Baboons, chimps enter the realm of cave.

Many anthropologists assume that until our evolutionary ancestors learned to control fire to keep predators at bay, primates avoided caves. Two separate studies in Africa now indicate that some groups of baboons and chimpanzees regularly enter caves, primarily to escape extreme cold and heat.

In one investigation, psychologist S. Peter Henzi of the University of Central Lancashire in Preston, England, and his coworkers tracked a baboon troop's forays into an underground limestone cave in South Africa.

One at a time, the animals would crawl down a narrow shaft and then move through a 40-yard-long passage that empties into the pitch-dark cave, which is always near room temperature.

Using soft grunts to communicate, the 31 to 50 animals would then get busy, copulating and grooming one another and splitting into small sleeping groups. Infrared video cameras that the scientists had placed inside the cave captured the action.

"These baboons took a leap of faith to find this utterly novel sleeping site," Henzi says. "It's a testament to the behavioral plasticity of the species."

His team's report appears in the February Journal of Human Evolution.

The study ran from June 1998, when the researchers first saw baboons entering the cave, to June 2003. The researchers rigged the cave with temperature and humidity sensors. A nearby weather station provided data on external temperature, wind speed, humidity, and rainfall for each hour of the day throughout the study.

Baboons most often entered the cave when nighttime temperatures dipped to near freezing in the winter or remained particularly hot in the summer. Two other baboon troops occasionally shared the cave with the study group. Baboons may also resort to the cave for protection from leopards and other predators, Henzi suggests.

Using implanted sensors, Henzi's team is now tracking the body temperatures of four baboons to identify physiological benefits of cave shelter on winter and summer nights.

In the second study, Jill D. Pruetz of Iowa State University in Ames is following a cave-using group of 30 chimpanzees in Senegal. People who live there had told Pruetz of seeing chimps annually entering caves in May and June. Since May 2001, feces and food remains retrieved from one of these caves indicate that chimps regularly use the shelter in the dry months, probably to escape extreme daytime heat, Pruetz says.

Although there have been several anecdotal reports of nonhuman primates visiting caves, Henzi's and Pruetz's projects are the first to document this behavior systematically, remarks anthropologist William C. McGrew of Miami (Ohio) University in Oxford.