Ozzie, the world’s oldest male gorilla, died at the Atlanta zoo Tuesday at the age of 61.

“This is a devastating loss for Zoo Atlanta. 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,” said Raymond King, president and CEO of the zoo. 

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The lowland gorilla was found dead by his care team at the zoo on Tuesday. Ozzie’s cause of death was not immediately known, though the gorilla began showing signs of low appetite last week.

The zoo’s veterinary team treated Ozzie for symptoms including facial swelling, weakness and inability to eat or drink fluids. A necropsy will be performed by the University of Georgia Zoo and Exotic Animal Pathology Service in the College of Veterinary Medicine.

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The primate was the world’s oldest male gorilla and the third-oldest gorilla in the world. It was the only surviving member of a generation that was brought to Zoo Atlanta in 1988 with the opening of The Ford African Rain Forest. In 2009, Ozzie became the world’s first gorilla in the world to participate in a voluntary blood pressure reading. Ozzie is survived by seven descendants at the Atlanta zoo and many other offspring at other zoos in the United States and Canada.

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Lowland gorillas are considered critically endangered because of poaching, habitat loss, and drastic population reductions from disease. According to the International Union for the Conservation of Nature, their numbers have decreased by 60% and as much as 90% in some parts of western Africa.

“Ozzie’s incredible longevity is a testament to the care, expertise, and significant innovations of the Zoo’s Gorilla Care Team, which has become renowned for the care of geriatric gorillas. The team is frequently sought out and shadowed by professionals at other organizations for their advancements in positive reinforcement training and for their leadership in devising habitat and space accommodations for senior gorillas,” Atlanta Zoo in a news release.