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The Cincinnati Zoo has sued the psychologist who famously taught the Western Lowland Gorilla Koko sign language and the nonprofit she founded, seeking the return of a male gorilla it loaned in hopes that he would mate with Koko.

Psychologist Francine “Penny” Patterson’s nonprofit, The Gorilla Foundation, received the male gorilla Ndume on loan under a 1991 agreement that was revised in 2015. Although the attempts to pair Koko and Ndube didn’t pan out, they lived within sight and smell of each other until Koko’s death in June of this year.

The Cincinnati Zoo claims since Koko’s death, the Redwood City-based animal sanctuary has reneged on the agreement to return Ndume to a facility approved by an organization charged with overseeing the well-being of gorillas in zoos in North America.

“TGF has breached the loan agreement and is keeping Ndume in violation of the law and to the detriment of Ndume’s health,” wrote the zoo’s lawyers, Simon Frankel of Covington & Burling in San Francisco and Aaron Herzig of Taft Stettinius & Hollister in Cincinnati.

“Western lowland gorillas require social interaction with other gorillas in order to be healthy,” they continued in the complaint, filed Thursday in the U.S. District Court for the Northern District of California. “TGF does not possess any gorillas besides Ndume, and does not have a reasonable prospect to imminently—and legally—obtain possession of another gorilla. Accordingly, if Ndume is not immediately transferred from TGF to [Cincinnati Zoo & Botanical Garden], he will continue to live in isolation from other gorillas, and thus, in a sub-optimal, at-risk situation.”

The suit seeks to force The Gorilla Foundation to be bound by the terms of the agreement to force the organization and its staff and veterinarians to coordinate a timely transfer of Ndube to the zoo, the facility chosen by the consortium of scientists.

Representatives from The Gorilla Foundation didn’t immediately respond to a request for comment.

According to correspondence between the zoo and TGF attached as exhibits to Thursday’s complaint, the sanctuary was initially cooperating with plans to relocated Ndume, but changed course in September claimed in a letter that a move to the zoo would provide “serious life and safety risks” given the gorilla’s 37-year-old age and the fact that he’s spent the past 27 years in TGF’s sanctuary environment.

But the zoo’s lawyers point out that TGF had already notified its donors and social media followers that it intended to find other female gorillas to live with Ndume at the sanctuary—a “direct contravention” of the parties’ agreement and scientists’ recommendation, according to the complaint.

“There is no scientific basis for TGF’s claim that Ndume is at any increased risk,” the zoo’s lawyers wrote. “And TGF offers no explanation for why it agreed to the transfer in the agreement if it believed such a transfer was harmful.”

  Read the complaint below: