05/04/2025
There has been a lot of excitement over a research article published this last week in Plos One describing a project in which captive-bred axolotls were introduced into a protected artificial environment, La Cantera Oriente, and reintroduced into a “restored” part of Lake Xochimilco - and there should be. Unfortunately, most of the commentary I’m seeing is misrepresenting both the purpose of the study and its conclusions. It is not a successful attempt to reestablish axolotls in their native habitat - and that is not the purpose of the study.
Edit: To avoid misinterpretation, the following discussion of previous reintroduction failures (of turkeys!) is not a critique of axolotl researchers. It is an explanation of the problems faced in reintroduction of animals into natural habitats. The axolotl researchers are fully aware of these issues, and are developing methods to track and monitor released animals so that these problems can be avoided.
I’ll have a link to the Plos One publication at the end of this post.
The purpose of this study was to evaluate “the viability of restored and artificial wetlands for axolotl conservation by comparing movement patterns, home range sizes, and habitat use.” In other words, the goal was to release some axolotls into controlled environments, then use radio tracking to see what they did after release. More specifically, the researchers want to know whether captive-bred and reared axolotls can survive and thrive long-term once released, and to see what parts of the environments they then choose to occupy.
The study is also a test of the tracking method used, in which tiny, high-frequency radio transmitters are surgically implanted in the axolotls. Each transmitter weighs about two grams, and has an effective battery life of about 100 days. Tracking those transmitters using directional antennas allows the researchers to pinpoint the location of each axolotl at any time.
The axolotls used in this study were bred, hatched and raised to adulthood in captivity at the Laboratorio de Restauración Ecológica (LRE) of the Instituto de Biología, Universidad Nacional Autónoma de México (UNAM) - which could be a problem for long-term release efforts.
These axolotls were raised without substrate or plants to minimize the risk of disease transmission, and provided with artificial shelters that were routinely sterilized for their security and comfort. They were fed live foods, all quarantined before feeding, including brine shrimp, small fish, crayfish, and earthworms, and given monthly nutritional supplements. An air-conditioning system kept their water temperature below 18ºC (64ºF).
A problem when trying to re-establish an endangered or exterminated species using human-reared animals is that much of an animal’s behaviour in the wild is learned, and much more is selected by environmental pressures. In other words, babies learn to fear dangerous stuff, and the stupid or fearless babies don’t survive to pass on their stupid and fearless genes. Animals reared in labs or hatcheries miss the learning experiences and culling dangers of their wild relatives. They never get attacked by crayfish, or see their siblings munched by herons, so they don’t avoid these things. Any of us who keep axolotls as pets know they can be pretty stupid when it comes to dangerous situations, swimming excitedly up to a housecat, casually floating around at the surface where a heron would be looking for them, shoving their heads into narrow gaps they can’t back out of, and trying to bite things they can’t possibly swallow. They also get regular meals of juicy worms, frozen insects, or scrambled food pellets, often delivered directly in front of them or placed neatly in a little dish for their convenience. As a result, they simply don’t know what to do when faced with predators and hard-to-find food sources that may even fight back in the wild.
Back in the 1950s there was an attempt to reintroduce turkeys to Southern Indiana (They had been wiped out by overhunting and habitat loss by about 1910). It failed miserably because the released birds casually walked up to dogs, tractors, and people with guns. Later attempts used birds that had been raised in such a way that they never saw humans. Some chicks were even deliberately chased by dogs. That worked, and we’re now up to our necks in turkeys around here.
Habituation to humans and rearing in protected conditions has been the downfall of many other attempts to reestablish populations of endangered or extinct animals, too. You can find wonderful videos of researchers using interesting and somewhat embarrassing ways to avoid these problems with whooping cranes, for example. So, there has been concern that lab-reared axolotls - even those produced by pairing animals taken directly from Xochimilco, would suffer the same fate as those stupid turkeys.
This tagging and tracking study is a first step in attempting to learn whether that is true, and to test a method for tracking the animals successfully.
Researchers tagged 18 adult axolotls by surgically implanting them with radio transmitters. They then monitored them for a while to make sure their surgery healed well and didn’t significantly impair them. The axolotls were then released at two locations.
The two locations were definitely not “wild”, unprotected environments. We already know that released axolotls don’t fare well in Lake Xochimilco. Just like the wild axolotls that were there in the past, they quickly die from poor water quality and predation by introduced carp and tilapia. One location is an artificial pond in the bottom of an old basalt mine not far from Xochimilco, called La Cantera Oriente (The Eastern Quarry). This site is being considered for use to reestablish stable populations of axolotls in the future, and is currently used by the University for aquatic studies. The other location is a small section of what was once Lake Xochimilco, between chinampas, that has been blocked off from the lake, cleaned of all carp and tilapia, and has a filtration system to reduce pollutants and increase dissolved oxygen. So, to be clear, these are both very artificial, protected habitats.
What the study found is:
(a) The released axolotls survived (mostly - Two in the chinampa of Xochimilco were eaten by egrets soon after the study ended).
(b) They were successfully tracked over an extended period of time, from March through April of 2018. Some were caught at the conclusion of the study, and their weight and length were recorded. They actually gained significant weight, showing that they successfully found food sources on their own.
(c) Tracking showed that they preferred certain parts of their new habitats, seeming to prefer areas with a narrow range of temperatures around 62 F (16.5 C).
(d) Females travel a bit more than males.
(e) The axolotls were more active in late afternoon and early evening than at other times of day.
All of this is very cool, because in spite of the 150ish years we’ve studied axolotls (assuming here that the Aztecs were mostly eating them rather than actually studying them), we really have learned very little about their natural behaviour in the wild. Technically, we still haven’t, since there was nothing natural about these axolotls or the environments they were placed in, but this study is a step in the right direction.
Using this tracking method to learn more about their behaviours and preferences we may be able to set up other protected bodies of water that may be used to establish stable populations of wild axolotls once again.
For anyone interested in reading the paper, here it is in Plos One:
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0314257&utm_source=pr&utm_medium=email&utm_campaign=plos006
