Why do some animals eat pebbles and stones?

Why do some animals eat pebbles and stones?

Birds do not have teeth to chew food, so some birds eat pebbles – but not for food and nutrition – they eat pebbles to help them digest food. The pebbles are stored in their gizzard, which is located under the stomach. All birds have a gizzard, but not all birds eat pebbles – for example, fruit-eating birds do not eat pebbles because fruit is soft and easily digested. 

The gizzard uses the pebbles like a flour mill, contracting the stomach muscles to grind larger pieces of food into smaller pieces that can be better digested. The stomach stones are called gastroliths. 

The stones will eventually be eliminated from the body through regurgitation (vomit) or excretion (poop).

Researchers at the University of California Museum of Paleontology (UCMP) in Berkeley, America, say that ground-living birds, such as chickens and ostriches, eat pebbles to help them digest food. Active swimmers, such as penguins, crocodiles, seals, and toothed whales have also been found with gastroliths in their bodies.

When the researchers studied crocodile gastroliths, they found that the pebbles make up only 2% of their body mass – so they do not swallow a lot of pebbles. They think that some pebbles are not used to aid digestion – the pebbles are just accidentally swallowed.  

Overall, the identification and function of stomach stones – gastroliths – in many animals is still uncertain. The UCMP, and paleontologists around the world, are still studying gastroliths.

Photographer: Martina Nicolls


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