Denver - Denver Zoo was mourning the death on Monday of Bertie, a 58-year-old male hippopotamus who was its longest resident and the oldest hippo accredited to a North American zoo, officials said.

Bertie was humanely euthanized after his keepers saw “a significant decline in his quality of life due to his advanced age,” the zoo said in a statement. Most hippos live about 30 to 40 years in the wild, and up to 50 in zoos.

“This is a very sad loss for Denver Zoo and our community. Bert was a member of our family for more than 50 years,” zoo President and Chief Executive Shannon Block said.

“He will be missed by all of us, including the many families and children who visited him and came to know his charismatic personality over the years,” Block said in the statement.

The zoo said Bertie's appetite had recently decreased noticeably, and that he had become less consistently interested in training sessions with his keepers.

He also became more reluctant to leave his pool and showed some difficulty walking.

The zoo said veterinarians were limited in their treatment options because he had not responded well to medications.

Diagnosing problems in hippos is particularly difficult because of their size and anatomy, the zoo said, but all of Bertie's symptoms pointed to severe old age.

Ultimately, the zoo said, his caretakers knew that his quality of life would not improve.

“This is never an easy decision, but it was the right one,” said the zoo's Vice President of Veterinary Medicine Scott Larsen.

“We'll all miss him very much, but were glad he lived such a long, happy life here at the zoo.”

Bertie arrived in 1958 from New York's Central Park Zoo.

He is the father of every hippo calf born at Denver Zoo, which said he had sired 29 offspring with two mates.

One of them, Bertie's 12-year-old son, Mahali, is the only hippo now at the zoo.