Tortoise Alex died at 100 years old at the National Zoo. (Courtesy of Connor Mallon, Smithsonian's National Zoo)

One of the National Zoo’s oldest and largest animals died Tuesday after caretakers found fluid inside his shell and around his heart.

Zookeepers euthanized Alex, a 100-year-old Aldabra tortoise who arrived at the Smithsonian’s National Zoo in Washington in the mid-1950s. Aldabra tortoises, among the world’s largest tortoises, have a life expectancy of 100 to 150 years.

Alex was considered an “ambassador” for his species, experts said. He was known as a social and personable tortoise who enthusiastically did training sessions. His favorite pastime was “relaxing in a cool and often muddy section of the outdoor yard,” the zoo said.

Zookeepers said Alex was a visitor favorite during daily demonstrations at the reptile center. He “seemed to enjoy interacting with the keepers” and volunteers who cared for him, a statement from the zoo said.

Alex came to the zoo’s Reptile Discovery Center in 1956 from the wild when he was about 30 years old. His health had recently declined and his behavior started to change.

He was sleeping more and seemed lethargic, showing little interest in food or training. Veterinarians kept a close watch on him as he was treated with antibiotics and anti-inflammatory medications.

On Tuesday, an exam found he had a “large volume of fluid inside his shell and around his organs, including his heart,” zoo officials said in a statement. Experts tried to remove the fluid, but his condition deteriorated.

“Due to Alex’s poor prognosis and quality of life, animal-care staff made the decision to euthanize him,” zoo officials said in the statement. A pa­thol­ogy report will be completed in the coming weeks.

Aldabra tortoises are native to Aldabra Island in the Indian Ocean. Considered vulnerable by the International Union for Conservation of Nature, they typically live in coastal dunes and mangrove swamp areas.

A male Aldabra tortoise can weigh up to 500 pounds with an upper shell that can measure four feet long. They like to eat fruits and leaves off small shrubs and trees.

The National Zoo has two other Aldabra tortoises — a male named Rulon and a female named Chyna.”