Gastrolith
A gastrolith, also called a stomach stone or gizzard stones, is a rock held inside a gastrointestinal tract. Gastroliths in some species are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. In other species the rocks are ingested and pass through the digestive system and are frequently replaced. The grain size depends upon the size of the animal and the gastrolith's role in digestion. Other species use gastroliths as ballast. Particles ranging in size from sand to cobbles have been documented.
Etymology
Gastrolith comes from the Greek gastro meaning stomach and lithos meaning stone.
Occurrence
Among living vertebrates, gastroliths are common among crocodiles, alligators, herbivorous birds, seals and sea lions. Domestic fowl require access to grit. Stones swallowed by ostriches can exceed a length of 10 centimetres (3.9 in). Amphibians such as the Axolotl are also known to deliberately ingest rocks that are presumed to be gastroliths. Apparent microgastroliths have also been found in frog tadpoles. Ingestion of silt and gravel by tadpoles of various anuran have been observed to improve buoyancy control.
