Geophagia (), also known as geophagy (), is the intentional practice of consuming earth or soil-like substances such as clay, chalk, or termite mounds. It is a behavioural adaptation that occurs in many animals and has been documented in more than 100 primate species. Geophagy in primates is primarily used for protection from parasites, to provide mineral supplements and to help metabolize toxic compounds from leaves.
Human geophagia is a form of pica – the craving and purposive consumption of non-food items – and is classified as an eating disorder in the Diagnostic and Statistical Manual of Mental Disorders (DSM) if not socially or culturally appropriate. Sometimes geophagy is a consequence of carrying a hookworm infection. Although its cause remains unknown, geophagy has many potential adaptive health benefits as well as negative consequences.
Animals
Geophagia is widespread in the animal kingdom. Galen, the Greek
philosopher and physician, was the first to record the use of clay by sick or injured animals in the second century AD. This type of geophagia has been documented in "many species of mammals, birds, reptiles, butterflies and isopods, especially among herbivores".
Birds
thumb|upright|Parrots eating earth
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.
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.
It is unclear which factor is driving avian geophagy. 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. Researchers Lee etal. 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,
Non-human primates
right|thumb|upright=1|[[Silky sifaka (Propithecus candidus) eating earth]]
There are several hypotheses about the importance of geophagia in bats and primates. 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.
thumb|Five Japanese macaques eating soil
Geophagy is a behavioural adaptation seen in 136 species of nonhuman primates from the suborder Haplorrhini (81%) and Strepsirrhini (19%). The most commonly ingested soils are soils from mounds, soils from tree bases, soils from termite mounds, 'Pong' soils, and forest floor. Studies have shown many benefits of geophagy such as protection from parasites (4.9%), mineral supplements (19.5%), and helping to metabolize toxic compounds from leaves (12.2%) nonexclusive. Geophagic behaviour plays an important role in nonhuman primates' health. In other cases, geophagy is used by the Ring-Tailed Lemurs as a preventive and therapeutic behaviour for parasite control and intestinal infection. The study was done at the salt licks in Veun Sai-Siem Pang Conservation Area, a site that is visited by various species of nonhuman primates. 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.
Humans
Anthropological and historical evidence
Evidence for the likely origin of geophagy was found in the remains of early humans in Africa: