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.