Geophagy and potential contaminant exposure for terrestrial vertebrates.

Geophagy, the ingestion of earth or earthlike substances, is a behavior that occurs in a wide range of vertebrate groups. Geophagy can be intentional: to acquire necessary nutrients from soil such as sodium or calcium or to acquire a stomach balm for parasites or toxins. It can also be incidental: acquired while foraging or grooming or from prey that have ingested soil. With the spreading effects of human "development," soils contaminated by anthropogenic products will be more frequently encountered by wildlife. Direct ingestion of such soils may be detrimental to the health of animals. In some cases information on the fraction of ingesta that is soil will enable adequate inferences about exposure and techniques are described to determine that fraction. In other cases, inferences about exposure require information on the daily rate of soil ingestion. The daily rate of soil ingestion can be estimated using the fraction of ingesta that is soil and the projections for food requirements. Also described here are methods to project mean daily food requirements and the mean daily rate of soil ingestion. Contaminant exposure by geophagy is affected by filtration of soil fractions, binding of some elements into compounds not absorbable through the gut wall, and neutralizing of toxicity after absorption. Bioavailability of contaminants in soil may also be related to taxon. Geophagy as an important mechanism of toxic exposure has been clearly demonstrated in several studies.