There may be more monkey business going on among chimpanzees than scientists once thought.

A study suggests that half of all chimpanzees may be conceived on the sly when females sneak off for risky trysts with males outside their social group.Female chimpanzees' secret sex lives come as something of a surprise to researchers, who previously thought that they almost always mated within their own group of 20 to 100 animals.

"When they can get away with it, they sneak off and they try to expand the pool of possible fathers," said Pascal Gagneaux, a professor at the University of California at San Diego.

Working with UCSD biologist David Woodruff and Christophe Boesch of the Basel Zoological Institute in Switzerland, Gagneaux painstakingly worked out the genetic family tree of a chimpanzee group living in the Tai Forest of West Africa's Ivory Coast.

Between 1991 and 1995, he and his colleagues collected DNA samples from all 52 members of the Tai Forest group. The DNA came from hair - collected from chimpanzee sleeping nests by researchers who climbed trees more than 100 feet tall - and from chewed fruit, which yielded cells from inside the mouth.

Paternity tests on the DNA yielded a shocking result: Of 13 infants, only seven were fathered by members of the Tai Forest group.

Richard Wrangham, a Harvard professor who studies chimpanzees in Uganda, said that because of the animals' ferocious territorial behavior, extra-group couplings might actually have a practical benefit: A female that has such a tryst creates the possibility that her offspring may be related to a neighboring male. That might lead the neighboring male to show mercy in a future encounter with her or her offspring.

In his own research, Wrangham has seen females lurking about a neighboring group's territory. So he was not surprised to learn that some infants among the Tai chimps have fathers from other groups.

"But even so, 50 percent seems extraordinarily high," he said. "There's still a lot that's mysterious about this."

Gagneaux's findings, published in Monday's issue of the journal Nature, give female chimpanzees a much more important role in the reproductive process. Evolutionary biologists often treat females as a prize to be won by the most deserving male. But Gagneaux said that's the wrong way to look at things.