A study published Tuesday in the journal PeerJ reveals that chimpanzees who are removed from early exposure to other chimpanzees, and raised by humans as pets or performers, are more likely to display long-term behavioral and social defects in the adult stage.

Researchers of the study, led by Dr. Stephen Ross of the Lester E. Fisher Center for the Study and Conservation of Apes at the Lincoln Park Zoo in Chicago, IL, collected data on 60 chimpanzees, 36 of whom were former pets or performers and are currently residing in accredited zoos and sanctuaries.

Results from the data suggest that chimpanzees who experienced less exposure to other chimpanzees in the first four years of life – due to being raised primarily around humans as opposed to a more natural environment – showed lower influence within social dynamics later on in life, such as grooming and sexual behaviors with groupmates.

“Unusually for a study on this topic, we looked at the degree of human and chimpanzee exposure on individual chimpanzees along a continuum,” said Ross in a recent statement. “This showed that those chimpanzees with more atypical beginnings to their lives, spending much more time with humans than with their own species, tended to behave differently than those that stayed with their family through infanthood.”

A startling observation was that the behavioral and social effects on these chimpanzees were demonstrated years (and even decades) after their lives as pets and performers had ended. “Chimpanzees which have found new homes in accredited zoos and good sanctuaries continue to demonstrate behavioral patterns that differentiate themselves from more appropriately-reared individuals,” said Ross. “As a result, the process of integrating them with other chimpanzees can be challenging, stressful and even dangerous at times.”

Authors of the study believe that these results highlight the need to consider how human interaction and/or lack of same-species interaction can have a considerable impact on animals living in captivity. “Chimpanzees are incredibly intelligent and sensitive animals,” said Ross. “Denying them access to members of their own species, during the critical infanthood period, results in behavioral outcomes that last a lifetime. Even with the best possible care as adults, they often can’t fit in with the other chimpanzees.”