Gastroliths – How I Learned to Stop Worrying and Love Gravel.

Axolotl_with_Grit_XRay

If you read anything about keeping the axolotl as a pet you are bound to encounter the stern admonition that you must never, ever, keep axolotls on gravel. Axolotls will accidentally swallow the gravel, get a gut impaction and die. Axolotls kept on sand, the “ideal” medium also can’t be trusted to not try to swallow anything else in the tank from plastic plants to pieces of your water filter. Keep that tank bare or they will die. Or will they? In all aspects of human life there is received wisdom. Some of it is very important. Look both ways before you cross the street. Cars can’t stop as fast as you can step out so if you don’t you might die. This was received wisdom of my childhood. Don’t open an umbrella in the house because it brings bad luck. That was also received wisdom. One is true, the other is just superstitious nonsense. The key question is how to tell the two apart and that is what being a scientist is all about.

Naturally, when I (Natalie) got my first axolotl I was determined to do everything right and so Cris Martin (the other person who helped set up that first axolotl colony at University of Manitoba) gathered oversized rocks to layer over the under gravel filtration. The axolotls had other ideas. We had rocks even as big as the axolotls but no matter how diligently we rearranged big rocks, the animals would dig and squirm and fuss until they got at the gravel. It drove us bonkers. They endlessly dug and fussed and dug until they could get at the gravel. After two years of this and no fatalities, we kind of gave up. The bigger rocks sunk to the bottom and the turkey grit ended up on the top and the axolotls got as much access as they wanted.

I felt guilty about the risks we were taking, so I contacted every single person I could find who was publishing the anti-gravel advice. Not one person could personally tell me of a single fatal case they knew of because they all avoided gravel and kept their animals on sand. I finally did find one person who could actually relate a death due to gut impaction. I asked her to describe how she determined this. The animal stopped eating, it died and when they did a necropsy, they found the gut full of gravel and mucus. I then asked the critical question. How did you know it was gut impaction and how did you know the gravel caused it? The animal stopped eating and then died and the gut was full of gravel. It was obvious. That was the answer. And here you see a failure to distinguish between correlation and causation.

Correlation is simply two things occurred at the same time. They may or may not be related. A correlation may indicate a cause but It might just be coincidence. Causation means doing A caused B to happen or in this case, eating gravel caused gut impaction. Or did it? There was no proof of causation only evidence of correlation. Meantime, generations of pet/lab axolotl keepers were assiduously keeping gravel from the animals based on what was, as far as I could tell, a single report of a single expert who had confused correlation with causation and published it. So we stopped worrying but, being mere undergraduate students, we also didn’t tell anyone who was “an expert” with  PhD about the gravel.

It all came to a head when our lab began a radiology experiment to learn about using ultrasound to view limb deformities in fetuses and very young children. The problem is that you need to be able to monitor limb development in certain situations but no one wants to expose kids to a whole bunch of x-rays. Also kids have soft bones that are not calcified which makes it harder to image them with x-rays. Axolotls regularly bite each other on the leg as part of their whole social structure and the loser often ends up having to regenerate a whole limb. Axolotls also have cartilage not bone, just like young humans. We took advantage of this natural behaviour to do a comparison of imaging with ultrasound versus x-rays of regenerating limbs. The results were that ultrasound works remarkably well. The work included the x-ray picture above. Look at that belly just packed full of gravel! We followed several animals over two years. Every time we took their x-ray or ultrasound, they had a belly full of rocks. They were also clearly healthy and normal. So what about the whole “Never keep axolotls on gravel in case they die of gut impaction” question?

As a scientist, one should never just accept received wisdom at face value, and so, since I was the axolotl expert in the collaboration, I got to do a full literature review on the topic. The result was that I learned there is absolutely no proof whatsoever that axolotls get gut impaction from eating gravel. I also learned all about gastroliths. A gastrolith is a rock ingested by an animal on purpose and carried around in the gut to provide stability, a weighted keel, while swimming. A clever pair of paleontologists explained that the little piles of rocks consistently found in the belly area of fossils of plesiosaurs were gastroliths. They pointed out an animal without a keel could compensate for the lack of a keel while swimming by having weight in the gut. Without such a weight, you end up rolling in the water as soon as you pick up any speed. Fish have a swim bladder that makes the top lighter than the bottom and they have fins. Plesiosaurs swallowed rocks. After that, I found examples all over the scientific literature of gastroliths including a complaint from a fellow who was dissecting frog tadpoles for an unrelated reason about how you had to be careful because the tadpoles had little rocks in their guts that can wreck the very expensive fine blade of a scientific microtome. Even seals apparently swallow gastroliths.

I did one final test. Baby axolotls swim, continuously hunting prey. We kept our baby axolotls in 25 gallon “pond tanks” chock full of things delicious (if you are an axolotl) to eat like Daphnia. My hypothesis was that the wee little 2-3 cm axolotl baby that has just grown front and back feet should really need gastroliths and therefore have a strong instinctive drive to find them. I put a tiny 3cm dish of fine coloured gravel in one bare corner of the 25 gallon tank, far from any food. Sure enough, I kept finding the little guys sitting on the gravel. When I offered them an assortment of sizes, each size a different colour, they would take the tiny rocks in their little mouths, wriggle their mouths in the cutest axolol style smile and swallow some and spit out others. I could see the coloured stones in their transparent guts and they almost always took stones of one colour. They even have a size preference. I also found mucus and waste with stones all over the bottom of the tank. So it appeared the baby axolotls ingested the gravel deliberately and passed it at random. So gastroliths don’t stay in the gut. They pass through the gut and the animal has to constantly seek out fresh gravel. Gizzards of birds probably evolved for more efficient retention of gastroliths. I also compared the swimming behaviour of baby axolotls who had access to gastroliths with those who did not. The ones with bright coloured gravel swam straight and fast. The others often missed prey because they rolled out of control on the big rush.

The take home message is this. Axolotls have a strong instinctive urge to swallow gastroliths. All those stories of pet keepers who have lost animals who swallowed plants and garbage probably had animals desperately trying to satisfy their instinctive urge to eat gastroliths. (I say “probably” because I don’t know for sure since I can’t read their minds.) Sometimes, even in science, received wisdom and the consensus of the experts is just plain wrong. Question everything, even and maybe especially, from the experts in science. Always go back and evaluate the original data for yourself. Scientists are first and foremost human beings and they are subject to all the failures of human beings. Oh, and you CAN keep your axolotl on gravel and they will probably be happier for it.

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About tumbleweedstumbling

I have three blogs, embryogenesis explained, tumbleweed tumbling AND fulltimetumbleweed. I am a retired scientist, and my husband and I have written a book which was published by World Scientific Publishing in Nov 2016 called Embryogensis Explained. Full time tumbleweed was my first blog which I worked on during five years of living full time in a travel trailer. I have now retired that blog in favour of Tumbleweeds Tumbling since we bought a stick house in April 2015 and are no longer full-time. I have a blended family of five sons and one daughter, all grown up now. I am (step)grandmother to nine boys and one girl. My husband and I have a dog and two cats. We live in Manitoba, Canada, in a 480 square foot house on a half acre of land in the tiny town of Alonsa on territory ceded, released, surrendered and yielded up in 1871 to Her Majesty the Queen and successors forever.

24 thoughts on “Gastroliths – How I Learned to Stop Worrying and Love Gravel.

    1. tumbleweedstumbling Post author

      People don’t have any need to ingest rocks. They don’t need them for digestion and they have no drive to do so. However there have been reports of people ingesting bizarre things and I think while mostly it wold pass right through a human system, it could kill a person if that person got a blocked gut.

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  1. Lottl lover

    So, this article is severely incorrect. Axolotls should only be on bare bottoms or sand-like substrate. You should not place your axolotls in an environment in which they can be exposed to harm if you truly care for your creatures, including rock bottoms. Lottls are not intelligent hunters (they are almost completely blind) and therefore, they think everything is food; that is why they swallow and pass rocks. Axolotls who eat rocks can experience blockages, tearing of their immune systems, and many more health problems across the course of time. They are not “happier” for having rocks that they can not distinguish from food (in fact, they are likely more stressed out); they would much prefer an environment where they can hunt for food without experiencing discomfort. This article is extremely dangerous and full of misinformation. I do hope that the person who wrote this chooses to take better care of their lottls in the future and switches to a lottl-friendly substrate. I apologize for my harsh tone, but I am extremely upset to hear of this dismissive behavior from someone who claims to own adults and babies. This is negligence. Why would you place your lottls in an environment where they are prone to eat rocks rather than giving them a safe environment simply in terms of experimenting for your own benefit to disprove genuine research? If you are purchasing a lottl for the first time, please do not listen to this article. Opt for a sand-like substrate instead, or a bare bottom. This will ensure your lottl is healthy and not stressed.

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    1. tumbleweedstumbling Post author

      Your response is precisely the kind of hysterical biased personal opinion that my entire article was about. You have no research to quote, no personal experiments and nothing to offer but your opinion. I advise you to go back and read what I wrote again very carefully. Obviously you missed everything but your personal beliefs being challenged and you are unable to process that challenge or consider you just might be wrong.

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      1. Gazz

        Hi, i actually run a rescue and this is massivly incorrect, i will be doing a video soon with my research and will send you it

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      2. tumbleweedstumbling Post author

        Okay I await the results of your experiment. I look forward to you providing me with properly researched and peer reviewed articles that show why my results are wrong. I will admit I am wrong and I will even post and link to your video if you do a good job. BTW running a rescue means absolutely nothing. There are no standards for rescues, no required training, no licensing procedure and no inspection by any kind of animal care committees. Anyone can say they run a rescue. So telling me you run a rescue is not any kind of qualification nor does it lend you any credibility.

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    2. tumbleweedstumbling Post author

      Oh and if you can quote any primary source scientific publications or provide any details on your own research into the topic I WILL post it right here so everyone can see for themselves and decide.

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  2. Matthew McClure

    I long suspected that this was precisely what my axolotl is doing. After all, axolotls WILL have access to rocks in the wild and I already knew that plenty of other animals ingest gastroliths. My “pot bellied muppet” appears completely happy and healthy. Has this research been published elsewhere?

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    1. tumbleweedstumbling Post author

      To the best of my knowledge, no. As we are retired now and the question of the gastroliths was not part of our main research when we were working we didn’t continue our research when it was obvious they were happy to us.

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  3. Jim

    It is a shame that this informative and well researched article is not receiving the attention it should be getting, I have long been worried about my axolotl that was living in gravel, I noticed that people tend to blame their own pet for being “bad hunters” as they swallow the food along with the food itself. I have realized that my axolotl really know the difference between the gravel and his food. But is there more information about choking on rocks for cause of deaths?

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    1. tumbleweedstumbling Post author

      I have not been able to find anything more than what I have shared here. If anyone can find more, I will happily post it. I’m glad to hear your axolotl is doing just fine on gravel.

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  4. Diana Harris

    Hi there. I’m interested in information based on facts like you have provided. Thank you. If the Axolotl naturally lives in rivers, wouldn’t they naturally be exposed to small stones or gravel? The pet shops and novices say no gravel or stones. That would make sense if the animal’s natural environment was free of gravel and stones, but it’s not. Why are people so insistent about not using gravel? Thanks again for your information that is based on facts. I was thinking about getting rid of the gravel but was dubious.

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    1. tumbleweedstumbling Post author

      It is my opinion you are thinking rationally. The response I get back to your comment about river bottoms is they live in places that are all muddy. This is simply not true. They live in lakes, streams, creeks and deep pools coming off mountains. (I should perhaps say Axolotls once did because they are almost extinct in the wild and their native habitat is mostly now Mexico City.) I have traveled all over North American and I have never once seen a pure mud bottom water system. Why do people always say no gravel? Because some person in authority somewhere had an axolotl die and they did a necropsy and found the rocks in the gut. They concluded that it must have been an intestinal block due to the rocks and published this. It soon became an accepted fact passed from one authority to another and published as such in all the books on care and feeding of axolotls. However, there is simply no evidence for it. How did this person know that the rocks in the gut caused the death? They didn’t. The just concluded that it did. Everyone repeats this “truth” over and over again. Except it isn’t truth. It’s just something someone in authority said once which got accepted as truth. No one ever questioned it. I would say think for yourself and do what you think is best for your pet. I can assure you of one thing. We had many many animals who lived very happily for years and years living on gravel with a lot of gravel in their guts.

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  5. Sasha

    Hi so glad I found this article it was a very interesting read. Iv been looking for more information on axolotle since I got mine last year. what size and colour did you find they preferred to use as a keel weight as young ones. Did the babies and adults prefer a certain type of stone to use as their gastroliths.

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    1. tumbleweedstumbling Post author

      We allowed our adult axolotls access to grit sold in feed stores for poultry. That is very cheap and easy to get from places like For the little wee guys, we offered them very small aquarium pebbles. They did not care about the color. We simply used different color to sort out the different sizes. A good rule of thumb is one half the size between the space between their eyes. If the space is 1 cm then no more than 1/2 cm gravel. If you offer many sizes they will pick the size they want. If you are unsure just put a small dish somewhere in their tank.

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      1. Sasha

        Thank you for your reply. I will pick up some poultry grit for mine and see if they’ll eat any. Think I’ll go with the little dish of gravel like you’ve recommended. He/she is being worms (my captive bred dendrobena sp) for food. But iv seen other things to feed would you recommend anything else I should be feeding. What foods were you feeding yours. Did you limit the amount of iodine.

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      2. tumbleweedstumbling Post author

        We fed ours all kinds of thing. When they are tiny, they only take live food or food falling in front of them moving like its alive. Once they have all four legs and spend more time sitting than swimming we would introduce commercial salmon pellet food. Before we found those we fed them raw beef heat but they did better on the salmon pellets. Those you can buy in huge pails very cheaply, usually by special order through your pet store. Everyone liked to give them treats. We added turtle food pellets, small pieces of raw beef heart, a bit of tuna or other fish (always cooked to avoid parasites), or a bit of hardboiled egg white as treats. They seemed to really appreciate a little variety. It’s been twenty years so maybe someone has made a specific axolotl food all balanced and with proper vitamins. We would offer treats on the end of big blunt tweezers and they soon learned to swim up to us and look for a treat. If we could find feeder fish we knew for sure were clean they LOVED those. Avoid live food like snails and minnows unless you are very sure of the source and you know those don’t have parasites. I don’t recommend catching live stuff outdoors. Too many pathogens and parasites. We even tried stuff like hot dogs and bologna but they always barfed up anything processed like that so no processed food. I never heard of avoiding iodine. This may be new research so you can check that yourself and make your own decision about that. I do know liver has too much Vitamin A and can be toxic if they get nothing but liver. We avoided liver except very rarely as an occasional treat. Be very careful to not overfeed and clean up the tanks of anything they don’t eat with an hour or so. We fed ours Monday Wednesday and Friday.

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  6. George

    Hi there!

    I’m extremely invested in your article! I just made an impulse purchase last week for my very first axolotl (wild type). He is ~5 inches; I transferred him into my ~8 gallon tank (my tank now registers between 66 and 70 F) previously occupied by my loach. It is then, naturally, currently full of river rocks ~cm in diameter. I have plans to extract as many said rocks as possible this weekend, and cover whatever remains with sand. I am hesitant to do so, though, as I ideally want to give him the most natural setting (to emulate a puddle he might occupy on a mountain top). I have already found that my axolotl spends almost the entirety of it’s day (apart from an occasional float) combing the bottom of his tank, digging his snout deep into gravel and then ‘hoovering’ up an incognito bloodworm. I’ve only witnessed him suck up one stone thus far, but he proceeded to spit it out, quickly, but still managed to hold a bloodworm in his mouth like a noodle. I have also witnessed my axolotl trying to ‘dig’ in the rocks, almost as if he wants to pull them up with his hands, as he roams around, seemingly hunting – have you noted such behaviour? I would have to assume an axolotl’s habitat contains rocks, as you discuss; or they are at least used to hunting in rocks, and I do not believe nature would evolve to develop a skill it does not require – as it would seem axolotl desire gastrolith, to an extent, I only question if a captive axolotl requires such means; where it only ingests a soft diet?

    I have massive respect for how you have approached this topic, but I have a couple queries:
    How old are your axolotl that are living with gravel? Have you had to deal with any impaction?
    I start to question your validity and morals when I read of you feeding your axolotl processed foods. Care to elaborate on that, so I understand those intentions? I doubt they have access to hot dog in the wild – and I thought your experiment was to determine a more natural state of an axolotl habitat. Otherwise I agree with just about everything you’ve stated, or I at least want to.
    Do you have any more slightly tangible evidence, rather than your own writings? (photos, videos, a youtube page- I’d be very interested in seeing that!)

    Otherwise, am curious what you think of my tank plans, or if you think I should scrap them until it is a reasonable time for me to buy my axolotl a 20 gallon tank. Thanks a ton. G.

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    1. tumbleweedstumbling Post author

      We had a small axolotl colony of ~100 animals for just over two decades. We were using the eggs of the animals for research. We did one experiment following their limb regeneration but otherwise we never used the adults for anything experimentally. We gave up our colony when my husband retired from active research so we have not had any in many years. We do not currently have any as pets. We moved the babies onto gravel substrate when they had all four limbs and were spending more time sitting on the bottom than swimming.

      We did not have any cases of death due to impaction even once during those years. Admittedly we did not do necropsies on every dead axolotl so it may be that we missed one but we never saw it. Our animals typically lived at least 12 years.

      We started out feeding our axolotls raw beef heart. At the time that was the standard. In our second decade we switched to turtle and then salmon pellets. The references to things like hot dogs occurred because we had students in the labs and they helped care for the animals. The animals appear to respond to their caregivers and swim up and “smile” at them. Students would sometimes offer an animal a bite of their lunch. This was not “normal” or authorized. That’s how I know they like other stuff but don’t like processed meat. I never encouraged students to do that. It just happened.

      I’m not sure about the axolotl with a loach though loaches are pretty benign. Generally if you house axolotls and fish together either the fish start nipping the gills or the axolotl eats the fish. Also while we never had trouble with gravel, axolotls are exquisitely sensitive to skin parasites that live with fish. So the axolotl often got sick from the fish parasites.

      As to whether or not you should strip your tank, my inclination is to leave it alone and feed the axolotl more. If it is digging around hunting it’s hungry. If it is seeking a correct size gastrolith it will be a rock about one half the size of the space between their eyes. Yo have not said what size your gravel is versus your axolotl. Of the gravel is large your axolotl might try to eat stuff too large and choke on it if it does not have gravel of the correct size available.

      Due to the fact that we retired and much of this dates back from before things like you tube, we don’t have any videos. You can see some of our animals by going to the Tyrell Museum youtube videos. When we disbanded the colony the majority of them went to Tyrell.

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  7. Pingback: Substrates for Axolotls & Gravel Guts: What Nobody Tells You • Fantaxies

  8. Natalie

    Have you ever had one sink because the ate too much gravel? My axelotel ate some gravel and is eating well but can’t swim to the top. Have you experienced this before? It has only been a few days so I can’t tell you if it has been expelled yet, but I can update it once I know.

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  9. Pingback: Gravel for Axolotls: What Nobody Tells You • Fantaxies

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