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Gastrolith (zoology noun)[GAS-truh-lith]: A small stone that is swallowed by an animal and held in its gastrointestinal tract, with the purpose of helping with digestion in the gizzard.
This fun word comes from the Latin words gastro, meaning stomach, and lith, meaning stone.
A surprising number of animals swallow rocks without the goal of digesting them. This is the case for many reptiles, fish, and even a few mammals. Of all the main groups of animals, gastroliths are most commonly found in birds, especially birds who spend a bit more time on the ground. The question now is, why?
Birds that spend more time closer to the ground, like ducks, gallinaceous birds (chickens, turkeys, grouse, and quail), emus, and doves, will more often than not eat hard foods like nuts and seeds and it can be rather challenging to break down that food without having any suitable grinding teeth. Therefore, in order to properly digest that food, these birds introduce rocks and other gritty materials into their digestive system. The rocks will work together with the thick and strong muscles of their gizzards, a second part to a bird’s stomach, to grind up and pulverize the food. You could even say that the gastroliths work like the pestle in a mortar and pestle!
FUN FACT: Many extinct animals, like herbivorous dinosaurs and some marine reptiles, have been found with gastroliths in their fossilized remains.