(CNN)Ozzie, the world's oldest male gorilla and the third-oldest gorilla in the world, died Tuesday at Zoo Atlanta, the facility announced. He was 61.
The 350-pound western lowland gorilla's cause of death is not yet known, the zoo said in a news release.
Ozzie was considered a true legend at the zoo and was the only surviving member of the original generation of gorillas who arrived with the opening of its Ford African Rain Forest in 1988, according to the release.
Last week staff noticed a decline in Ozzie's appetite and were encouraging him to eat, according to the zoo.
"Over the past 24 hours, the teams had been treating him when he presented symptoms including facial swelling, weakness, and inability to eat or drink," reads the release. A necropsy will be performed to determine the cause of death.
Ozzie's death comes just days after his habitat-mate, Choomba, was euthanized due to a decline in her health. She was 59. The two lived together in the same habitat area at Zoo Atlanta and had been together for more than 15 years.
Ozzie was frequently seen by visitors napping in his favorite space -- his wood-wool pile. His favorite treats included oranges and cabbage. And he communicated with staff by using "reply" vocalizations and a series of characteristic raps on doors or windows, according to his biography on the zoo's website.
"This is a devastating loss for Zoo Atlanta. While we knew this time would come someday, that inevitability does nothing to stem the deep sadness we feel at losing a legend," Raymond B. King, president and CEO of Zoo Atlanta, said in the news release.
"Ozzie's life's contributions are indelible, in the generations of individuals he leaves behind in the gorilla population and in the world's body of knowledge in the care of his species. Our thoughts are with his care team, who have lost a part of their lives and a part of their hearts."
The western lowland gorilla is listed as critically endangered, according to the World Wildlife Fund (WWF). Due to poaching and disease, the species population declined more than 60% over the last 20-25 years, according to WWF.
Ozzie has more than 20 descendants, according to the zoo. He is survived by daughter Kuchi; sons Kekla, Stadi, and Charlie; granddaughter Lulu; great-granddaughter Andi, and great-grandson Floyd, all of whom live at Zoo Atlanta, according to the release. The rest of his offspring live at other accredited zoos in the US and Canada.