Geophagia

Many species of South American parrots have been observed at clay licks, and sulphur-crested cockatoos have been observed ingesting clays in Papua New Guinea. Analysis of soils consumed by wild birds show that they often prefer soils with high clay content, usually with the smectite clay families being well represented.[8]

The preference for certain types of clay or soil can lead to unusual feeding behaviour. For example, Peruvian Amazon rainforest parrots congregate not just at one particular bend of the Manu River but at one specific layer of soil which runs hundreds of metres horizontally along that bend. The parrots avoid eating the substrate in layers one metre above or below the preferred layer. These parrots regularly eat seeds and unripe fruits containing alkaloids and other toxins that render the seeds and fruits bitter and even lethal. Because many of these chemicals become positively charged in the acidic stomach, they bind to clay minerals which have negatively charged cation-exchange sites, and are thereby rendered safe. Their preferred soils have a much higher cation-exchange capacity than the adjacent, rejected layers of soils because they are rich in the minerals smectite, kaolin, and mica. The preferred soils surpass the pure mineral kaolinate and surpass or approach pure bentonite in their capacity to bind quinine and tannic acid.[7]

In vitro and in vivo tests of these soils and many others from southeastern Peru indicate that they also release nutritionally important quantities of minerals such as calcium and sodium. In the Manu River example cited above, the preferred soil bands had much higher levels of sodium than those that were not chosen. Repeated studies have shown that the soils consumed most commonly by parrots in South America have higher sodium contents than those that are not consumed.[9][10][11]

Parrots eating earth

It is unclear which factor is driving avian geophagy.[12] However, evidence is mounting that sodium is the most important driver among parrots in southeastern Peru. Parrots are known to eat toxic foods globally, but geophagy is concentrated in very specific regions.[13] Researchers Lee et al. show that parrot geophagy in South America is positively correlated to a significant degree with distance from the ocean. This suggests that overall lack of sodium in the ecosystem, not variation in food toxicity, is a better predictor of the spatial distribution of geophagy. This work, coupled with the recent findings of consistently high sodium levels in consumed soils,[9][10][11] make it highly likely that sodium is the primary driver of avian geophagy among parrots (and possibly other taxa) in the western Amazon Basin. This supplemental nutrients hypothesis is further supported by peak geophagy occurring during the parrots' breeding season.[14]

There are several hypotheses about the importance of geophagia in bats and primates.[15]^: 436[16] Chimpanzees in Kibale National Park, Uganda, have been observed to consume soil rich in kaolinite clay shortly before or after consuming plants including Trichilia rubescens, which possesses antimalarial properties in the laboratory.[17]

There is debate over whether geophagia in bats is primarily for nutritional supplementation or detoxification. It is known that some species of bats regularly visit mineral or salt licks to increase mineral consumption. However, Voigt et al. demonstrated that both mineral-deficient and healthy bats visit salt licks at the same rate.[18] Therefore, mineral supplementation is unlikely to be the primary reason for geophagia in bats. Additionally, bat presence at salt licks increases during periods of high energy demand.[18] Voigt et al. concluded that the primary purpose for bat presence at salt licks is for detoxification purposes, compensating for the increased consumption of toxic fruit and seeds.[18]

Evidence for the likely origin of geophagy was found in the remains of early humans in Africa:

The oldest evidence of geophagy practised by humans comes from the prehistoric site at Kalambo Falls on the border between Zambia and Tanzania (Root-Bernstein & Root-Bernstein, 2000). Here, a calcium-rich white clay was found alongside the bones of Homo habilis (the immediate predecessor of Homo sapiens).

— Peter Abrahams, Geophagy and the Involuntary Ingestion of Soil[15]^: 446

A five-year-old girl in Peru with a desire to eat earth (geophagia). Wood engraving by E. Riou, 186-.

Geophagia is nearly universal around the world in tribal and traditional rural societies (although apparently it has not been documented in Japan or Korea).[15] In the ancient world, several writers noted the phenomenon of geophagia. Pliny is said to have noted the ingestion of soil on Lemnos, an island of Greece, and the use of the soils from this island was noted until the 14th century.[15][19] The textbook of Hippocrates (460–377 BCE) mentions geophagia, and the famous medical textbook titled De Medicina edited by A. Cornelius Celsus (14–37 CE) seems to link anaemia to geophagia.[19]

Early explorers in the Americas noted the existence of geophagy amongst Native Americans, including Gabriel Soares de Sousa, who in 1587 reported a tribe in Brazil using it in suicide,[15] and Alexander von Humboldt, who said that a tribe called the Otomacs ate large amounts of soil.[19] In Africa, David Livingstone wrote about slaves eating soil in Zanzibar,[19] and it is also thought that large numbers of slaves brought with them soil-eating practices when they were shipped to the New World as part of the transatlantic slave trade.[15] Slaves who practised geophagia were nicknamed "clay-eaters" because they were known to consume clay, as well as spices, ash, chalk, grass, plaster, paint, and starch.[20]

In more recent times, according to Dixie's Forgotten People: the South's Poor Whites, geophagia was common among poor whites in the Southeastern United States in the 19th and early 20th centuries, and was often ridiculed in popular literature. The literature also states, "Many men believed that eating clay increased sexual prowess, and some females claimed that eating clay helped pregnant women to have an easy delivery."[21] Geophagia among Southerners may have been caused by the high prevalence of hookworm disease, of which the desire to consume soil is a symptom.[22] Geophagia has become less prevalent as rural Americans assimilate into urban culture.[20] However, cooked, baked, and processed dirt and clay are sold in health food stores and rural flea markets in the American South.[23]

Several different rocks of clay-like material being sold at a local market in Kabwe, Zambia. These are usually purchased and consumed by pregnant women.

In Africa, kaolinite, sometimes known as kalaba (in Gabon[24] and Cameroon[25]), calaba, and calabachop (in Equatorial Guinea), is eaten for pleasure or to suppress hunger.[25] Kaolin for human consumption is sold at most markets in Cameroon and is often flavoured with spices such as black pepper and cardamom.[26] Consumption is greatest among women, especially to cure nausea during pregnancy, in spite of the possible dangerous levels of arsenic and lead to the unborn child.[27][28] Another example of geophagia was reported in Mangaung, Free State Province in South Africa, where the practice was geochemically investigated.[29] Calabash chalk is also eaten in west Africa.

In Haiti, poor people are known to eat mud biscuits made from soil, salt, and vegetable shortening. These biscuits hold minimal nutritional value, but manage to keep the poor alive.[30] However, long-term consumption of the biscuits is reported to cause stomach pains and malnutrition, and is not recommended by doctors.[31]

Bentonite clay is available worldwide as a digestive aid; kaolin is also widely used as a digestive aid and as the base for some medicines. Attapulgite, another type of clay, is an active ingredient in many anti-diarrheal medicines.[20]

Haitian mud cookie being prepared

Clay minerals have been reported to have beneficial microbiological effects, such as protecting the stomach against toxins, parasites, and pathogens.[32][33] Humans are not able to synthesize vitamin B12 (cobalamin), so geophagia may be a behavioral adaption to obtain it from bacteria in the soil.[34] Mineral content in soils may vary by region, but many contain high levels of calcium, copper, magnesium, iron, and zinc, minerals that are critical for developing fetuses which can cause metallic, soil, or chewing ice cravings in pregnant women. To the extent that these cravings, and subsequent mineral consumption (as well as in the case of cravings for ice, or other cold neck vasoconstricting food which aid in increasing brain oxygen levels by restricting neck veins) are therapeutically effective decreasing infant mortality, those genetic predispositions and the associated environmental triggers, are likely to be found in the infant as well. Likewise, multigenerationally impoverished villages or other homogenous socioeconomic closed genetic communities are more likely to have rewarded gene expression of soil or clay consumption cravings, by increasing the likelihood of survival through multiple pregnancies for both sexes.[33][35]

There are obvious health risks in the consumption of soil that is contaminated by animal or human feces; in particular, helminth eggs, such as Ascaris, which can stay viable in the soil for years, can lead to helminth infections.[36][37] Tetanus poses a further risk.[36] Lead poisoning is also associated with soil ingestion,[38] as well as health risks associated with zinc exposure can be problematic among people who eat soils on a regular basis.[29] Gestational geophagia (geophagia in pregnancy) has been associated with various homeostatic disruptions and oxidative damage in rats.[39]