Believe it or not, more people are keeping chimpanzees for pets in the U.S. than you think. One scientist decided to begin studying chimps abandoned by their owners. According to a study published on PeerJ, he found that while scientists already advised against keeping the primates as pets, they didn’t have any scientific reasons for doing that—until now.

Wired, Business Insider and other publications spotted the study.

Chimpanzees Suffer Behavioral Problems If Raised By Humans

Study identifies behavioral problems in chimpanzees

Stephen Ross heads up the Chimpanzee Species Survival Plan for the Fisher Center for the Study and Conservation of Apes at Lincoln Park Zoo in Chicago. He’s been studying the behavioral habits of chimps that were raised by humans for about a decade. He said that approximately ten years ago, a lot of chimpanzee owners were calling him to inquire about getting rid of their pets.

Ross and fellow researcher Hani Freeman studied 60 chimps that were living in sanctuaries or accredited zoos. Of those 60, they said 36 of them were previously performers or pets.

Problems for chimps

Ross said usually chimpanzees that are bred to be performers or pets are taken away from their mothers extremely early, sometimes within 24 hours of their birth. Once taken away from their mothers, they usually don’t have any “normal chimpanzee stimuli or experiences,” according to Ross.

“They don’t see other chimpanzees,” said Ross. They aren’t exposed to chimpanzee culture; they’re in a completely human world.”

Measuring human – chimp interactions

The researchers developed what they call the Chimpanzee – Human Interaction Index, which measures the proportion of chimp versus human interactions in a chimpanzee’s early life. In the chimps with very little interaction with other chimps early in life, they discovered some social and behavioral problems.

For example, chimpanzees that were raised by humans spent a lot less time grooming other chimps. This practice is an important part of chimp culture, as it helps them maintain their social bonds, according to Ross.

They found that even years or decades after pulling a chimp out of its human environment and placing it with other chimps, the social and behavioral problems continued. Ross says these chimps are unable to figure out how to live and interact with their own kind.

Ross and his team will continue to study chimps in this manner and hope to provide more data on why private chimpanzee ownership isn’t good for the primates. They also want to change policies regarding private ownership of chimps.