Silverback gorilla Ndume picks up a toy at The Gorilla Foundation's preserve in California's Santa Cruz mountains. Photo: Ron Evans/Cincinnati Zoo and Botanical Garden via AP

A Bay Area animal research nonprofit is fighting to keep PETA out of a legal dispute over the fate of the former companion of Koko, the western lowland gorilla who famously learned sign language.

The Cincinnati Zoo sued Koko's longtime caretaker, psychologist Francine “Penny” Patterson, and her Redwood City-based nonprofit, The Gorilla Foundation, in October seeking the return of Ndume, a male gorilla it loaned the foundation back in 1991 in hopes that he and Koko would mate.

The Gorilla Foundation's lawyers at the Casselman Law Group and Cotchett, Pitre & McCarthy have argued that Ndume's age, 27 years of experience living under the foundation's care, and history of troublesome behavior while in zoo captivity would make any move a serious risk to the gorilla's health. On Friday, they sought to block PETA from weighing in on the case.

“PETA is not a party to the contracts at issue in this case,” wrote the foundation's lawyers. “No one from PETA claims to have even once seen Ndume during his 37 years.” The lawyers pointed out that no one at PETA has visited the foundation or even asked to visit.

PETA lawyers this week submitted an amicus brief backing the zoo's motion for summary judgment. In their own brief, the PETA lawyers argue that the foundation's “sloppy and inaccurate” characterization of the Endangered Species Act could negatively impact their organization work in protecting animals from substandard facilities.

“Just in the past six years PETA has helped re-home more than 120 captive wild animals in the United States from substandard conditions to reputable facilities, including ten great apes and many other endangered animals,” the PETA lawyers wrote. “Notably, despite many of them having compromised health and/or being of advanced age, none of these animals suffered harm during transport.”

The zoo's lawyers at Covington & Burling and Taft Stettinius & Hollister are seeking a court order forcing the foundation to return Ndume under the terms laid out in a 2015 agreement claiming that the terms of the contract are “unambigous.”

“The foundation agreed in 2015 that when Koko died certain experts would decide what was best for Ndume,” said Covington's Simon Frankel in a phone interview Friday. “This idea that this transfer would harm Ndume is simply not factually supported.”

David Casselman of the Casselman Law Group said in a phone interview Friday that PETA's attempt to join the case was “more about PETA than it is Ndume.”

“Ndume is an individual and he is entitled to be evaluated as an individual,” Casselman said. Casselman said that the foundation is concerned that if the decision is made solely on the basis of the contract without considering Ndume's individual needs it could “ destroy the object of the contract in the process.”

For his part, U.S. District Judge Richard Seeborg, the federal judge in San Francisco who is overseeing the dispute, has urged the parties to come to some sort of agreement about Ndume's future outside the courtroom, perhaps with input from other organizations with animal welfare expertise.

“Without prejudging the merits of the arguments presented by either side, it seems probable that deciding the dispute based solely on whatever legal principles would govern on the factual record would not necessarily lead to the result that is in Ndume's best interests, regardless of which side may be correct regarding those interests,” wrote Seeborg, in a Dec. 17 order directing the parties to meet concerning expedited alternative dispute resolution. “The forum of the courtroom may not allow for a full, and fully informed, evaluation of all of the potential impacts to Ndume of any particular course of action.”

Despite the judge's entreaty, the parties have completed briefing on the zoo's summary judgment motion. A hearing is set on the matter for Jan. 24.