Joining the long list of behaviors we share with chimps, a new study published in PLOS ONE last week details how chimpanzees in the Budongo Forest of Uganda have been observed eating clay in order to access its detoxifying minerals.
"A chimpanzee's diet is mostly leaves, fruits and the occasional monkey. They sometimes eat other things—bark, rotting wood and even soil," Cat Hobaiter, a researcher at University of St. Andrews in the U.K. and co-author of the study, told NPR's The Salt. Typically, chimps rely on decaying swamp trees for a range of minerals they don't get elsewhere. As deforestation limits the availability of edible wood, chimps have upped their intake of clay and clay-rich water in order to supplement their diets.
After first noticing this behavior years ago, researchers began more closely observing the chimpanzees in 1990. They noted instances of both eating clay directly from the ground and drinking clay-rich water via "leaf sponges," or chewed-up leaves that are dipped into water holes to collect liquid.
Clay, which has always been a part of the chimp diet to some degree, has an additional benefit in larger doses: It works to detoxify tannins. The mature leaves that make up most of what chimps eat are full of the bitter polyphenols found in tea, chocolate, and wine, which can have harmful effects when consumed in large quantities.
The mineral-binding structure of the kaolin clay consumed by chimps neutralizes this acidity. Vernon Reynolds, a professor emeritus of biological anthropology at Oxford University and the lead author of the study, said that there's no indication the chimps were suffering from digestive problems: "They're all perfectly healthy, and so [clay-eating] was preventative rather than curing."
These findings are particularly interesting in light of recent research into the disorder known as pica, in which people compulsively crave things that aren't food, such as starch, charcoal, ice—and dirt, or more specifically, kaolin clay.
Cornell nutritional anthropologist Sera Young, author of Craving Earth, theorizes that clay's ability to act as a "mud mask for the gut" might be behind this baffling compulsion. There's evidence that our ancestors were eating dirt at least 2 million years ago, indicating that there is something innately appealing about clay consumption. And Young has found that pregnant women—whose immune systems are slightly surpressed—and people living in hot, humid areas, where pathogens multiply and spread rapidly, are the most susceptible to pica.
"I can assure you that no one has said, 'Actually, Dr. Young, I'm picking up this box of Argo corn starch to protect myself from the pathogens in my environment.' They're saying what the impetus is, the smell and the taste," Young told NPR last year. In healthy humans, the desire to eat clay is more harmful than the tannin-neutralizing properties are worth—the binding properties of the clay don't just flush out the toxins, they absorb the useful nutrients as well.
But the new evidence of chimps consuming kaolin clay supports the theory that pica isn't purely random—it could have evolved as an early protective measure.