For years, chimpanzee owners have been calling Stephen Ross to see if he can take their pets off their hands.

Ross is the director of the Fisher Center for the Study and Conservation of Apes at Lincoln Park Zoo, and he launched the zoo's Project ChimpCARE, an initiative that assesses the status of chimps living in the United States as pets and performers.

The goal of the project is to ensure appropriate care for all chimps, and a study published this week in PeerJ highlights the negative effects private ownership can have on chimpanzees.

Ross studied 60 chimpanzees living at U.S. zoos and sanctuaries — 36 of which were former pets or performers — and rated them on a metric he calls the Chimpanzee-Human Interaction Index.

This index measure the amount of time a chimpanzee spent with humans and other chimps during its early life.

Using the index, Ross and his colleagues then looked at what high exposure to humans and little exposure to their own kind does to chimps in the long-term, and they found that chimps kept privately had lost their ability to form strong social bonds with other chimps.

"Chimpanzees raised to be pets or performers have very atypical lives," Ross told Wired. "They don't see other chimpanzees. They aren't exposed to chimpanzee culture. They're in a completely human world."

Chimps kept as pets or performers are often bred for this purpose and are taken away from their mothers very early, but in the wild, young chimps receive 24-hour care from their mothers.

The chimpanzees in the study who were raised by humans exhibited behavioral and social deficiencies even years after being introduced to other chimps. This was also true of those who seemed to thrive.

"They've had the opportunity to learn to be chimps, and they do okay in some circumstances," Ross said. "But there's also lots of examples of chimps who, on some level, have been suffering because they just can’t figure it out."

The most noticeable difference was in how little human-reared chimps engaged in social grooming.

"Grooming is the glue that holds chimpanzee society together," Ross said. "We found chimpanzees that were around humans a lot early in life tended not to do a lot of this behavior. They just weren't good at maintaining these social bonds, and that was expressed by these lower rates of grooming."

Ross hopes his data will help change policies about private chimpanzee ownership, as well as better equip zoos and sanctuaries to take in these animals.

Most zoos won't accept human-reared chimps because they often don't fit into established social groups, according to the Jane Goodall Institute.

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