Stones in the stomach of crocodilians: new light on a weighty problem

Crocodiles have been in the news recently and the topic has been one in which I have a proprietary interest.

Ever since the discovery of stones in the abdomen of fossil crocodilians in 1831 by Geoffroy Saint-Hilaire there has been discussion as to the function they serve. Stones—gastroliths—found in the stomach of living crocodilians have usually been considered to have a hydrostatic function, in other words as ballast to increase the specific gravity of the animal in water, or to assist in the trituration of food in the stomach, like grit in the gizzard of birds.

The other question was whether the gastroliths had any function at all, but simply there by dint of their being ingested accidentally with their food.

When I came into the picture, as a small bit player, Hugh Cott (1900-1987) then in Cambridge had published in 1961 his work on the Nile Crocodile in East Africa. He made a strong case that the stone ballast enables the crocodile to stay under water and to pull the struggling large mammals they often feed on under the surface. The position of the stones in the gut raises the head end of floating crocodiles and stabilises them against rolling, he posited.

In the early 1960s, it was unresolved how crocodilians acquired the stones. Did they seek them out and swallow them?

I have written before on the American, usually then called the Mississippi, Alligator I had in the early 1960s. My father and I converted the end of a greenhouse to accommodate it. We built a concrete pond and left a basking area. We could not leave the ‘dry’ area as natural soil since the water would have been dirty in no time. We had a fairly small amount of mixed builders’ gravel which we spread across the dry area. The pond was filled, emptied and refilled each day for about a week to wash out soluble salts, the heater was installed and the 51 cm Alligator moved in from its bare temporary tank.

A short time later, while doing something else, I turned round to see the Alligator picking up pieces of gravel (plus some adhering earth) and swallowing them. It picked up each piece with the left side of the jaws held towards the substrate, then, when it had the stone in its mouth, straightened its head and swallowed. Each piece was about 1.5 cm in diameter.

I did not want to disturb the newly-installed animal that day since I wanted him to feed undisturbed. However, several days later I picked it up and could feel the stones in the stomach.

Apparently, I was the first to observe that the acquisition of stomach stones is a deliberate action and later authors have either credited me with that discovery, forgotten all about it or have not been diligent in searching the literature.

My alligator - from its size it was about a year old

Whether the gastroliths serve a hydrostatic function as ballast has been the subject of a good few papers and reviews since the 1960s but with comparatively little experimentation. Similarly, the efficiency of digestion in crocodilians with or without gastroliths has not been tested.

Cott’s views appeared to take a beating from mathematical modelling, the increase in specific gravity caused by a typical weight of gastroliths, as found by Cott in large crocodiles, not emerging as sufficient to have any marked effect on buoyancy. The conclusion was reached that control of buoyancy is achieved by the degree of filling of the lungs with air.

Expanding the lungs to dive could, though, increase the buoyancy to such an extent that the animal could not dive. Therefore, there were suggestions and some evidence, starting with Cott, that gastroliths could enable a greater volume of air in the lungs and therefore enable crocodiles to stay submerged for longer. The recently published paper tested this hypothesis directly—in my old friend, the American Alligator.

Juvenile alligators were used. First, all had any stones washed out of their stomachs. Then the duration of each of 21 voluntary dives in excess of 1 minute they made in an aquarium was recorded over two days. Then, granite stones no bigger than 1 cm were placed in the aquarium at a weight equal to 2.5% of body-weight. Four of the seven animals ingested all the stones. The three that swallowed only some were gently persuaded to ingest the rest by placing them at the back of the mouth.

The dives of the this time ballast-loaded alligators were recorded again. The results were very clear. The average duration of dives increased by 88% (5.9 to 11 minutes) and the maximum duration increased by 117% (11.7 to 25.3 minutes).

From Uriona et al 2019

The authors suggest that because juvenile crocodilians are subject to a high death rate through predation, a behaviour ‘that enhances the ability of juvenile crocodylians to remain submerged may have high selective value. Furthermore, juveniles have cartilaginous tissues that have lower specific gravity than the bone that will replace these tissues in later life, and the rate of depletion of oxygen stores during diving is relatively higher in juveniles than in larger animals making the increase in diving lung volume enabled by lithophagy potentially more important in juvenile animals than in adults.

A question that arises and one I have now wondered about for nearly 60 years after my alligator picked up and swallowed stones is: how do crocodilians know they need to find and ingest stones? Do they sense in some way how high they float in the water or whether they need to use muscles to maintain their characteristic pose on the surface with just the tip of the nose and eyes above the water?

The function of gastroliths in crocodiles has wider ramifications. This is because they are found in those extinct reptiles, the plesiosaurs but not in ichthyosaurs. Twenty-five years ago, Michael Taylor, a plesiosaur expert then at the Leicestershire Museums, wrote an excellent survey and comparative analysis of gastroliths and their possible function in marine tetrapods. I show the entire summary of his paper below because it puts the acquisition and occurrence of gastroliths into context:

Gastroliths or ‘stomach stones’ occur frequently in some, but not all, groups of fossil and living marine tetrapods. Comparative analysis of gastrolith distribution suggests a role in buoyancy control rather than food processing. Once accidental ingestion by bottom-feeding animals is excluded, gastroliths occur in most tetrapods which ‘fly’ underwater with hydrofoil limbs, including plesiosaurs, penguins, and otariid pinnipeds, but not the marine chelonians. They do not usually occur in cetaceans, ichthyosaurs, mosasaurs, and odobenid and phocid pinnipeds, which swim with a caudal fin or the equivalent. Occurrence in amphibious forms is variable; crocodilians often have gastroliths, but nothosaurs and placodonts do not. The correlation of gastroliths and underwater flight is corroborated by a comparative analysis which takes phylogenetic factors into account. There is no correlation with diet. Consideration of function and occurrence in terrestrial forms suggests that the use of gastroliths in digestion would not be useful, and might even be harmful, to a carnivorous marine tetrapod. Gastroliths are more efficient than skeletal bone (as in pachyostosis) in terms of sinking force per unit of added mass or volume. As well as driftwood and ice, marine tetrapods should be considered as a potential source of erratic stones in freshwater and marine sediments. Gastroliths may have evolved by the accidental ingestion of stones, the retention into adulthood of stones used by juveniles to process insect or plant food, or as a compensatory replacement for dense bones habitually filling the stomach. Their presence or absence should be more carefully recorded and further studies should be carried out on their function. 

Over the years I have seen and watched in the wild 9 of the currently-recognised 24 species of crocodilian: American Alligator (A. mississippiensis), Spectacled Caiman (Caiman crocodilus), Yacare Caiman (C. yacare), Black Caiman (Melanosuchus niger), American Crocodile (Crocodylus acutus), Australian Freshwater Crocodile (C. johnstoni), Nile Crocodile (C. niloticus), Mugger (C. palustris), Saltwater (Estuarine) Crocodile (C. porosus). As I have done so, I have pondered the question of whether they have gastroliths (the Spectacled Caiman is rumoured not to have) and whether or not they were making use of their presence as I watched.

It does finally appear that Humboldt—he of the eponymous current and penguin—was wrong. When told during his expedition to South America at the turn of the 19th Century that natives believed crocodilians augmented their weight with stones, he dismissed the notion as an “absurd hypothesis”.

The new paper:

Uriona TJ, Lyon M, Farmer CG. 2019. Lithophagy prolongs voluntary dives in American alligators (Alligator mississippiensis). Integrative Organismal Biology 1  doi.org/10.1093/iob/oby008

My old note:

Peaker M 1969. Active acquisition of stomach stones in a specimen of Alligator mississippiensis Daudin. British Journal of Herpetology 4, 103‑104

Two very useful reviews:

Taylor MA. 1993. Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapods. Philosophical Transactions of the Royal Society B 341, 163-175.

Wings O. 2007. A review of gastrolith function with implications for fossil vertebrates and a revised classification. Acta Palaeontologica Polonica 52, 1-16.