The deliberate ingestion of soil, or 'geophagy', has important health benefits for chimpanzees, according to Sabrina Krief and her colleagues from the Muséum National d'Histoire Naturelle in Paris, France. Far from being a dysfunctional behavior, geophagy has evolved as a practice for maintaining health amongst chimpanzees. In this particular study (1), to be published online this week in Springer's journal Naturwissenschaften, geophagy increases the potency of ingested plants with anti-malarial properties.
Although geophagy is widespread throughout the animal kingdom, in humans it is perceived as a curious behavior, even linked by some to mental health issues. The paper looks at the consequences of soil ingestion on the health status of chimpanzees in the Kibale National Park in Uganda. These chimpanzees have been observed to ingest soil shortly before or after consuming plant parts, such as the leaves of Trichilia rubescens, which have anti-malarial properties in the laboratory.
The research team collected fourteen samples of soil eaten by chimpanzees as well as T. rubescens leaves from young trees in the same area. They designed a digestion model to replicate the digestive process of mastication, gastric and intestinal digestion in the laboratory. The samples were then analyzed for bioactive properties. The soil and leaves were examined both individually and as a mixture.
Before being mixed with the soil, the digested leaves showed no significant anti-malarial activity. However, when the leaves and soil were digested together, the mixture had clear anti-malarial properties.
The researchers also compared the composition of the soil eaten by chimpanzees with the content of soil used by the local healer to treat diarrhea amongst his patients. All the soil samples were rich in the clay mineral kaolinite, the principal component of some anti-diarrheal medicines. Furthermore, samples used by chimps and humans had exactly the same external aspects, were collected in a similar place and show a comparable physical and chemical profile, indicating similar content.
On the basis of these observations, Krief commented, "This overlapping use by humans and apes is interesting from both evolutionary and conservation perspectives - saving apes and their forests is also important for human health."
Krief and her colleagues discount mineral supplementation, stress-induced behavior and the search for anti-diarrheal effects of clay as the reasons behind the chimpanzees' geophagy observed at the field site during this study period. They propose geophagy's ability to enhance the pharmacological properties of plants as a novel argument to explain motivation for chimpanzees to ingest soil. They conclude that geophagy is a practice for maintaining health which may explain why it has persisted through evolution.
1. Krief S, Klein N & Fröhlich F (2008). Geophagy: soil consumption enhances the bioactivities of plants eaten by chimpanzees. Naturwissenschaften (DOI 10.1007/s00114-007-0333-0)
The full-text article is available to journalists as a pdf.