The Gorilla Foundation in California says returning a 37-year-old gorilla named Ndume to the Cincinnati Zoo would cause the animal harm and possibly lead to an early death.
"The transfer (the Cincinnati Zoo) seeks will harm Ndume physically, emotionally and could very easily be the direct cause of his premature death," the foundation writes in a court filing.
Ndume was loaned to the foundation in 1991 as a companion for Koko, the gorilla famous for communicating with humans. Koko died earlier this year, and the Cincinnati Zoo is now suing the foundation because it says the foundation refuses to return Ndume as required by a contract.
Cincinnati is asking a federal court in California for a summary judgment. The foundation's response filed Thursday says Ndume has a record of poor behavior and health in public zoos.
"Ndume would repeatedly throw his own feces and regurgitated food at zoo keepers and patrons," the filing states. "Now an elderly gorilla at age 37, (Ndume) still responds poorly to strangers approaching him - which would again become a serious problem should he be forced once again to live in a public zoo environment."
The foundation adds it believes Ndume would likely redevelop a Balantidium coli infection - an intestinal protozoan parasite - if removed from the private sanctuary.
Ndume is currently the only gorilla at the Gorilla Foundation. Cincinnati Curator of Primates Ron Evans told WVXU in October that it isn't fair to keep Ndume in isolation.
"That is just not acceptable," he said. "That's kind of the ironic part ... the only reason we left him out as long as we did is so Koko wouldn't have to be in isolation, but now they're choosing to do that to Ndume."
Cincinnati has a week to respond. The sides are currently scheduled to appear in a San Francisco courtroom Jan. 3.