Prince, ZooMontana's 18-year-old Amur tiger, died unexpectedly Tuesday afternoon, probably due to complications from an aggressive cancer.
"We're devastated," said Jeff Ewelt, the zoo's executive director. "It's been a really tough day for us here."
Two weeks ago, zoo staff noticed Prince limping and, after medications didn't have any result, decided to put him under anesthesia on Tuesday to investigate, thinking he might have an issue with one of his toes.
X-rays showed that he had a "pretty significant bone cancer" that staff didn't know was there, Ewelt said.
As Prince began to recover from the anesthesia, he began to have breathing problems and died.
"The cancer had literally riddled those lungs of his, and we think that's why he had trouble breathing," Ewelt said.
After further examination, zoo staff said they believed the tiger had only a few weeks left to live.
"They were shocked at how fast the cancer spread," Ewelt said. "It was a lot worse than any of us thought it would be, and when we saw those X-rays, we were heartbroken.
"Remarkably, he put on a good front. Other than that limp, there weren't really any other signs. That tells us it was probably a really aggressive cancer."
Before his death, the 460-pound tiger was eating, active and had actually gained a little weight over the last few weeks, Ewelt said.
Amur tigers, also known as Siberian tigers, are an endangered species found in the wild in the northern forests of Russia, China and the Korean peninsula. As the world's largest cats — weighing as much as 600 pounds — their average lifespan in the wild is about 14 years, while they live an average of about 20 in captivity.
Only about 500 of the animals remain in the wild.
Prince was born in captivity and came to the zoo in February 2004, donated by the Henry Doorly Zoo in Nebraska, after it sent another male Amur, TJ, to the St. Louis Zoo as part of a breeding program.
Since then, Prince has spent more than a decade as one of ZooMontana's centerpiece animals and was often seen pacing the edges of his large enclosure, brushing up against the glass window separating him from groups of delighted onlookers.
"He was such a laid-back cat, super easygoing," Ewelt said. "He liked to interact with guests and he very much loved the outdoors. He really fell into his own because he had that yard to himself most of the time."
When the zoo lost its accreditation from the Association of Zoos and Aquariums in 2011, the Philadelphia Zoo removed two other tigers from ZooMontana. Officials from that zoo said they had no concerns over the tigers' health and that they appeared healthy, but were worried about ZooMontana's financial situation at the time.
Prince, however, remained at the zoo.
His passing is not the first time a tiger has died at the zoo. In June 2009, a 10-year-old female Amur tiger named Luna died unexpectedly. While a necropsy wasn't able to pinpoint her cause of death, a veterinarian said the tiger may have had an aneurysm.
Ewelt said Wednesday morning that Prince had been cremated and that the zoo hopes to hold a memorial of some sort, although details have not yet been worked out.
In addition, the zoo hopes to bring a new tiger in, when possible.
"We're actively searching for another tiger," Ewelt said. "We want to get another tiger in here. We're always so honored to have these tigers here."