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.