In her prime, Rio the river otter liked to bound out of her favorite little cave and scamper along the rock work at Shedd Aquarium -- charming countless visitors.
On Tuesday, the aquarium announced that Rio -- arguably among the world's cutest critters -- had to be euthanized, the Chicago Sun-Times is reporting.
"Rio has lived a long, wonderful life," said Shedd spokeswoman Andrea Rodgers.
Rio, Shedd's only river otter, was 21 years old -- a true geriatric. In the wild, river otters typically don't live longer than about 12 years, Rodgers said.
Like any geriatric, Rio had begun to slow down and show signs of arthritis, Rodgers said. The aquarium says she was euthanized Tuesday because her quality of life had diminished.
Staff could tell "she was feeling very poorly."
Ken Ramirez is vice president of animal care and training. He says "Rio became a testament to the zoological community's emerging expertise in geriatric animal care and welfare."
Rio, the Spanish word for river, came to the Shedd in 1992 from the Little Rock, Ark., zoo.
Widespread trapping, loss of habitat and pollution pushed the river otter close to extinction in Illinois. The species was added to the state's endangered species list in 1989. Conservation efforts have since led to the river otter's comeback and they can now be found in every county in the state.
The Associated Press contributed to this report.