12/23/2025
True!
Axolotls are not “Wolverine”.
Axolotls are famous for their ability to heal. They not only heal quickly after an injury, and do so without scarring, but can regrow entire organs, including muscles, bones, and even brain tissue. A juvenile that loses a limb to a predator (or other axolotl) may regrow the entire limb right down to the correct number of toes! Basically, as long as the injury isn’t immediately fatal, and infection is limited, they will heal.
Unfortunately, this has led to the disturbing misconception that their healing is perfect, that they are little superheroes that heal completely and perfectly after injuries. This belief leads to the idea that there is nothing wrong with housing young juveniles together, and nothing unethical about selling animals with missing limbs, shredded tails and gill stalks, and more serious injuries.
Nope.
Axolotls under three weeks old are capable of inflicting damage on each other, but that is rare unless there is a tremendous difference in growth rate within the clutch. Axolotls do not have a “cannibal morph” (Look that one up - It’s kinda amazing), as many other species of Ambystoma salamanders do, but they will take advantage of weaker, smaller siblings when there is opportunity.
Once the front legs become obvious, the risk of injury greatly increases. Juveniles nip anything that brushes against them when in feeding mode, and tiny feet and legs are easy targets. Feeding aggression increases once the front legs are fully formed, around two weeks after hatch. Experienced breeders have noticed that there are always a few juveniles that are outgrowing their siblings at this stage, and a careful examination of the clutch is likely to reveal that a LOT of the smaller siblings are bitten. Legs, toes, and tails are very nutritious.
You might think that providing a lot of food prevents biting. It doesn’t. That’s like thinking that the way to swim safely with sharks is to throw a lot of meat into the water first to distract them. At three weeks old, an axolotl has a brain about the size of a grain of sand. They aren’t really thinking things through at that age - They just snap at whatever moves so they get it before their siblings do. Survival of the fittest, and all that…
Also, and I hear this a LOT: People tend to be misled by survivor bias: They think that because someone else kept babies together and made grandiose claims about them never biting that this means they can do it too. NO. I’ve seen entire clutches of mutilated axolotls in tubs at trade shows, where the sellers just tell potential buyers that they will heal. I've been contacted countless times by people who saw 50% or more of their clutches shredded when housed together and just couldn't understand why that happened, or (worse) thought that was just normal (It’s not). Those people don’t post pics of their babies on social media.
Note: I separate babies at around 18 days after hatching. Minor injuries acquired before 18 days old are inevitable for some, but those do, in fact, heal completely and perfectly (with rare exceptions). I don’t put them back together until they are at least seven months old, though that is probably an overabundance of caution. When you see older juveniles in pics together on my page, they have been put into a “glamour shots tank” just for those pictures, and are separated immediately after. Once they start to approach sexual maturity (6-8 months old), and their little brains have become a bit more complex, the random biting behaviour generally ends.
Axolotl healing ability is amazing, and certainly is far beyond our own healing ability, but it is not at all perfect. The success of limb regrowth is dependent on many factors, like the type of injury (tear or clean bite?), its position on the limb (at a joint or mid-limb?), and the age and health of the axolotl.
Juveniles that are repeatedly injured by the siblings they are housed with grow more slowly never completely recover. A body part that is repeatedly bitten off may never regrow at all, forming a stub.
And just to be clear on a couple of points: (1) They can and do die of serious injuries and infected wounds, and (2) older axolotls do not heal as easily or as quickly as juveniles, and sometimes not at all - just like us.
It should be no surprise that the amazing healing ability of axolotls has been documented in many, many studies over the last 150 years or so. Some of those studies, particularly those done in the 19th century, are pretty disturbing, involving inflicting wounds to observe the results, grafting body parts from one animal to another, etc.
Under ideal conditions both larvae and juvenile axolotls will regrow completely normal limbs, tails, gills, etc. Ideal conditions means a clean, uniform cut that does not “rearrange” tissue, and sterile conditions to prevent infections. Unfortunately a bite is rarely a clean cut, and conditions are not usually sterile. For older juveniles and adults, researchers find that they have to do some very careful cutting to get a new, perfect limb to form.
Cells at the exposed end of severed limb have all of the instructions they need to build new tissue and organs written on their DNA, but they require information from surrounding cells in order to follow those instructions. Cells communicate with the cells around them to figure out what to do. They have to know what part of the limb they are in, and what type of cells they need to become and produce.
If a bite is jagged, and tissue is torn, there is likely to be some confusion about exactly what needs to be done to fix the damage. If cells on one hanging bit start forming a new leg, and cells on another hanging bit also start forming a leg, then the axolotl will end up with two legs where there should be one. If a bit of skin gets spread over the stub end of a severed toe, the cells beneath it will “assume” there is already a toe there, and will simply heal that skin into place - so a toe never grows there. A bacterial infection may destroy some of the cells that have already taken on the task of making part of a new limb, resulting in a healed-over stub, missing toes, etc.
So where’s the evidence to support my claim that axolotls should be separated as soon as their little front legs form? Well, here comes the lengthy part…
Early studies showed that axolotls not only heal well, they can accept tissue and organ transplants from one another without risk of rejection. This made them ideal for the study of tissue and organ formation in developing embryos, and for understanding organ transplantation. While we humans don’t heal the way the axolotls do, and definitely have a problem with tissue rejection from poorly matched donors, we do have the same basic body parts with similar functions.
More recently, as we’ve developed the tools needed to understand the genetic and molecular machinery axolotls use to achieve healing, this has led to further research into methods to improve healing in humans - particularly in the fields of neurology and immunology.
All of that research requires lots of axolotls. The need for axolotls in research requires production under controlled conditions in laboratories.
Colonies of lab-reared axolotls are kept at many locations, including the National Autonomous University of Mexico (Which is the only facility maintaining captive axolotls in their native environment for conservation efforts and studies on reintroduction into the wild, headed by Dr. Luis Zambrano.), Whitehead Institute (Cambridge, MA, USA), University of Heidelberg (Germany), Tohoku University (Japan), Institute of Molecular Biotechnology (Austria - a research lab focused on regeneration with axolotls), and the Centre for Regenerative Therapies Dresden (Germany).
The largest facility for breeding and rearing axolotls is the Ambystoma Genetic Stock Center (AGSC) at the University of Kentucky (Which was originally the Indiana University Axolotl Colony, formerly run by Dr. George M. Malacinski, and now headed by Dr. S. Randall Voss). The AGSC is the primary supplier of axolotls to researchers globally.
The AGSC maintains close to 1000 adult axolotls. There are axolotls with every phenotype and genotype you can imagine there, as well as several you can’t imagine :)
When researchers require axolotls of a specific age, phenotype, and/or genotype, the AGSC produces them, raises them to the desired age, and sends them out. As you might suspect, that means that thousands of hatchlings and juveniles have to be housed for extended periods, which requires a lot of space and manpower.
Please note: The following is a rough description of how axolotls are raised at AGSC and other lab settings in order to provide animals for research. These are standard methods of lab husbandry that have been in place for more than 40 years. This is neither a recommendation or detraction of these methods.
At the AGSC, one week after hatching, about 50–100 larvae about ⅓ of an inch long from each clutch are housed in the same tub. As they grow, the per-tub density of larvae is reduced, so that after about three months each of the tubs have 10–15 two-inch babies.
At about three months after hatching, the juveniles are separated and reared independently to generate stocks for users and to replace aging members of the adult breeding population. At approximately 18 months of age, females at AGSC are moved into paired group housing. Males are reared alone (which seems to make them more receptive to mating when needed).
Just a reminder, here, for those with short attention spans: This is NOT the way to raise axolotls at home. Pet breeders and hobbyists should NEVER house juvenile axolotls over three weeks old together precisely because they WILL bite and get bitten. Juveniles that bite others are unlikely to stop doing that as adults! Also note that healthy, unbitten axolotls will be significantly larger at three months of age (approximately three inches or more) if they are raised individually and fed properly.
So, if what I have said is true, and juvenile axolotls under around six months old are likely to bite each other, and AGSC houses juveniles together, then there should be lots of evidence at the AGSC!
Spoiler alert: There is.
Studies have been done at AGSC to determine the probability and degree of healing in axolotls. Here is a summary of what has been found:
One study looked at all of the adult axolotls at AGSC. About half of those adults had four normal legs with four toes on the front feet, and five on the back feet back. Which, of course, means that half of them didn’t have four normal limbs. About 4% of those adults had abnormalities of all four of their legs!
The most frequently observed abnormalities were syndactyly (fused toes), ectrodactyly (missing toes), and brachydactyly (missing part of a toe). Curiously, adult males and females were missing toes with about the same frequency, but fused toes were twice as common among males. The males were much more likely to have backward legs, extra legs, or missing legs. In other words, the males tended to have more serious injuries than females.
Some researchers I have spoken to regarding axolotl husbandry think that males are more aggressive than females, and that this is, in part, why the males are separated when housed in lab conditions. Based on the information in that last paragraph, that just doesn’t make sense. When male and female juveniles are housed together the males suffer the most damage! Anecdotally; I have seen that pattern in juveniles that other breeders have raised together, as well.
Just speculating here, but I suspect that older juvenile males get bullied more than females in mixed groups because the males tend to be a bit smaller than their female siblings, and have a lower calorie requirement.
By far, lab-reared juveniles had more injuries than adults. By the time the juveniles were separated from group living, 80% had at least one abnormal limb. Almost half of them had all four limbs damaged one way or another! Worse: 65% of all legs had recent bite injuries - and most of those were severe, missing pieces of legs, whole feet, or whole legs!
Obviously, putting all those numbers together, this means that most of the injuries the juveniles get early on will heal enough after they are separated (at about three months old) not to be noticeable by the time they are mature adults.
Even so, the majority of the adults still have noticeably abnormal limbs. That means they healed, but not always perfectly.
Missing only part of a toe was found only among the adult females. The obvious reasons: (1) Toe injuries usually heal completely, and (2) The females are kept in pairs as adults. Any males with nipped toes heal after they are separated, but cohabitating females keep getting nipped now and then by their roommates.
I would also like to point out that in my experience an axolotl that bites successfully as a juvenile is very likely to keep biting as an adult. This may be learned behaviour, or it may simply be that more aggressive juveniles housed together tend to be the ones that survive to adulthood. It may be that raising juveniles together is actively breeding aggression.
Even “friendly” females will accidentally nip when sucking in food, so a toe is taken now and then. That is not an act of aggression, it is just the natural result of having two vacuum-feeding predators stuck together in a small space. The solutions to this are simply to provide more space, and to provide food distributed over a wide area to reduce the probability that two animals will be feeding in the same spot. That is one of many reasons I like to use floating pellets for cohabitating axolotls. The pellets drift around on the surface, so the axies are unlikely to go for the same one at the same time, or to confuse a toe with a drifting pellet.
Personally, I generally keep my adults in same-sex pairs, and have noticed no significant or consistent difference in levels of aggression between adult males and females. In fact, a quick check of the axolotls I currently have housed together in pairs - 14 females and 8 males - shows no abnormalities at all, not even a bent toe.
Request: If any of you with large numbers of adults in your care have any anecdotal evidence regarding differences in aggression between the sexes, please share in the comments!
I think this quote from one study (Laura Muzinic) is most revealing: “We note that in reviewing records where larvae are reared independently after hatching, which is provided as an AGSC service, the frequency of observing abnormal limbs is less than 0.5%.” In other words: If you separate your babies as soon as their front feet form, they won’t get bitten, and they won’t have messed up toes, legs, tails, gills, etc., as adults.
Here are a few choice Sources, if you’re in need of some more light reading:
Bowerman, J., Johnson, P.T.J. & Bowerman, T. (2010). Sublethal
predators and their injured prey: linking aquatic predators and
severe limb abnormalities in amphibians. Ecology, 91,
242–251.
https://doi.org/10.1890/08-1687.1
Bryant, S.V., French, V. & Bryant, P.J. (1981). Distal
regeneration and symmetry. Science, 212, 993–1002.
https://doi.org/10.1126/science.212.4498.993
Johnson, P.J., Preu, E.R., Sutherland, D.R., Romansic, J.M., Han,
B. & Blaustein, A.R. (2006). Adding infection to injury:
synergistic effects of predation and parasitism of amphibian
malformations. Ecology, 87, 2227–2235.
https://doi.org/10.1890/0012-9658(2006)87[2227:aitise]2.0.co;2
McCusker, C.D. & Gardiner, D.M. (2013). Positional
information is reprogrammed in blastema cells of the
regenerating limb of the axolotl (Ambystoma mexicanum).
PLoS One, 8, e77064.
https://doi.org/10.1371/journal.pone.0077064
Semlitsch, R.D. & Reichling, S.B. (1989). Density-dependent
injury in larval salamanders. Oecologia, 81, 100–103.
https://doi.org/10.1007/BF00377017
Thompson, S., Muzinic, L., Muzinic, C., Niemiller, M. L., & Voss, S. R. (2014). Probability of Regenerating a Normal Limb After Bite Injury in the Mexican Axolotl (Ambystoma mexicanum). Regeneration (Oxford, England), 1(3), 27–32.
https://doi.org/10.1002/reg2.17
