I. ANIMALS THAT EAT DIRT
Known variously as pica or geophagy, eating soil is widespread among many animals on every continent. It is also frequently observed among people, especially traditional societies. In wild animals, eating dirt seems to be a weapon inthe ancient competition between plants and animals. Geophagy is an animal weapon in the struggle between plant reproduction strategy and the animal desire for food.
A way of understanding geophagy is to consider the strategies involved. Many plants use animal mobility to spread seeds by enticing animals with tasty fruit. The animal digests the pulp, but spits out the seed or passes it unharmed through its digestive system to be defecated--freshly potted and fertilized. Plants fend off the animals by discouraging consumption until the seeds are ready to sprout by making the pulp bad tasting or even poisonous. Plants manipulate us by making us wait until the pulp of fruit is suitable to eat--and the seeds are ready for dispersal.
Seeds possess a concentrated supply of carbohydrates, proteins, and fats to help them germinate and grow. Bad tastes or poisons are added to the seeds--or unripe fruit--to fend off animals. How to overcome them? Parrots will eat soils containing minerals that bind plant toxins effectively. In fact, they are quite selective in choosing the most effective soils for their purpose. (My comment: parrots are very intelligent creatures. Some have learned to speak and understand over a hundred English words. Trial and error seems to be part of their behavioral repertoire in nature.)
Are you acquainted with the place name 'lick' such as French Lick in southern Indiana? It is a place where animals gather to eat soil. North American wild hoofed animals visit licks, as do bears and many small mammals. Feeding selected soils to cows, sheep, goats and pigs results in enhanced growth--by 20% or more.
Antler-growing deer eat soil rich in calcium and magnesium. Some animals seek out sodium. So, with animals (and humans), in some cases a particular element is sought. The most compelling need, however, seems to be the need to detoxify plant products in the diet.
II. HUMAN GEOPHAGY
People seem to use geophagy to protect themselves against plant toxins. We are accustomed to eating domesticated potatoes. In South America, however, some Indians regularly eat
bitter, toxic wild potatoes capable of producing stomach pains and vomiting. They have learned to make the potatoes safe and palatable by eating them with an alkaloid-binding clay. (My comment: we have not met plant alkaloids yet; we will see their plant kingdom distribution and medical significance when we get to rainforest pharmacology.)
California Native American Indians harvested acorns as a major staple in their diet. Before contact, they routinely burned the forest undergrowth to encourage growth of the some twenty species that they exploited in their environment. To make the acorns edible, the naturally occurring tannic acid had to be removed. This was done by soaking the whole acorns in a running stream or percolating water through ground acorn flour. Another technique was to mix the acorn flour with a clay that reduced the tannic acid content by up to 77 percent. They achieved this know-how by trial and error. We can verify its efficacy today with chemistry.
Geophagy is especially popular with pregnant and nursing mothers, who have an increased need for minerals (and mineral supplements). In Zambia and Zimbabwe the main sources of soil,
which 90% of rural women consume while pregnant are giant termite mounds that also attract cows and giraffes. When asked why, they might answer "I feel good when I eat it" or "I like the taste." This doesn't answer the question for us with a physiological perspective, but craving for trace elements may be the reason for eating--dirt!
..... CJ '98
Diamond, J. "Eat Dirt!" Discover. February, 1998 pp 70-75.
Goodhart, R. and Shils, M. Modern Nutrition in Health and Disease. Philadelphia: Lea and Febiger, 1976.
Overfield, T. Biologic Variation in Health and Illness. Boca Raton: CRC Press, 1995.