PORTLAND, Ore. -- Denali, a 26-year-old grizzly bear at the Oregon Zoo, was euthanized Thursday morning, officials said.

Ore. Zoo photo

Denali at the Oregon Zoo.

Bug, as he was affectionately known, had been showing signs of lameness and stiffness in his legs. He had been put on multiple medications to control his pain, but his condition did not improve significantly.

A veterinary exam Thursday morning revealed joint deterioration in his right hip, resulting in a bone-on bone situation, and part of the socket had worn away.

He also had multiple bone chips and deterioration in both feet, making it constantly painful for him to walk, according to zoo veterinarian Mitch Finnegan.

"When we saw the radiographs, we knew no matter what medications we put him on, his level of pain would be unacceptable," Finnegan said.

"Bug was a great bear," said Oregon Zoo General Curator Chris Pfefferkorn.

"He was easy to work with and train, and he taught us a lot."

Several members of the zoo's animal-care staff worked with Bug for more than 20 years; they said his death leaves a large void in their lives.

Denali and his brother and sister were born at the Denver Zoo on Christmas Day in 1981, earning the nicknames Bah, Hum and Bug. Bah and Bug came to Portland in February 1985 to reside in the zoo's newly constructed Alaska Tundra exhibit, which opened in March of that year. Bah died in 2001.

Grizzly bears are considered a threatened species in the U.S. In the wild, their life expectancy is between 20 and 25 years.