Alligators apparently swallow stones for ballast

Alligators and crocodiles are known to swallow stones, which are called “gastroliths” when they settle in the creature’s stomach. Some birds do this, too, but to help grind up their food. (Other marine creatures like seals also swallow stones.) Why do crocs and alligators do it? In the paper below, published in Integrative Organismal Biology (click on screenshot), the authors proffer several hypotheses and then test them. It turns out that swallowing rocks reduces the buoyancy of the tested species (American alligators), and that could have several advantages.

The authors’ hypotheses are all based on the fact that if you have rocks in your belly, you’re going to be less buoyant (gators without air in their lungs have a specific gravity of 1.04, close to that of water [humans have 1.07]). Why would a gator want to be heavier? Here are several of Uriona et al.’s hypotheses:

  • Adult gators, by being heavier, could more easily drag prey underwater to kill them.
  • A heavier body could help the creature maneuver better underwater, or stay still in a current when waiting for food or basking.
  • Smaller gators, who are subject to predation, might be less visible if they were heavier and stayed underwater.
  • By allowing a gator to put more air in its lungs without surfacing, a heavier body could allow them to stay underwater longer (this also feeds into the third hypothesis).

The authors note that none of these hypotheses have been properly tested. In fact, one site claims, citing supporting research, that this hypothesis cannot be right because density couldn’t increase that much with stones in the belly:

For years, it was hypothesized that eating stones might also help a crocodile stay under the water longer and to dive deeper. Many crocodilians like to float in the water with just their eyes and nostrils showing so they can ambush their prey. It was thought that a stomach full of rocks might help the crocodile keep their bodies under the water and out of sight.

New research shows that this hypothesis might be wrong! Recent research by a paleontologist named Don Henderson has shown that for the rocks to help stabilize the buoyancy of the crocodile’s body, the rocks would have to account for at least 6% of the body mass of the crocodile.  They have now measured it, and the rocks only account for about 2% of body mass. Below 6%, the filling and emptying of the lungs has a much greater effect on the buoyancy of the crocodile than the stones. However, a low number of rocks might keep the crocodile from rolling from side to side. Also, with fewer rocks, they might not have helped grind up their food either. It is surprising what we are finding out today!

This shows us that even a hypothesis that seems obvious needs to be tested! It seems that the scientific method we all learned in elementary school would have some merit here! #crocodiles #crocodileseatingstones

Now I haven’t read Henderson’s paper, but his conclusion isn’t based on direct observation of crocodiles diving underwater. It would be salubrious to actually see if gators or crocs (both swallow stones) do have reduced buoyancy with gastroliths inside, enabling them to stay underwater longer. And so the authors of the present paper did the empirical test.

Uriona et al. used seven juvenile American alligators from a refuge in Louisiana, transported to a lab in Salt Lake City.  They then proceeded to remove any stones already in the gators’ stomachs by “gastric lavage” (poor animals!) and then X-rayed them to ensure that no rocks remained.  Then then tested the rockless juveniles by observing them in aquaria, watching them dive for 30 minutes, and then collected data until seven timed dives were recorded for each alligator.

After a night’s recovery, the same gators were placed in individual aquaria containing granite stones totaling about 2.5% of the gators’ body mass. Four of them voluntarily gobbled all the rocks, and, well, the other three were forced to swallow them (poor gators!). Then they repeated the same diving protocol, observing each gator for seven dives.

Here are the data for time spent diving, including the average duration of a dive and the average maximum duration of dives among the seven gators:

(From paper). Box and whisker plot of mean duration of dives, and of the mean of the maximal dives, with and without gastroliths. Boxes, interquartile range; whiskers, the lower and upper quartile; + mean; − median line, ♦ shortest and longest mean dives or shortest and longest maximal dive.

As you can see, swallowing stones increases the length of time you can stay underwater, both in terms of average dive length (660 seconds with stones, 351 seconds without), and maximum dive duration (1518 seconds with stones, 699 seconds without). The conclusion: stones weigh down the gators, allowing them to stay underwater longer.

What does this all mean? What it does is confirm the author’s notion that if you have stones in your belly, you stay underwater longer, not only because you’re weighted down, but mainly because this allows you to put more oxygen in your lungs and thus stay under longer before coming up for air. This contradicts the conclusion of Henderson cited above.

Is reduced buoyancy useful or “adaptive” for a gator? Probably, and it’s probably why they do swallow stones. But we’re not sure. They could be swallowing the stones simply because they mistake them for food. This could be tested by giving food-sated gators both food and stones. And there are of course no tests about whether gators with ballast survived or thrived more than gators without gastroliths, a necessity if you think the behavior is adaptive. I do suspect that gators swallow stones as an adaptive behavior, but this is only a hypothesis.

You might be asking yourself, as one of my friends did when we discussed this, why alligators don’t simply increase their body density via evolutionary change rather than swallow stones? (I assume, as I note below, that swallowing stones it itself an evolved behavior.) If acquiring gastroliths is adaptive, there are several possible answers.

The most likely is that it’s “easier” in an evolutionary sense to evolve stone-swallowing than to increase body density. That is, there may be genetic variation in both traits, but if you have a greater tendency to swallow stones than your comrades, you instantly get a huge advantage over them, for swallowing stones instantly and strongly decreases your buoyancy.

In contrast, a slight decrease in buoyancy via body modification might give you only a slight selective advantage, and there may be maladaptive byproducts of changing your morphology or physiology this way. Further, once the genes for stone-swallowing have spread throughout a population, there’s simply no additional selective advantage to be had by decreasing your buoyancy by jiggering with your body density or physiology. (I am assuming here that swallowing stones is an evolved behavioral trait rather than a learned trait, which could itself be tested by giving stones to food-sated, naive juvenile alligators in the lab.)

Here’s last question, which I’m sure has occurred to many readers: What happens to the swallowed stones?  Good question! If they’re too big to poop out, I guess they stay in the gators’ bellies for life. That’s okay, but is there then a cue for a gator to stop swallowing stones?

Stone goes down the hatch of a Nile crocodile (photo from here):

____________________

Uriona, T. J., M. Lyon, and C. G. Farmer. 2019. Lithophagy Prolongs Voluntary Dives in American alligators (Alligator mississippiensis). Integrative Organismal Biology 1, 11 January 2019, oby008, https://doi.org/10.1093/iob/oby008

71 Comments

  1. ThyroidPlanet
    Posted February 4, 2019 at 8:56 am | Permalink

    “put more oxygen in your lungs”

    That makes the most sense.

    Also I suppose they don’t need to consume energy and oxygen and produce CO2 by working against the buoyant force.

    I bet they discover this by accident…

    So they can stay under for generally what, 10% more?… and what are they doing down there? Obviously I need to read the paper….

    • ThyroidPlanet
      Posted February 4, 2019 at 8:58 am | Permalink

      Or perhaps the swimming dynamics are better with ballast, better for catching prey, so there might be some who stay under the same length of time?… still didn’t read yet…

  2. Mark Sturtevant
    Posted February 4, 2019 at 9:23 am | Permalink

    So it looks like both alligators and crocodiles swallow stones.
    You can always distinguish an alligator from a crocodile by observing whether they will see you later or after a while.

    • Cate Plys
      Posted February 4, 2019 at 9:47 am | Permalink

      Fabulous use of the scientific method, Mark.

      • JezGrove
        Posted February 4, 2019 at 11:03 am | Permalink

        I just hope t authors credited Elton John for his seminal work in this area.

  3. Michael Fisher
    Posted February 4, 2019 at 9:24 am | Permalink

    Do the Alligators expel all the air from their lungs before diving? I would guess/hypothesize they rely on oxygenated blood entirely during dives.

    I wonder how close in design is the cardiovascular system of crocs & alligators? Crocodiles have an extraordinary blood supply system – I had to read the link twice to grok what’s going on.

    • chascpeterson
      Posted February 4, 2019 at 11:44 am | Permalink

      “The Almighty, who creates everything with his wisdom, lowers the blood oxygen ratio and the metabolic speed of crocodiles by creating a valve that combines the two aortas.” Well that was gratuitous!

      Crocodilians (and turtles) generally dive with air in the lungs. They can float at the surface, sink to the bottom, or swim at neutral buoyancy by controlling the volume of air in the lungs. Most turtles have exceptional bone mass that they counter buoyancy-wise with controllable lung air. Crocs, despite osteoderms, seem to use swallowed stones for that purpose.

      However, the gastroliths also serve a digestive function. Sequestered in the gizzard (crocs have this muscular forestomach giblet as do their archosaur relatives, turkeys), from which they are probably relatively easily vomitable, they are used to grind up food swallowed in large chunks.

      • Posted February 5, 2019 at 11:39 pm | Permalink

        Wikipedia: “The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including archosaurs (pterosaurs, crocodiles, alligators, and dinosaurs, including birds), earthworms, some gastropods, some fish, and some crustaceans. This specialized stomach constructed of thick muscular walls is used for grinding up food, often aided by particles of stone or grit. In certain insects and molluscs, the gizzard features chitinous plates or teeth.”

        Any chickens I have “processed” have grinding material remaining in their gizzards at the time of departure. I don’t know enough about crocs and alligators to know what happens to their stones.

        The diversity of methods for converting foods in different lifeforms is fascinating. Think of cows with three stomachs, for example.

        • ThyroidPlanet
          Posted February 6, 2019 at 5:50 am | Permalink

          I love the word “gigerium”

  4. Cate Plys
    Posted February 4, 2019 at 9:28 am | Permalink

    Re what happens to the stones: If any are too big to get pooped out, I would think they would get stuck in—and block—the intestines. That’s based on a cat who I got at the Humane Society and who has since cost me more than an award-winning purebred for operations to remove household objects blocking his intestines, and even a ginormous hairball that almost killed him. So eating stones above a certain size would presumably be a maladaptive behavior for alligators and crocodiles, which they would evolve to avoid? On the other hand eating anything on the floor in my home is a habit that has not been bred out of my cat. Frankly I almost never can think of anything to comment on for a scientific post, so I had to bring this up.

    • Keith
      Posted February 4, 2019 at 10:17 am | Permalink

      This is what came to my mind, too. Intestinal blockage seems likely unless the animals have a way to safely sequester them until the stones grind down enough to pass. I think this is essentially what happens to grit in a bird’s crop. If stones eventually wear down, the gators would need to replenish them from time to time.

      • chascpeterson
        Posted February 4, 2019 at 11:50 am | Permalink

        Not the crop (storage only), but rather the muscular gizzard. Think giblets. Crocs, birds, dinosaurs: all have them. And gastroiths are sequestered there.

        • chascpeterson
          Posted February 4, 2019 at 11:52 am | Permalink

          …and, I’d hypothesize, relatively easily vomited from there.

        • Posted February 4, 2019 at 12:33 pm | Permalink

          Yes, I thought that gastroliths of crocodilians are to assist digestion, like in birds and other dinosaurs.

          • Dave
            Posted February 4, 2019 at 2:47 pm | Permalink

            I’m not convinced by the idea that crocs swallow stones to aid digestion. As far as I know, birds that process food in grit-filled crops generally eat seeds or other tough plant matter, i.e. tough, fibrous stuff that needs to be abraded or ground up to release the nutritious content. Crocs, on the other hand are purely carnivorous. Meat is easy to digest, it doesn’t need to be ground up. Even bones will disappear without help if the stomach is sufficiently acidic. Carnivorous mammals don’t chew their food very much, and none of them seem to need gastroliths. I think the ballast idea is a much more likely explanation for stone-swallowing crocs.

  5. Randall Schenck
    Posted February 4, 2019 at 9:35 am | Permalink

    Isn’t the word we are avoiding in this study/finding ballast. It is used in ships for stability and improved control. A submarine uses the water itself to dive and to surface. The animals are nautical engineers.

    • ThyroidPlanet
      Posted February 4, 2019 at 10:27 am | Permalink

      I think that’s what I suggested above : improved swimming dynamics.

      • Randall Schenck
        Posted February 4, 2019 at 10:31 am | Permalink

        Yes you did. I did not read your second comment. I saw the word in the title to the post but did not see it again in the study.

  6. Merilee
    Posted February 4, 2019 at 10:13 am | Permalink

    Sub (as it were)

    • ThyroidPlanet
      Posted February 4, 2019 at 10:28 am | Permalink

      Excellent! I missed a rare chance to joke about sub!

      • Blue
        Posted February 4, 2019 at 10:56 am | Permalink

        +1.
        me as well.

        heh heh.
        Blue

  7. Posted February 4, 2019 at 10:15 am | Permalink

    The original thinking that the animal needs to add 6% of its weight for negative buoyancy seemed potentially flawed from the beginning. That is if this is the figure to hold down the entire body. But they often sit with their rear half only under water, while the tops of their heads are at the surface. So the ballast could be to just hold their rear down.

    • James
      Posted February 4, 2019 at 10:45 am | Permalink

      That was my thinking. The issue may not be staying underwater, but rather stability while underwater. Where the weight is positioned can have powerful effects on how well a thing can move through the water, as any sailor knows. Being able to push the right part of you in the right direction without expending metabolic energy would certainly have some advantage.

    • ladyatheist
      Posted February 4, 2019 at 11:38 am | Permalink

      Actually, the center, since the tail would not be weighted.

      This occurred to me too — alligators & crocs are horizontal in the water, so calculating their change in weight would be different from calculating the change in weight of a human, who would presumably dive vertically.

  8. Posted February 4, 2019 at 10:19 am | Permalink

    Just DO NOT SWALLOW PUMICE!

  9. Danny Kodicek
    Posted February 4, 2019 at 10:38 am | Permalink

    This is fascinating, and comes mere hours after I learned another new thing about alligators, which is that they can survive freezing conditions by sticking out their snouts and allowing the ice to freeze around them, then dropping into hibernation.

  10. Posted February 4, 2019 at 10:42 am | Permalink

    That photo, good grief! I was envisioning smooth stones at least, not jagged rocks!

  11. James
    Posted February 4, 2019 at 10:50 am | Permalink

    I wonder if Spinosaurus did this as well. Or the ancestors to whales, for that matter. If it really does affect buoyancy to the extent that it becomes advantageous, you’d expect other semi-aquatic critters to do it as well.

    For that matter, I wonder if gastroliths have been found with ancient crocodilians. Not all were semi-aquatic, and it’d be interesting to see how this trait spreads across the cladogram.

    • Nicolaas Stempels
      Posted February 4, 2019 at 11:41 am | Permalink

      We should not forget that terrestrial herbivorous sauropods (which lacked the awesome chewing equipment of hadrosaurs and ceratopsians) also had gastroliths.
      So we must surmise there are (at least) 2 different reasons for swallowing stones: helping digestion in some terrestrial herbivores and ballast in aquatic animals.
      It looks like the study shows at least one reason why ballast in aquatics is advantageous. We are beyond speculation now, very interesting!

      • Posted February 4, 2019 at 12:26 pm | Permalink

        There is this area, I think in Utah where there are incredibly rich dinosaur bone beds. A large museum has been put up to display a dinosaur graveyard, but the ranch lands all around it also have ton of dino bones. Some let in tourists to pick up fossils. I did that as a kid, and remember seeing hundreds of smooth, polished rocks everywhere. Someone told me they are gastroliths. I have a few, along with some dino bone bits.

        • Grania Devine
          Posted February 4, 2019 at 1:04 pm | Permalink

          I have a couple of dinosaur gastroliths, too, also from Wyoming. Irregular, but polished – very amazing – false teeth!

        • James
          Posted February 4, 2019 at 1:46 pm | Permalink

          Probably the Morrison Formation. I spent a happy few hours lost in that formation in college. Not “lost in thought” or “lost in wonder”–actually lost. Tried to take a short cut through a wash, not realizing it had a number of twists and turns and intersections with other washes. Not my brightest bit of orientering.

          Sadly, I never saw any dinosaur bones in it. Apparently we were too far south.

      • James
        Posted February 4, 2019 at 1:51 pm | Permalink

        Agreed.

        I wonder if degree of rounding and/or polishing (two different concepts in geology) would help here. I would imagine–naively, to be fair–that gastroliths would be worn more smooth and round by the digestion of food. Rocks swallowed for other reasons wouldn’t grind as much food, and therefore would be rougher and more angular.

        Of course, to test this we’d have to know or make assumptions about degree of roundness and roughness prior to ingestion.

        • Raskos
          Posted February 4, 2019 at 5:24 pm | Permalink

          I believe that gastroliths are identified by this rounding by palaeontologists, when they aren’t found within the position that would correspond to that of the stomach in fossil archosaur skeletons.

  12. Posted February 4, 2019 at 10:58 am | Permalink

    I would guess that it is very difficult for evolution to change body density significantly. Virtually every change that might affect density would likely be maladaptive in other ways. Swallowing stones is, therefore, a much more efficient process for increasing density. The short answer, they got tired of waiting for evolution to do it. 😉

    • chascpeterson
      Posted February 4, 2019 at 11:48 am | Permalink

      Yes, because the only dense tissue that vertebrates can make is bone. Crocs have osteoderms, but evidently they don’t add up to something like a turtle shell in terms of negative buoyancy.

      • infiniteimprobabilit
        Posted February 5, 2019 at 6:43 am | Permalink

        The specific gravity of bone is 1.85 (according to the first Google ref I found). (Water is 1.0)

        The SG of most rocks is around 2.7.

        So stones are far more effective weights, volume-wise, than bone.

        cr

        • ThyroidPlanet
          Posted February 5, 2019 at 6:51 am | Permalink

          Good work

          Also for the record:

          Calcium is a metal. This, bone is, largely, metal.

          Corollary: teeth are not bones – they are mostly hydroxylapatite

        • ThyroidPlanet
          Posted February 5, 2019 at 6:54 am | Permalink

          “specific gravity ranging from 2.65 to 2.67. Inorganic clay’s generally range from 2.70 to 2.80”

          ^^^^ Hasty Google search result

          • ThyroidPlanet
            Posted February 5, 2019 at 6:54 am | Permalink

            Sand!

          • ThyroidPlanet
            Posted February 5, 2019 at 6:56 am | Permalink

            “Sand particles composed of quartz have a ” was missing

            I told you it was hasty

  13. Posted February 4, 2019 at 11:19 am | Permalink

    What’s the evidence against them using the rocks for grinding food in their stomachs? If birds do this why assume that crocs have a different reason? Aren’t birds ‘dinosaurs’ anyway?

    • Posted February 4, 2019 at 11:24 am | Permalink

      Apparently some dinosaurs did have gastroliths – and not just aquatic ones:
      http://www.ucmp.berkeley.edu/taxa/verts/archosaurs/gastroliths.php

    • chascpeterson
      Posted February 4, 2019 at 11:45 am | Permalink

      They are used that way. Gastroliths live in the muscular gizzard, a food-grinding organ for those without teeth.

    • Posted February 4, 2019 at 12:37 pm | Permalink

      I thought the same, and still do.

    • James
      Posted February 4, 2019 at 3:22 pm | Permalink

      Birds are dinosaurs (specifically therapods). Crocs are not. Crocodillians are quite distinct from dinosaurs and birds, splitting off well before the Mesozoic. In fact, they are one of my favorite under-appreciate groups of animals. Sure, the Mesozoic had its share of things we’d recognize as crocodiles, but they had things we’d never imagine. Gracile crocs that could run. Bipedal crocks that make the daedroth from The Elder Scrolls games look downright tame. 8-ton fresh-water monsters that could eat our modern salties for breakfast.

      These were animals that gave dinosaurs a serious run for their money while dinosaurs were in their prime!

      To get back on topic: That’s why we can’t simply assume they use gastroliths the same way as birds. We can assume other therapods likely used them for the same reason because birds ARE therapods; it’s like assuming a fossil monkey used its hands for grasping. As organisms get further apart, it becomes far less likely that they evolved the same trait for the same reason (unless there’s compelling physical reasons to do so). We can assume a fossil monkey used its hands for grasping, but we cannot assume a T. rex did.

      Obviously an argument can in fact be made that if it looks like a food-processing device, walks like one, and quacks like one, it is one. But that line of evidence is weaker the more the two taxa diverge from one another. So we have to look for corroboration. And we must always remember that sometimes, biology does weird things and sometimes, animals are just plain stupid. Sometimes there’s NOT a good explanation for the phenomenon other than “The fish thinks the red dot is sexy”.

      • Merilee
        Posted February 4, 2019 at 3:49 pm | Permalink

        Thanks for that info, James.

        • Mark R.
          Posted February 4, 2019 at 5:49 pm | Permalink

          I’ll second that.

  14. Steve
    Posted February 4, 2019 at 11:20 am | Permalink

    Is this another example of an animal which is “perfectly designed” for its habitat? Or one that has adapted over time?

    • Michael Fisher
      Posted February 4, 2019 at 11:27 am | Permalink

      Another example? That means you have other examples in mind already that are “perfectly designed” for their habitat. What are they? I can think of none.

      • Nicolaas Stempels
        Posted February 4, 2019 at 11:43 am | Permalink

        I think Steve was being sarcastic.

        • Michael Fisher
          Posted February 4, 2019 at 11:54 am | Permalink

          Well spotted – I guess I’m thicker today than most days.

          • Steve
            Posted February 4, 2019 at 12:25 pm | Permalink

            Nicolaas gets a prize! 🙂

          • Merilee
            Posted February 4, 2019 at 1:06 pm | Permalink

            But thicker in the middle or the tail or snout? Hmm, sounds like a good theory (which is mine…)

            • Michael Fisher
              Posted February 4, 2019 at 1:20 pm | Permalink

              I think you’re in fine form today! Sub etc.

              • Merilee
                Posted February 4, 2019 at 3:50 pm | Permalink

                Half-dozey…

  15. ladyatheist
    Posted February 4, 2019 at 11:52 am | Permalink

    Hmmm human divers use lead weights to fight against buoyancy, and it seems to be a trial-and-error thing for them (based on my intensive research of 10 minutes of googling).

    Alligator & crocodile tails are so strong, would there be a downside (literally?) to eating too many rocks?

    • Posted February 4, 2019 at 11:57 am | Permalink

      Two things:

      1. They’d have to eat a lot of rocks before buoyancy becomes negative. Probably die of stomach problems first.

      2. The desire to ingest rocks is undoubtedly tied to their sense of buoyancy.

  16. Charlie Jones
    Posted February 4, 2019 at 11:55 am | Permalink

    I wonder what the density of an average manatees is? They have solid rib bones and dense skin that helps them stay submerged for a long time while they graze.

    Interestingly, the solid (marrow-free) manatee bones are quite brittle, which makes manatees more easily injured by speeding boats.

    So, if the alligators had gone for solid bones, they may have ended up with more brittle bones, which I assume would not be useful in an aggressive predator. This would leave dense skin as something to help them submerge. Anyone know the density of alligator skin, or how much denser it could potentially become?

    • rickflick
      Posted February 4, 2019 at 11:07 pm | Permalink

      As a scuba diver myself, I know how hard it would be to dive without lead weights. Humans are very buoyant. I’d have to guess that Alligators and Crocks evolved to be dense enough to dive pretty well without rocks, but that the rock swallowing simply goes beyond what tissue density could manage.

  17. Posted February 4, 2019 at 12:13 pm | Permalink

    I hope the animals were not too distressed by this experiment. I hate it when animals are used to satisfy idle curiosity, interesting though the results may be.

    • Posted February 4, 2019 at 12:36 pm | Permalink

      Crocodilians are not known to be thin-skinned. But I also pitied the objects of another experiment, in which the left aorta was surgically obstructed to see what happens when systemic and pulmonary circulation are completely separated in a crocodilian. The hearts enlarged, indicating overload.

  18. Merilee
    Posted February 4, 2019 at 1:11 pm | Permalink

    I made a special side trip to that dinosaur museum/park in Utah maybe 12 years ago and arrived to find it CLOSED for preventative earthquake maintenance. Very disappointed. Did do some nice hikes in the area, though,

  19. Posted February 4, 2019 at 4:04 pm | Permalink

    Amazing work in cognitive ethology. (I think that’s the right field, no?)

  20. Mark R.
    Posted February 4, 2019 at 5:46 pm | Permalink

    Dinosaurs also swallowed stones. I wonder if marine dinos did it for the ballast effects as well.

  21. Torbjörn Larsson
    Posted February 6, 2019 at 5:41 pm | Permalink

    As you can see, swallowing stones increases the length of time

    Not without the context of the text, since the plot is poorly labeled.


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