I have never liked zoos. To me, they are prisons and do not provide healthy environments for the animals locked in small places (relative to their natural habitats). Suffering does not have to involve physical pain or torture. For animals, like chimpanzees, who have highly developed emotional and intellectual brains, suffering can be severe merely because of captivity.
Project Nim, available on DVD, chronicles a 1970s (totally misguided) attempt to take an infant chimp away from its zoo mother, and teach it sign language while living among humans. The movie depicts the damage done by the various humans involved in this bizarre experiment, and the hubris and carelessness of those humans.
The film graphically depicts the horror when the helpless chimp, after attaining adulthood and fluency with sign language, but never having seen other chimpanzees, is summarily transported back to the zoo from whence he came. The lead researcher in the original study, Columbia University Professor Herbert Terrace, was (and is) a clearly a self-interested and despicable human being, and he is featured throughout voicing self-important, and false, self-justifications for his inhuman behavior, including a sexual relationship with at least one of of his teaching assistants.
To add insult to injury, a year after leaving Nim back at his zoo, in horrible conditions, Terrace shows up for a photo op with Nim. Nim reacts with relief, as if he will be released from prison. He was not released. And eventually, a medical researcher targeted Nim and moved him to a New York Univiersity medical research center. You can guess the rest, but it is not a totally grim ending.