When Sandra and Jerome Herold adopted Travis, he was just three days old. He didn't have a history of violence, but one day when Sandra's friend Charla Nash was visiting, the 200-pound Travis lashed out. He tore Nash's face off of her skull. Sandra couldn't save her friend, even by hitting Travis with a shovel. Travis wasn't a troubled human child; he was a chimpanzee.

When the police arrived, Travis smashed into their car before being shot. He crawled, bleeding, into his cage, where he eventually bled to death. Nash's injuries were so pervasive that hospital workers who treated her required counselling.

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When kept as pets or used as performers, chimpanzees often wind up becoming violent and destructive as they age – even with the best of care. Image: Donald (Don) Harrison, Flickr

When kept as pets, even with the best of care, chimpanzees often wind up becoming violent and destructive as they age out of childhood and become adult apes. In situations like these, it isn't so much a question of whether a chimpanzee will become violent, but when and how.

Anyone who has watched Super Bowl commercials over the last few years remembers the CareerBuilder and E*Trade commercials that featured chimpanzee actors, dressed in human clothes, bouncing around on camera. Travis himself was featured in commercials for Old Navy and Coca Cola. It's easy enough to point out that chimpanzees do not belong in human homes raised as if they were furry children, nor should they be 'hired' as performers. But primatologists Hani D. Freeman and Stephen R. Ross from the Lincoln Park Zoo's Lester E. Fisher Center for the Study and Conservation of Apes found that it isn't enough to rescue chimpanzees from those sorts of unnatural environments and rehabilitate them in more appropriate settings. That's because the first few years of life are critical to set up a chimpanzee for lifelong success. 

And they should know. As Director of the Lester Center and chair of the Chimpanzee Species Survival Plan for the Association of Zoos and Aquariums, Ross oversees the 259 chimpanzees who reside in AZA-accredited North American zoos. Since 2009, he has aided in the transfer of more than thirty chimps from private homes and 'animal actor' facilities to habitats and social groups more appropriate for the species in accredited zoos and sanctuaries.

By studying chimpanzees who had previously been pets or performers, the researchers revealed the pervasive ways in which atypical early experiences go on to affect the rest of a chimpanzee's life. Freeman and Ross weren't the first ones to highlight the importance of early life experiences. For years, researchers have noted the ways in which enriched or deprived childhoods affect species ranging from humans and primates to rats and lizards.

“By studying chimpanzees who had been pets or performers, the researchers revealed how these atypical early experiences go on to affect the rest of the animals' lives.”

But most studies of primate development have categorised individuals as either human-raised, peer-raised or mother-raised; Freeman and Ross hoped to add a bit more texture to the story by classifying chimpanzees on a continuous scale that accounts for the shades of grey in between. The broad categories, they argue, "do not encompass the actual experience[s] of chimpanzee[s], such as those who performed and spent significant time in full contact with both humans and other chimpanzees."

Chimpanzee performers, after all, do spend significant time with humans, but they may also be exposed to other chimpanzees. That's different from pet chimpanzees, who are more likely to be exposed to humans alone.

The researchers created a measurement that they called the Chimpanzee-Human Index, or CHI. It was designed to account for both the amount of time spent with humans and with other chimpanzees over the first four years of life. Wild chimpanzees would naturally be placed on one extreme of the CHI, while pet chimps would sit on the opposite end. In between are performing chimps, who have lots of human interaction and a bit of chimp exposure as well, and chimps housed in zoos, who have lots of chimpanzee interaction and some human exposure too. Freeman and Ross assessed 61 chimpanzees, 36 of which were former pets or performers, from three sanctuaries and six accredited zoos. In all, they collected nearly 1400 hours of behavioural observations.

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The early years of life are critical to set up a chimpanzee for lifelong success – and the research emphasises that the animals fare best when raised by others of their own species. Image: Chris de Visser, Flickr

They found, perhaps unsurprisingly, that chimpanzees who interacted primarily with humans in their infancy and childhood were less socially competent as adults, even for individuals who had spent years or even decades in social groups of their own species. They spent far less time engaging in social grooming, a critical behaviour that not only helps with hygiene, but is also the main driver in what primatologist Frans de Waal has called the chimp 'marketplace of services'. Grooming is the currency that determines access to food and friends. Chimpanzees are more likely to share food with those who have previously groomed them, for example. It also serves to ease social tensions after a fight, perhaps because grooming stimulates the release of feel-good endorphins. Chimps raised by their mothers, and even those with a mixture of human and chimpanzee exposure in their early years, also had more sexual competence than those raised by humans alone. Freeman and Ross published their findings in September in the journal PeerJ.

One surprising result of their observations was that chimps who spent most of their childhoods with other chimps showed higher levels of coprophagy, the eating of feces. That wasn't expected, since coprophagy is typically thought of as evidence of poor welfare. But Ross thinks that while coprophagy is abnormal insofar as it is quite rare in wild chimpanzee communities, its link to welfare is unsubstantiated. Instead, he thinks that coprophagy is simply a cultural behaviour, passed from chimp to chimp through social learning. "So individuals are likely to do it because they see other chimpanzees doing it, not because they are experiencing an environmental deficiency," he told me. For these chimpanzees, "it may actually reflect the heightened ability of the mother-reared chimps to socially learn from their group-mates, though in this case they are learning a somewhat off-putting habit."

The findings reinforce the notion that chimpanzees fare best when raised by others of their own species. Those with too much human interaction early in their lives remained left out, even after being placed into more reasonable social settings. "Nothing compares to the benefits of mother-rearing when it comes to the behavioural development of infants," says Ross.  

In cases where people must intervene, such as when a chimpanzee mother is neglectful, then the human caregivers should maximise the influence of other chimpanzees and minimise their own. That's built into the Chimpanzee Species Survival Plan, which prescribes that infants who do require human intervention be integrated back into their social groups as early as possible, and preferably to surrogate chimpanzee mothers. "It's important that chimps (and really all apes) be around others of their own species as much as possible in those critical early years," he adds. 

Top header image: Gemma Stiles, Flickr