A DANGEROUS COUSIN: This chimp from the Knoxville Zoo bares its teeth to visitors, who observe from behind glass. Chimpanzees have been known to bite off fingers from behind bars.
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Earlier this week, a 14-year-old, 200-pound (90-kilogram) pet chimpanzee in Stamford, Conn., left a woman in critical condition after attacking her—mutilating her face and hands. The owner, Sandra Herold, who tried to stop the attack, was also injured and briefly hospitalized. The victim remains in critical condition.

The chimp, Travis, who was shot and killed by police officers at the scene, was apparently a friendly fixture around the neighborhood. He appeared in television commercials and had a sapiens-level CV that included using a computer, bathing and sipping wine from a stemmed glass, according to The New York Times. Reports, however, are starting to surface that Travis might have bitten another woman in 1996 and that Herold had been warned by animal control that her pet could be dangerous.

Chimpanzees, with a genetic profile that's 98 percent like ours, can seem like cute, hairy iterations of people. But periodic violent attacks on humans, including one in Havilah, Calif., in 2005 in which a man was maimed by two chimps at an animal sanctuary, are reminders that the animals have at least one big difference: brute strength.

So why would an allegedly acclimated chimpanzee turn on a human—especially one whom he had known? Travis was reportedly suffering from Lyme disease, caused by a tick-borne bacterium and known to cause fatigue, joint problems and mental difficulties—including trouble focusing and poor memory in humans. Some have suggested that the attack was spurred by Xanax, a prescription drug used to treat anxiety disorders in humans, with side effects that can—but rarely—include depression, confusion and problem behavior. Travis’ owner claims to have given him a Xanax-laced tea the day of the attack.

To find out more about chimpanzee attacks, we spoke with Frans de Waal, lead biologist from the Yerkes National Primate Research Center. He is affiliated with the Living Links Center at Emory University in Atlanta where he is a professor of psychology, and is also author of The New York Times notable book of the year, Our Inner Ape.

[An edited transcript of the interview follows.]


Are captive chimpanzee attacks on humans common?
Yeah, definitely common. Most of the time they attack through cage bars. They bite off fingers. It happens more often with people they don't know very well and people who aren't familiar with chimpanzees. But it has happened to many of the best scientists and researchers, who are now missing digits. The reason we have them behind bars in zoos and research settings is because chimpanzees can be very dangerous—it's to protect ourselves. This was a sort of free-ranging chimp, which is much more dangerous.

But chimps in the wild are not used to people—they're afraid of them. That's why Jane Goodall had to habituate them. So, really wild chimps don't attack people. But in captivity, they have learned in the meantime that they are stronger than humans.

How strong are they?
The chimpanzee has strength for a human that is utterly incomprehensible. People watch pro wrestlers on TV and think they are strong. But a pro wrestler would not be able to hold a chimpanzee still if they wanted to. Chimpanzee males have been measured as having five times the arm strength as a human male. Even a young chimpanzee of four or five years, you could not hold it still if you wanted to. Pound-for-pound, their muscles are much stronger. And the adult males, like Travis—unless his were filed down—have big canine teeth. So you have a very dangerous creature in front of you that is impossible to control.