Researchers say they have obtained the first video-based observation of swimming and diving by apes, a behavior previously thought distinct to humans.

Zoos have for many years used water moats to confine chimpanzees, gorillas or orangutans, who often drowned if they ventured into deep water, leading some researchers to conclude swimming was an exclusively human activity among primates.

Renato Bender of Wits University in South Africa and Nicole Bender of the University of Bern have studied a chimpanzee and an orangutan in the United States that were raised and cared for by humans and have learned to swim and to dive.

"We were extremely surprised when the chimp Cooper dived repeatedly into a swimming pool in Missouri and seemed to feel very comfortable," Bender said. "It was very surprising behavior for an animal that is thought to be very afraid of water."

An orangutan named Suryia, filmed in a private zoo in South Carolina, also exhibited this rare swimming and diving ability, the researchers said.

Instead of the usual dog-paddle stroke used by most terrestrial mammals -- a mode of locomotion that they employ instinctively -- both primates use a leg movement similar to the human breaststroke of "'frog kick," they said.

Humans do not swim instinctively but are attracted to water and can learn to swim and dive. The findings suggest the same may be true of some of our primate relatives, the researchers said.