But the practice is relatively common among animals and is now understood to yield important health benefits.
In parrots and macaws, for example, clay particles are thought to help neutralize toxins present in some favored food sources.
Soil-eating may also provide a soothing coating to the animals' digestive tracts.
In Uganda, however, Krief and colleagues suspected that the chimps were eating soil for other reasons than simply to combat indigestion.
Like humans, chimps can suffer the potentially fatal effects of malaria, although the types of Plasmodium parasites that cause the disease in chimps are different from the four known to infect people. (See a close-up image of malaria parasites attacking a human's red blood cells.)
In previous laboratory studies, Krief's team had found that extracts of the Trichilia plant were effective in fighting the parasite that causes malaria in chimps.
"I noticed that the chimps often eat soil just before or after eating Trichilia leaves," Krief said. "I wondered what might be the effect of mixing the two substances."
To investigate, Krief and colleagues constructed a laboratory model that duplicated chimpanzees' chewing and digestion.
The researchers ground up leaves and soaked them in an acid solution similar to the chimps' digestive fluids. The mixture was then tested for its ability to kill the malarial parasite.
Solutions of artificially digested leaves without soil showed no anti-malarial activity.
But when samples of the soil consumed by the chimps were added to the mix, the researchers found a strong disease-fighting effect.
Chimps and other apes may treat themselves for a variety of ailments by consuming plants, earth, and other materials, experts say.
In another part of Africa, chimps are known to swallow certain leaves to help rid themselves of intestinal parasites.
Previous work by Krief and others had shown that of the 163 plant species Kibale chimps are known to eat, at least 35 are used in humans' traditional medicine.
"The shared use of plants by humans and apes shows how the tropical forest is a unique resource for wildlife, local communities, and western medicine," Krief said.
Jim Moore is an anthropologist at the University of California, San Diego, who was not involved with the new study.
"It's been known for a long time that chimpanzees selectively eat certain plants in ways and at times that only really make sense if they are self-medicating," Moore said.
"This paper is the first I'm aware of to suggest synergistic action of soil and a putatively medicinal plant, and that's important."
But Moore noted that the anti-malaria effect has only been shown in the researchers' model and not yet in living chimps.
"It isn't clear that chimpanzees are seeking the synergy [between leaves and soil]," Moore said. "The paper isn't persuasive in eliminating other hypotheses for geophagy."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES