Chimps Use Caves to Beat the Heat, Scientists Find

Scott Norris for National Geographic News

April 9, 2007

Chimpanzees in the West African nation of Senegal take shelter from the scorching heat in caves, researchers have found. The discovery has raised chatter among primate researchers, who say it's the first known case of regular cave use by an ape species. The unusual behavior is shown by the same chimpanzees recently found to hunt small mammals using sharpened sticks. Jill Pruetz, an anthropologist at Iowa State University, led the research team behind the new discoveries. Hers is the first long-term observational study of a savanna-dwelling chimpanzee population. Pruetz said that when she began fieldwork in 2001, at a site known as Fongoli, local Malinke people showed her the caves and told her they were often occupied by chimpanzees during the hottest part of the year. She was intrigued by the claim, but observing the chimpanzee behavior proved difficult. "It took years and years for the chimpanzees to get habituated [to the researchers' presence]," Pruetz said. "As soon as we would walk anywhere close, it would scare them out of the caves." Even with few direct observations, Pruetz's team was able to assess the extent of cave use using clues left behind on sandy cave floors: tracks, droppings, and food remains. The research showed that cave use was concentrated at the end of the dry season in May and June. "The behavior appears to be an adjustment to heat stress," Pruetz said. "No one has ever before published reports of apes in caves," noted William McGrew of Cambridge University in England. "This is one of those cases in which the apes genuinely surprise us, exceeding our expectations and imaginations." Pruetz's paper on cave use by the Fongoli chimpanzees will be published in a forthcoming issue of the journal Primates. Dry Country Apes Much of what is known about wild chimpanzee behavior comes from studies conducted in forests. But in Senegal chimpanzees occupy arid savanna habitat dominated by open grassland and sparse woodland. Chimpanzees in these areas exhibit a range of behaviors not found elsewhere. Pruetz noted that cave use is just one of several strategies the chimpanzees use to cope with their difficult environment, where both shade and water are critical resources. In April and May maximum temperatures in open grassland near the caves can reach 107.6 degrees Fahrenheit (42 degrees Celsius). Temperatures in the largest of the three small caves used by the chimpanzees never exceeded 84.2 degrees Fahrenheit (29 degrees Celsius). Lack of a nearby water source earlier in the dry season may have prevented the caves from being used more extensively throughout the year. Pruetz said that while her new paper is based solely on data collected at the Fongoli study site, her team has also observed cave use in other nearby chimpanzee populations. At a place called Baniomba, she said, "[chimpanzees] seem to use caves even more than at Fongoli. But we don't yet have any systematic data." Pruetz's graduate student Peter Stirling reported that the deeper Baniomba caves appeared to be occupied on a daily basis over a two-week period in the summer. "It's almost like they're using the caves there as a home base," Pruetz said. Give Me Shelter The adaptations of savanna chimpanzees are particularly interesting to researchers because early humans are thought to have occupied similar environments. "The finding would be notable in itself, but the implications for reconstructing the evolutionary origins of shelter in our ancestors make it even more so," said Cambridge's McGrew. Some monkeys use caves to stay warm at night, he noted. What is intriguing about the new study is that it shows "not the nocturnal use of caves for overnight sleeping but rather [daytime use] for siestas, socializing, and picnicking. No one expected this." University of California San Diego anthropologist Jim Moore said that while cave use may seem to be an obvious strategy for avoiding the midday sun, the behavior documented by Pruetz's team is unusual. "Primates very often don't use available shelter, even to get out of the rain," Moore said. "The finding opens up a whole set of questions, given that this behavior isn't seen in other regions of Africa," he continued. "Are they right at the edge of what chimpanzees can handle in terms of temperature … or is it a cultural thing?" Long-term studies of forest chimpanzee populations have shown that "historical and fine-scale ecological differences can result in very different cultures," Moore noted. "There have been a number of attempts to study savanna chimpanzees," he said, "but [Pruetz's study] is the first to get enough close-up observations to start seeing things like this. "By building up our understanding of how such environments shape [modern human relatives], we can better model our early ancestors," Moore added.