitali, hooked the man with a claw and pulled the man's arm through the inner fence.
The second man called a security person he knew for help. His arm was also injured, but he initially refused treatment.
Mr. Newton said there were four security guards on duty, but they have several responsibilities in the course of a shift, including checking on the animals.
"We don't know what their intention was," said Newton. "I think it is fair to say that if anybody puts their mind to it, they can breach any kind of security and that certainly seems to be the case here."
Police are investigating and say they are considering charges of break and enter against the two men. They haven't determined whether alcohol was involved.
Tim Sinclair-Smith, curator of the zoo's Eurasia centre, said the tiger was acting instinctively and was to remain part of the exhibit.
Vitali was born at the zoo in July 2007 to much fanfare because there are only an estimated 450 left in the wild.
Siberian tigers are an endangered species found in the Amur Valley of northeastern Russia. They are threatened by habitat destruction and overhunting for their luxurious pelts and for body parts used in traditional medicines.
They are the largest species of cat in the world. Males weigh between 180 and 305 kilograms and can grow to be between three and 3 1/2 metres in length from their nose to the tip of their tail.
In the wild, the tigers eat mainly wild boars, deer and antelope. They ambush their target, sneaking up on it and then stopping within a few metres until ready to attack - sometimes waiting for up to two hours.