Stomach stones are hard masses found inside the digestive tracts of animals and humans, but the term covers a surprisingly wide range of objects with very different origins. In the animal kingdom, many species deliberately swallow rocks and gravel, called gastroliths, to aid digestion, store minerals, or adjust their buoyancy underwater. In humans, “stomach stones” usually refers to bezoars, which are compacted masses of hair, plant fiber, or medication that form in the stomach and can cause serious medical problems. The biology behind these objects stretches from crayfish recycling calcium for their shells to alligators diving longer with a bellyful of pebbles, and understanding them means crossing between veterinary medicine, paleontology, and gastroenterology.
Why Animals Swallow Stones on Purpose
The most familiar stomach-stone users are birds. Many species, from chickens to ostriches, swallow grit and small pebbles that collect in a muscular organ called the gizzard. The gizzard contracts rhythmically, and those stones act like grinding teeth, crushing seeds and tough plant matter that a bird’s beak alone cannot break down. Research on domestic chickens has shown that chicks ingest roughly 3 grams of grit over the course of an experiment, but only about 1 gram stays in the gizzard at any given time. The rest passes through and is excreted. Stones retained in the gizzard tend to be larger and become noticeably rounder and smoother over time, worn down by constant abrasion against food and other stones.
1PubMed Central. Effects of diet and gizzard muscularity on grit use in domestic chickensCrocodilians also swallow stones, though their reasons appear to be different from those of birds. Crocodiles and alligators lack a muscular gizzard built for grinding, and the stones they swallow are sometimes too few or too small to function as an effective mill. Some researchers have proposed that these animals use gastroliths primarily for buoyancy control, essentially weighting themselves down so they can stay submerged with less effort. A study on American alligators tested this directly by comparing dive times with and without gastroliths. Without stones, the alligators averaged about six minutes per dive. With gastroliths, that jumped to about eleven minutes, an increase of roughly 88 percent. Their maximum dive durations more than doubled, going from about twelve minutes to over twenty-five.
2PubMed Central. Lithophagy Prolongs Voluntary Dives in American alligators (Alligator mississippiensis)That said, the buoyancy hypothesis is not universally accepted for all aquatic stone-swallowers. Computational modeling has suggested that the relatively small amounts of gastroliths found in most aquatic animals, typically less than two percent of body mass, are too light to meaningfully change buoyancy or stability. The lungs, which can be inflated or deflated, are a far more powerful buoyancy tool.
3Canadian Journal of Zoology. Effects of stomach stones on the buoyancy and equilibrium of a floating crocodilian: a computational analysisHow Marine Animals Complicate the Picture
The debate over digestion versus buoyancy gets more interesting when you look across the full range of marine animals that carry gastroliths. A broad comparative study found a striking pattern: gastroliths tend to show up in animals that “fly” through water using flipper-like limbs, such as penguins, plesiosaurs, and sea lions, but are largely absent in animals that swim with a tail fin, like whales, ichthyosaurs, and true seals. The researchers argued this pattern fits a buoyancy-control explanation better than a digestion one, since flipper-propelled swimmers face different hydrodynamic challenges than tail-propelled ones.
4Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences. Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapodsThe real answer is probably that gastroliths serve different purposes in different animals. A chapter reviewing the evidence across living and extinct vertebrates concluded exactly that: some crocodiles and marine mammals appear to use gastroliths for buoyancy, but all other gastrolith-bearing vertebrates seem to use them for digestion. The function depends on the species, its anatomy, and its ecological niche. Lumping every stone-swallower into one explanation oversimplifies the biology.
Crayfish and Lobsters Use Stomach Stones to Build Their Shells
Crustaceans have evolved an entirely different use for stomach stones that has nothing to do with grinding food or staying underwater. Lobsters and crayfish form paired gastroliths inside their stomachs before they molt. These are not swallowed from the environment but built internally from calcium carbonate extracted from the old exoskeleton as it is being resorbed. The gastroliths grow rapidly during the pre-molt phase, acting as temporary calcium warehouses. Once the animal sheds its old shell, the gastroliths dissolve, releasing calcium and bicarbonate ions that are shuttled to the new, still-soft exoskeleton to harden it.
5IntechOpen. Functions and Regulation Mechanisms of Calcium in the Molting Cycle of Decapod CrustaceansThis process is tightly synchronized with molting. Gastrolith formation begins the moment the pre-molt stage kicks off, and the stones reach peak size just before the old shell splits open. After ecdysis, the dissolution happens quickly, because a soft-shelled crayfish is extremely vulnerable to predators and needs to mineralize its new armor as fast as possible. The gastroliths of various decapod crustaceans have been studied as a window into how these animals manage calcium regulation and growth more broadly.
6Journal of Crustacean Biology. Molting behavior, gastrolith formation, and mandibles of Enoplometopus debelius Holthuis, 1983 (Decapoda: Astacidea: Enoplometopidae) in an evolutionary contextThe Dinosaur Stomach Stone Mystery
Gastroliths get a lot of attention in paleontology because clusters of polished stones are sometimes found near fossilized dinosaur skeletons, and the natural question is whether dinosaurs used them the way modern birds do. The answer, at least for the giant long-necked sauropods, appears to be no, or at least not in the same way. A study comparing gastrolith function in ostriches to what has been found with sauropod fossils concluded that sauropods did not possess an avian-style gastric mill. The stones found with sauropod remains are too scarce and too small relative to the animals’ body size to have served as an effective grinding system.
7PubMed Central. No gastric mill in sauropod dinosaurs: new evidence from analysis of gastrolith mass and function in ostrichesA separate survey of the Late Jurassic Morrison Formation in the western United States reinforced this conclusion. Genuine gastrolith clusters associated with sauropod skeletons are remarkably rare in the fossil record, and the taphonomic evidence simply does not support the idea of a bird-like grinding organ in these animals.
8Fossil Record. The rarity of gastroliths in sauropod dinosaurs – a case study in the Late Jurassic Morrison Formation, western USAInstead, sauropods may have compensated for their inability to chew by simply keeping food in their guts for a very long time, letting fermentation and chemical digestion do the work rather than mechanical grinding. That strategy is a real departure from the bird model, and it is a reminder that not every large herbivore solves the same problem the same way.
Marine reptiles tell a different fossil story. An elasmosaurid plesiosaur specimen from Antarctica was found with a collection of gastroliths whose rock types could be traced to geological formations within about 100 kilometers of where the animal died. The stones included volcanic rocks, quartz vein material, and sandstone, and their composition suggests the animal picked them up from coastal outcrops or seafloor exposures during its life.
9Cretaceous Research. Gastroliths associated with an Aristonectes specimen (Plesiosauria, Elasmosauridae), López de Bertodano Formation (upper Maastrichtian) Seymour Island (Is. Marambio), Antarctic PeninsulaTelling Real Gastroliths From Random Rocks
One persistent challenge in paleontology is distinguishing genuine gastroliths from stones that simply ended up near a skeleton by geological chance. Researchers have developed quantitative methods to make this distinction. A study examining suspected gastroliths in a fossil marine turtle found that the stones inside the body cavity were statistically more circular than randomly transported clasts from the same rock formation. They also fell within a tighter, more consistent size range. In both shape and size, the suspected gastroliths resembled confirmed gastroliths from other species far more closely than they resembled naturally weathered stones from the surrounding sediment.
10PLoS ONE. First evidence of marine turtle gastroliths in a fossil specimen: Paleobiological implications in comparison to modern analoguesThis kind of analysis matters because gastroliths become polished through constant contact with stomach walls and other stones, producing a shape signature that random erosion does not replicate. Without these statistical tools, claims about fossil gastroliths would remain educated guesses.
Bezoars in Humans
When people think of stomach stones in a human context, they are usually thinking of bezoars, which are masses that accumulate in the stomach from indigestible material. Unlike animal gastroliths, bezoars are not swallowed intentionally and are not beneficial. They come in several varieties depending on what they are made of.
Phytobezoars, composed of indigestible plant fibers, are the most common type. They tend to form in people who eat large quantities of fibrous foods, particularly persimmons, celery, or pumpkin, and are more likely in people with reduced stomach motility, such as those who have had gastric surgery or who have diabetes. The undigested fibers tangle together, compact, and gradually harden into a mass that can fill a significant portion of the stomach.
Trichobezoars, made of hair, are rarer and more dramatic. They occur almost exclusively in young women with underlying psychiatric conditions, particularly trichotillomania (compulsive hair pulling) and trichophagia (eating the pulled hair). Because hair is not broken down by stomach acid, it accumulates over months or years into a dense, dark mass that conforms to the shape of the stomach. In extreme cases known as Rapunzel syndrome, the hair mass grows a long tail that extends from the stomach into the small intestine.
11PubMed Central. The rapunzel syndrome: an unusual trichobezoar presentationRapunzel syndrome is extremely rare, but when it occurs, the symptoms are severe: vomiting, upper abdominal pain, weight loss, and sometimes a palpable mass in the abdomen. Patients often deny the hair-eating behavior, which can delay diagnosis.
12PubMed Central. Case Report on Rapunzel syndrome: a large gastric trichobezoar extending to the proximal jejunum in a young adult femaleA third category, pharmacobezoars, form from medications that clump together in the stomach instead of dissolving properly. Drugs reported to cause these include aluminum hydroxide gel, enteric-coated aspirin, sucralfate, nifedipine extended-release tablets, and psyllium-based fiber supplements. Contributing factors include dehydration, low gut motility, and concurrent use of drugs that slow the bowel, such as opioids or anticholinergics. In some cases, the insoluble coating of a pill is the culprit rather than the active ingredient itself.
13PubMed Central. Pharmacobezoar—a rare case presented as gastric outlet obstructionComplications and Treatment
Left untreated, bezoars can cause ulcers, bleeding, and intestinal obstruction. Elderly patients with diabetes are especially vulnerable because reduced stomach motility allows bezoars to grow larger before causing symptoms, and when pieces break off, they can block the small intestine. There are no precise incidence figures for these complications, but case reports consistently describe multiple ulcers in the gastrointestinal tract surrounding large bezoars.
14International Journal of Surgery Case Reports. Small intestinal obstruction caused by the gastric bezoars in elderly diabetic patients: A case reportTreatment depends on the type and size of the bezoar. For phytobezoars, one of the most widely used first-line approaches is surprisingly low-tech: drinking Coca-Cola. The combination of carbonic acid, phosphoric acid, and carbon dioxide bubbles helps dissolve and fragment the compacted plant material, either breaking the bezoar apart enough for it to pass on its own or softening it enough for endoscopic removal. Coca-Cola ingestion, alone or alongside endoscopy, has a high success rate for phytobezoars and is used globally as a cost-effective initial treatment.
15PubMed Central. Is Coca-Cola Useful for Treating Gastric Bezoars?Trichobezoars almost always require surgical removal because hair does not respond to chemical dissolution. Large pharmacobezoars may also need endoscopic or surgical intervention. In cases where a gallstone erodes through the gallbladder wall into the duodenum and becomes impacted, a condition called Bouveret’s syndrome, endoscopic techniques including laser lithotripsy can be used to break the stone into smaller pieces. Stones larger than about two centimeters are at risk of getting stuck, and in roughly one to three percent of gallstone ileus cases, the obstruction occurs in the duodenum.
16PubMed Central. Successful endoscopic treatment of a large impacted gallstone in the duodenum using laser lithotripsy, Bouveret’s syndrome: A case reportEnteroliths in Horses
Horses develop their own version of stomach stones, though the term used is enteroliths, mineral concretions that form in the large intestine rather than the stomach itself. These stones are built up layer by layer around a small foreign object, typically a bit of wire, sand, or a pebble, as dissolved minerals precipitate out of the gut contents. They are most common in certain parts of the United States, particularly California, and can grow large enough to block the intestine entirely, which is a surgical emergency.
Diet plays a major role. A study of risk factors in California horses found that feeding more than half the diet as alfalfa hay was significantly associated with enterolithiasis, while feeding grass hay or oat hay and providing daily pasture access were protective. Alfalfa is high in magnesium and protein, and the alkaline environment it creates in the hindgut appears to promote mineral crystallization.
17Research in Veterinary Science. Evaluation of dietary and management risk factors for enterolithiasis among horses in CaliforniaHorses with enteroliths are also far more likely to have gastric ulcers. A study comparing enterolithiasis cases to horses with other types of large-intestinal blockage found that about 68 percent of horses with enteroliths had gastric ulcers, compared to 31 percent of controls. In Thoroughbreds, the association was even stronger, with an odds ratio above 22.
18PubMed Central. Prevalence of Gastric Ulceration in Horses with Enterolithiasis Compared with Horses with Simple Large Intestinal ObstructionGeophagia and Deliberate Earth-Eating in Humans
Humans do not carry gastroliths, but the practice of deliberately eating soil, clay, or chalk, known as geophagia, is widespread across cultures and has been documented for centuries. It is especially common among pregnant women and is thought to help with nausea, though the exact reasons people engage in it remain unclear. Some researchers have proposed that geophagia supplies trace minerals like iron or zinc, while others suggest the clay acts as a binding agent that neutralizes plant toxins or soothes an irritated gut lining.
19PubMed Central. Geophagia: Benefits and potential toxicity to human-A reviewThe practice also carries risks. Depending on the source, the soil may contain heavy metals, parasites, or pesticide residues. In areas with contaminated soils, habitual geophagia has been linked to lead exposure and intestinal infections. The tension between potential benefit and potential harm makes geophagia a genuinely complicated subject, and the line between cultural practice, nutritional strategy, and medical concern is blurry. It is not the same as carrying grinding stones in a gizzard, but it sits in an interesting space between human behavior and the broader animal pattern of ingesting non-food materials for physiological advantage.