Naturally very social animals who can live in troops of up to 120 in the wild — though their troop size is often lower due to habitat loss — vervets in captivity are prone to developing severe psychological problems due to lack of natural environment and family structure, David Du Toit, director of the Vervet Monkey Foundation in South Africa, told The Dodo. When they are raised from infancy by humans instead of their primate families, they may seem agreeable — but as they age, they can grow aggressive and territorial from natural instinct and a lack of socialization, he said.
“Being very social animals, deprivation of family members can lead to depression,” Du Toit said. “Fear and stress are also great contributing factors, as people tend to like showing off their pet vervets by taking them out to malls or shopping centers, which is highly unnatural for them.”
In the wild, Du Toit explained, troops fight for territory and normally a monkey who finds himself in another troop’s territory will face a chance of being killed. “So imagine the fear when the monkey finds himself surrounded by thousands of unfriendly faces making threatening sounds and bearing their teeth, or as humans see it, laughing and smiling,” Du Toit said. “The poor animals are petrified, clinging for their life to the inconsiderate person who brought it into this dangerous environment.”