Why do crocs eat rocks?

In the years following the Scramble for Africa, European hunters killed millions of Nile crocodiles to strip their skins for shoes and handbags… and also to search their stomachs for diamonds.

Colonial prospectors thought they had a shot at finding gemstones in croc bellies because crocodilians (the group of animals including crocodiles and alligators) engage in a behavior called lithophagy. In other words, they eat rocks. (Lots of rocks.) Biologists have spent centuries debating why in the world crocs do this – but now, we might be closer to an answer.

A summary of Farmer et al.’s experiment. Credit: Manafzadeh

A New Study

In a study published today in Integrative Organismal Biology, Dr. C.G. Farmer at Trinity College Dublin and colleagues at the University of Utah tested the hypothesis that lithophagy allows American alligators to dive for longer periods of time. The researchers (1) timed the dives of seven young gators with empty stomachs, (2) fed the gators a set number of rocks, and then (3) timed their dives again.

After gators ate gastroliths – stomach stones – their maximum dive times increased by 117%, and their average dive times increased by 88%. Farmer et al. explain that when a gator eats even a few gastroliths, it can hold 20-40% more oxygen in its lungs and still be dense enough to sink in water. This increased lung volume means that the gator can hold its breath for longer, allowing longer dives.

Farmer et al.’s experiment elegantly demonstrates that lithophagy allows crocs to take longer dives. But is that really why they eat rocks?

Another Possibility?

Back in the 1960s, Dr. A.W. “Fuzz” Crompton and his colleagues at the Yale Peabody Museum recorded an X-ray video of a caiman (another crocodilian) eating a dead mouse. Crompton reported that for a while, the caiman’s gastroliths were sitting neatly in its stomach. Then, after 36 hours, “all hell broke loose” and the rocks started tossing around, tearing the mouse to pieces — suggesting that gastroliths might be a key part of how crocs digest their food.

croc gastrolith x-ray.jpg
An x-ray showing gastroliths (bright white) inside a croc’s stomach. Credit: Weigmann

About fifteen years ago, Farmer and her colleagues formally tested lithophagy’s role in digestion by following an experimental design a lot like the one in today’s new study. The researchers crafted some handy gator-diapers to (1) measure how well seven young gators without gastroliths processed their food, and then (2) fed the gators a set number of rocks and (3) analyzed the diaper contents again. In the end, they didn’t find a significant difference in the gators’ ability to process crickets, mice, and fish with or without gastroliths.

Future experiments may turn out differently, but for now, it seems like eating rocks has a larger effect on diving ability than on digestion in crocs.

Moving Forward

Farmer et al. set a strong example of how experimental studies can be used to test hypotheses about gastrolith function. As research on lithophagy continues, we should aim to integrate this kind of rigorous lab work with observational data about wild crocs.

For example, we know that baby crocs spend lots of time in shallow water – and eat lots of crabs and shrimps – but adult crocs venture out into deeper water – and eat mostly fish. These developmental shifts in habitat and diet suggest that the importance of gastroliths for dive time and digestion changes over a croc’s lifespan. Do crocs start eating rocks at the same age that they head out into deeper water? Can gastroliths increase dive times equally in crocs of all ages? Might experiments with older crocs reveal that lithophagy is critical to digestion in adults?

And finally, if we truly want to understand why crocs eat rocks, we’ll need to piece together the evolution of this behavior. Plenty of extinct reptiles have been found with fossil gastroliths, including sauropods (giant, long-necked dinosaurs) and plesiosaurs (flippered marine reptiles), and we’re pretty confident that lithophagy has evolved several times.

Using data from the fossil record, we’ll have to think about when lithophagy evolved in croc ancestors, and about how those animals lived their lives. Do we ever find gastroliths associated with purely terrestrial croc ancestors, supporting early importance in digestion? Or do they only ever show up with the fossils of aquatic ones, favoring a diving-related origin?

It’s clear that we still have some work to do before we can be confident about why crocs eat rocks, but one thing’s for sure – today’s new study is a step in the right direction.

By Armita R. Manafzadeh

Armita R. Manafzadeh is a PhD candidate studying the evolution and development of joint mobility at Brown University. Her interests include functional morphology, vertebrate paleontology, and biomechanics.

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