CINCINNATI (AP) — Attorneys for an Ohio zoo suing a conservatory for the return of a gorilla are asking a judge to rule in the zoo's favor without going to trial.

The Cincinnati Zoo sued The Gorilla Foundation in October for the return of the silverback gorilla who served as a companion to Koko, the late ape famed for mastering sign language. The 37-year-old Ndume was loaned to the California-based foundation in 1991 under a contract that was revised to guarantee his transfer after Koko's death.

After Koko died in June, the Association of Zoos and Aquariums' Gorilla Species Survival Plan recommended Ndume move back to the zoo where he was born. The Cincinnati Zoo's lawsuit alleges the foundation violated the contract when they refused to coordinate his planned return.

The zoo's lawyers filed for summary judgment Thursday, saying the case was "uncomplicated" and no factual issues remain to be tried. In their response to the lawsuit, the foundation's lawyers argued it would be unlawful to enforce a contract that causes harm to the gorilla.

Zoo officials, who claim Ndume has lived in isolation for months to his detriment, want to integrate him into a family of gorillas. Ron Evans, curator of primates at the Cincinnati Zoo, said being with other gorillas is an "unarguably basic need."

But the foundation said a transfer would pose unnecessary risks. Francine Patterson, an animal psychologist who cared for Koko and co-founder of The Gorilla Foundation, wrote in a September letter addressed to zoo officials that a move would hurt Ndume by causing undue stress and exacerbate an "ongoing suffering after the loss of Koko."

Zoo attorneys argued The Gorilla Foundation is contractually obligated to facilitate Ndume's transfer, but lawyers for the conservatory said it's impractical to fulfill the agreement, which prioritizes Ndume's wellbeing, by acting against what they consider to be his best interests.

In a recent counterclaim, the foundation asserts Ndume would not thrive in a public zoo. Officials allege the original transfer agreement in 1991 prevented Ndume from participating in human-gorilla communication research because zoo officials worried he would become capable of indicating he'd been treated poorly at zoos in the past.

"The mere possibility that a gorilla might communicate extreme dissatisfaction with the way s/he had been treated in a zoo was viewed as an entirely unacceptable risk," the foundation said of zoo officials' reasoning. Cincinnati Zoo attorneys denied those claims.

Foundation officials also allege the zoo purposefully aimed to embarrass the conservatory by claiming it was not accredited and blocking attempts to bring Ndume other gorilla companions. Kristen Lukas, chair of the AZA's Gorilla Species Survival Plan, said the association doesn't place animals in facilities - including The Gorilla Foundation - that do not have AZA accreditation.

A federal judge in San Francisco will decide Ndume's future.

Western lowland gorillas like Ndume are considered to be a critically endangered species, with fewer than 175,000 found in the wild.

Cincinnati Zoo officials killed a gorilla named Harambe in 2016 after a 3-year-old boy climbed into the enclosure. Harambe’s death inspired global mourning, criticism and satire.