A 14-year-old lion at the Brookfield Zoo near Chicago mysteriously fell into her enclosure's dry moat and died less than two weeks after her mate was put down for age-related issues, officials said.
After the female African lion named Isis didn't show any signs of recovery from the 15-foot fall, the zoo's veterinary staff decided to euthanize her, The Chicago Tribune reported.
Zoo staff said her death was unexpected, mysterious and especially difficult.
"There was no sign of depression because of Zenda not being there," Brookfield Zoo SVP of Animal Programs Bill Zeigler said. "Again, knowing that she was acting normal and she was physically okay, it still begs the question: what happened?"
The lion was fine when staff checked on her at 9 a.m. Monday, an hour before the zoo opened, but she was found unresponsive at the bottom of the moat a half-hour later, said William Zeigler, senior vice president of animal programs. Her fall wasn't seen by anyone or caught on camera.
The design of the moat allows a lion to climb back out on the habitat side, but Isis essentially didn't move during roughly 24 hours of treatment and observation before her death.
Staffers who cared for the big cats watched Isis closely after her the death of her longtime mate, Zenda, and although she did call out more frequently, Zeigler said she seemed to get over the death within a few days.
"I've been in this business over 44 years, and I cannot tell you of any time that I would've thought to have considered something like that in the action of an animal," Zeigler said. "They don't respond that way."
A CT scan showed that Isis had no broken bones but that she did have seizures indicating possible swelling of the brain. An autopsy will be conducted to determine what killed her.
In the meantime, Brookfield Zoo, which already planned to add more lions, will step up its efforts to do so.
"Most of us aren't going to have the opportunity to go to Africa or even Australia, but if we can come here and see the lions or hear them roar or we can see the bears or whatever, it prompts us to think about conservation and to contribute so that wildlife will continue," zoo visitor Janet Coats said.