The big male chimpanzee picked up a heavy rock, the size of a large melon, and rushed aggressively towards us. The guide shouted at us to retreat, and we hastily scrambled back to a safe distance.
The chimp, an alpha male named Amadeus, spun around and hurled the rock at another chimp, Nikki, who leaped away as the rock sped towards him.
The tour guide had hesitated to take us here. The other two enclosures at the Jane Goodall chimp sanctuary in South Africa was considered safe, with their mix of females, males and young chimps. The third enclosure, occupied only by adults, was known to be more dangerous and unpredictable. And the females were in heat, making the males more aggressive and territorial.
In the rock-hurling incident, about 10 days ago, we escaped unscathed. But just a few days later, the same two chimps – Amadeus and Nikki – grabbed a U.S. researcher and dragged him under a fence, mauling him repeatedly and leaving him in critical condition in the intensive care ward of a local hospital.
The 26-year-old researcher, Andrew Oberle, had been at the sanctuary for just a few weeks. He made the mistake of breaking the rules and stepping past one of the two fences around the adult enclosure, leaving only a single electrified fence between him and the chimps as he talked to a tour group.
Mr. Oberle, a University of Texas graduate student in anthropology and primatology, was dragged by the chimps for several hundred metres and bitten repeatedly. He lost parts of an ear and fingers and suffered other deep wounds, according to local reports. The sanctuary's manager was forced to fire gunshots to drive the chimps away from the critically injured student.
The student is now in a drug-induced coma at a hospital in Nelspruit, about 12 kilometres from the sanctuary, known as Chimp Eden, which holds 33 rescued chimps from across Africa. The chimps have been featured on an Animal Planet series, "Escape to Chimp Eden," which emphasizes the violent tendencies of the abused apes as they are rescued.
Many of the rescued chimps were badly abused by circuses, nightclubs or pet-owners who dressed them up, kept them in small cages, and taught them to smoke and drink.
After the attack last week, the sanctuary insisted the two attacking chimps were behaving naturally: defending their territory from an intruder who had stepped too far. But the inmates of Chimp Eden also illustrate the complex mysteries of primate psychology – especially when their natural behaviour is mingled with the trauma of abuse from humans.
One of the rescued chimps, Cozy, had been confined and abused for so many years that he was mentally damaged. Despite years of rehabilitation, he still routinely throws stones or other objects at the sanctuary's visitors, who are told to duck behind a fence whenever he begins his aggressive displays.
Nikki, one of the two chimps that attacked Mr. Oberle, was orphaned when his parents were killed for the bushmeat trade in Liberia. His owner raised him as a human infant, shaving his body (except for a boy's haircut), dressing him in children's clothes and a gold watch, and teaching him to eat at a table with cutlery. He arrived at the sanctuary clutching a suitcase with spare clothes.
The second chimp, Amadeus, was a bushmeat orphan from Angola. On the sanctuary's website, he is described as a "quiet thinker" who never threatens the staff – but has developed physically and "is not to be underestimated." He has learned how to hunt and kill wild birds at the sanctuary, the website says.
What had amazed me – and our guide at the sanctuary – was the physical strength of the chimps. Amadeus had tossed the heavy rock as if it was as light as a baseball. Chimps are said to be six times stronger than humans, and much more powerful than most of their owners realize.
In a notorious incident in 2009 in Connecticut, a pet chimp named Travis inflicted horrendous injuries on Charla Nash, a friend of his owner. She was blinded and lost most of her face, including her lips and nose, and was later given a full-face transplant. Travis had been raised by humans since birth, and he was considered friendly and gentle until the day of the attack.
Last week's attack on Mr. Oberle was the first in Chimp Eden's six years of existence. The sanctuary has now been temporarily closed to visitors, and Amadeus and Nikki placed in isolation, while the centre investigates the attack. The chimps are showing "remorse," by behaving submissively, and will not be euthanized, the centre says.
"This is a terrible tragedy that should never happen," said David Oosthuizen, executive director of the South African branch of the Jane Goodall Institute.
Mr. Oberle's friends, who are raising money for his medical treatment, are convinced that he will return to his chimpanzee research in the future.
"Andrew was meant to work with animals," said a friend, Marissa Reimherr. "This is what he loves to do. I know Andy, and I know that he will continue his work when he recovers. Yes, this is tragic, but Andy will not see this as a tragedy. He will see it as an opportunity."