In the fall of 2009 a group of biology students at Tufts University sat down together and ate some dirt. They ground up small clay tablets and swallowed the powder to find out, firsthand, what clay tastes like. This unusual taste test was part of a Darwinian medicine class taught by one of us (Starks). The students were studying the evolution of geophagia—the practice of eating dirt, especially claylike soils, which is something animals and people have been doing for millennia.
The standard reference guide for psychiatrists—the fourth edition of the Diagnostic and Statistical Manual for Mental Disorders (DSM-IV)—classifies geophagia as a subtype of pica, an eating disorder in which people consume things that are not food, such as cigarette ash and paint chips. But as the students would learn, studies of animals and human cultures suggest that geophagia is not necessarily abnormal—in fact, it may well be adaptive. Researchers are taking another look at dirt eating and discovering that the behavior often provides people and animals with vital minerals and inactivates toxins from food and the environment.
One way to decide whether geophagia is abnormal or adaptive is to determine how common the behavior is in animals and across human societies. If many different species and cultures demonstrate the same behavior, then it is probably beneficial in some way.
Today it is clear that geophagia is even more widespread in the animal kingdom than previously thought. Investigators have observed geophagia in more than 200 species of animals, including parrots, deer, elephants, bats, rabbits, baboons, gorillas and chimpanzees. Geophagia is also well documented in humans, with records dating to at least the time of Greek physician Hippocrates (460 B.C.). The Mesopotamians and ancient Egyptians used clay medicinally: they plastered wounds with mud and ate dirt to treat various ailments, especially of the gut. Some indigenous peoples in the Americas used dirt as a spice and prepared naturally bitter foods such as acorns and potatoes with a little clay to counteract the acerbic taste. Geophagia was a frequent practice in Europe until the 19th century, and some societies, such as the Tiv tribe of Nigeria, still rely on cravings for dirt as a sign of pregnancy.
A common explanation for why animals and people eat dirt is that soil contains minerals, such as calcium, sodium and iron, which support energy production and other vital biological processes. The fact that an animal’s need for these minerals changes with the seasons, with age and with overall health may explain why geophagia is especially common when an animal’s diet does not provide enough minerals or when the challenges of the environment demand extra energy. Mountain gorillas and African buffalo that live at high altitudes may, for example, ingest earth as a source of iron that promotes red blood cell development. Elephants, gorillas and bats eat sodium-rich clays when they do not get enough sodium in their diet. One elephant population is known to continually visit underground caves where the animals dig up and eat salt-enriched rock.
Among human populations in Africa, those who have ready access to calcium do not practice geophagia as often as those deprived of calcium. The need for calcium may also partly explain why geophagia is most commonly associated with pregnancy: a mother needs extra calcium as the fetal skeleton develops.
Mineral acquisition does not fully explain geophagia, though. In an extensive review paper published in the 2011 Quarterly Review of Biology, Sera L. Young of Cornell University and her colleagues conclude that eating earth rarely adds significant amounts of minerals to one’s diet and, in many cases, interferes with the absorption of digested food from the gut into the bloodstream, sometimes resulting in nutrient deficiency.