One of the Saint Louis Zoo's most popular orangutans, and its oldest primate, has died.

Zoo officials said "Junior" was euthanized Tuesday after his health had been declining. He was thought to be about 44 years old.

Junior was the third-oldest male in the national Orangutan Species Survival Plan population. He was born in Sumatra, a western Indonesian island, and came to the U.S. in 1963. He first lived at the Tulsa, Okla., and Memphis, Tenn., zoos.

Zoo officials said Wednesday that veterinarians had been treating Junior for severe arthritis for three years. In recent weeks, pain medication did not appear to be helping and Junior became reluctant to move and eat.

"It's like losing a really close friend. They share so many characteristics with us and we developed very close bonds," the zoo's primate curator, Ingrid Porton, said. "It's a very difficult decision to make, but I think we made the right and kind decision."

He came to St. Louis in 1992 as part of the national species survival plan and quickly became a favorite among zoo keepers for his gentle nature.

"You had to prove yourself to Junior and show you were trustworthy," Porton said. "When he let you in his world, you saw what a magnificent guy he was. You wouldn't say that about his female counterpart or some other male orangs."

Zoo officials said orangutans from Junior's part of the world are critically endangered due to widespread habitat destruction.

Exacerbating the problem is the fact that female orangutans breed only once every seven to nine years, Porton said.

Of all the male orangutans at the Saint Louis Zoo, Junior had the fewest offspring, a son and a daughter with his mate Merah, Porton said.

He was the most valued because he came from the wild and was the oldest of all the primates at the zoo.