Autopsy results are pending, but zoo staff say Juara died Aug. 3 from respiratory arrest due to anesthesia veterinarians used to examine the animal, the St. Louis Post-Dispatch reported Wednesday.
"The vets felt he was somewhat slow to wake up," but Juara seemed to be all right as the staff watched him until the early evening, zoo spokeswoman Janet Powell said.
The U.S. Department of Agriculture, which regulates animal welfare at zoos, was notified of Juara's death, Powell said.
In December, the zoo lost a baby chimpanzee that died soon after birth because of an umbilical cord infection.
Juara, one of three orangutans at the zoo and the only male, weighed 375 pounds. He was born at the Toronto Zoo in 1985 and lived at zoos in Columbus, Ohio, and Little Rock, Ark., before arriving at the St. Louis facility in 2006, the Post-Dispatch said. Orangutans can live an average of 35 years in the wild and up to 60 years in captivity.