The Cincinnati Zoo is suing for the return of a gorilla loaned to a California conservatory as a companion for Koko, the late gorilla famed for mastering sign language.
In a complaint filed Thursday against The Gorilla Foundation, attorneys for the Cincinnati Zoo said Ndume, a 37-year-old silverback gorilla, has lived in isolation to his detriment since Koko's death in June.
The zoo sent Ndume to the foundation in 1991 under a contract that was revised to say he would be transferred after Koko's death. In the months that followed, the Gorilla Species Survival Plan recommended Ndume move back to the zoo where he was born.
The zoo's complaint alleges the foundation violated the contract when they refused to coordinate Ndume's planned return.
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 harm Ndume by causing unnecessary stress. She said would also exacerbate an "ongoing suffering after the loss of Koko."
In the letter, Patterson claimed Ndume screamed, banged and shoved objects for 14 consecutive hours after overhearing talk of a transfer - behavior the gorilla had never before exhibited at the foundation. She said gorillas' ability to understand human speech is underestimated, and the foundation's "decades of experience communicating with them confirms their ability to do so."
Ron Evans, curator of primates at the Cincinnati Zoo, acknowledged every transfer has its risks, but Ndume's isolation presents a greater concern.
"To be near and/or with other gorillas is a foundational, natural-history-proven and unarguably basic need for all gorillas," he wrote in a reply.
Patterson acknowledged Ndume's need for a social group, but emphasized he is not technically alone. "He is with a strong family support group of human great apes, from whom he takes great comfort," she wrote.
She added The Gorilla Foundation will continue trying to bring Ndume gorilla companions, after what she calls many "blocked attempts" by the Association of Zoo and Aquariums. The foundation does not have AZA accreditation.
A federal judge in San Francisco will decide Ndume's future.
A message seeking comment was left Friday for The Gorilla Foundation, most widely known for purchasing Koko from the San Francisco Zoo in 1977. Patterson began teaching the gorilla sign language that became part of a Stanford University project in 1974.
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.