This undated photograph, provided by the Facebook group HelpAndrewOberle, shows American student Andrew Oberle sitting with a chimp. The two chimps that mauled Oberle in South Africa will not be killed, according to park rangers, because they were "defending their territory".

Johannesburg - Two chimpanzees that mauled an American student in South Africa will not be killed because authorities found the animals defended their territory against a trespasser, a park director said on Tuesday.

Andrew Oberle, 26, lost an ear, several fingers and toes, and a testicle in the attack on Thursday when he entered a restricted enclosure at Jane Goodall Institute Chimpanzee Eden in the north-east of the country, the national parks board found.

“They don't believe the chimpanzees should be put down because they were in defence of their territory and they did not escape or try to climb over the fence with the intent to hurt anyone or to attack any tourist,” said Chimp Eden director Eugene Cussons.

Oberle, a Masters student in anthropology and primatology at the University of Texas at San Antonio, was doing research at Cussons' sanctuary for abused and orphaned chimps.

While taking tourists on a visit of the facility he crossed an outer fence to a restricted zone in front of the 4.5-hectare encampment fence to fetch a stone that the apes might try to throw at people.

“He himself climbed over the safety fence and into the no-go zone,” said Cussons.

“There was a rock that could have been used as a projectile and out of human nature Andrew felt that he should intervene,” he said.

“Andrew was never trained to do so. There's only a few people that's trained at the highest level that actually go into the no-go zone to do maintenance on the fence.”

Cussons shot and injured one of the primates, called Nikki, while trying to rescue Oberle, then stabilised the student when the primates retreated.

“He was conscious, he was speaking to me. He was badly injured. He had deep lacerations. He had injuries to his feet and hands and lacerations all over his body,” Cussons told AFP.

The student was being kept unconscious in hospital, said Cussons.

“Andrew is in a drug-induced coma for his own comfort. He is in a stable condition now.”

His family has flown to the country from the United States.

Chimp Eden gained fame through an Animal Planet TV series called Escape To Chimp Eden. - Sapa-AFP