|
Very deliberately, the chimpanzee selects a branch, breaks it off, strips away leaves, and uses its teeth to sharpen the end to a point. Then, spear in hand, it begins to hunt. A scene from a science fiction movie? No, one of several startling behaviors Jill Pruetz's fieldwork has revealed for the first time.
It was not the scorching 110 degree Fahrenheit (43 degree Celsius) temperatures, poisonous snakes, or ever present risk of malaria that drew Pruetz to the savannas of Senegal, but the similarity between this habitat and that of our earliest human ancestors, hominids.
"Environment," Pruetz explains, "has triggered significant changes throughout the evolution of our own human lineage. In the savanna, food sources are few and far between, which can exert pressure to make more sophisticated tools and develop more innovative hunting behaviors. We see it today with the chimps at my site, and hominids may have responded to this challenging habitat in the same way."
Pruetz's study was also the first to report that chimps, our closest animal relatives, seek shelter in caves, behavior reminiscent of early hominids. "Again, this is environmentally driven," she notes. "They use caves only during the hottest times of day." Another surprise came when she observed chimps at her site cooling off by soaking in pools of water. Chimpanzees have long been thought to dislike and even be afraid of water.
But the most striking difference between the savanna chimps and those living in forests lies in how they hunt. Savanna chimps fashion "spears" and jab them into hollow tree cavities to kill and extract bush babies, a small primate and a source of protein that is ignored by forest-dwelling chimps. "In the forest, chimps have a much more plentiful supply of monkeys," Pruetz says. "But for chimps at my site, hunting bush babies is a creative way to get protein. Forest chimps don't have to go to the extremes savanna chimps do."
Even more surprising, the spears are predominately used by females. Previously, hunting was considered an almost exclusively male behavior in chimps. "To me, this shows that the females have to be a little more innovative," Pruetz observes. "Males are able to run down monkeys. That's much more difficult for a female, especially since most have an infant clinging to them."
Although anthropologists and biologists have long been interested in studying savanna chimps, Pruetz is the first to successfully habituate them in this unique habitat, a process that took four years and still is not complete.
Fieldwork in this environment is particularly challenging because of the chimps' enormous home range—more than 23 square miles (60 square kilometers) compared to less than four square miles (ten square kilometers) at many forested sites. "I usually spend 13 to 15 hours in the field. I get up before dawn, hike to where the chimps are sleeping—sometimes an hour and half away—and wait for them to wake up. Then we follow them all day until they stop for the night. Because their territory is so gigantic, if we left before they stop there's a good chance we'd never find them the next day."
When she's not traversing the savanna, Pruetz is an associate professor of anthropology at Iowa State University. In the future, she plans to conduct a distance education course from her Senegal site, giving students a chance to observe the chimps live from the field via satellite computers and phones.
Nearly seven years after beginning her Senegalese study, Pruetz is primed for more surprises. "The most exciting findings here have been completely unexpected. And even now, although I've seen some of these behaviors dozens of times, it's still exciting. Of course the chimps are completely oblivious that they're doing anything extraordinary, but I want to say to them, 'Wow, don't you realize how important and amazing you are!'"
