The Super-cute Axolotl Is Also a Ruthless Carnivore

By: Jesslyn Shields  | 
The axolotl, a threatened species, is known as a "walking fish" but is actually an amphibian. Given that picture, you can see why they're so popular in the pet trade, too. MaxPixels

In Julio Cortázar's short story "Axolotl," first published in 1954, a Latin American man living in Paris becomes infatuated with the axolotls living at the zoo, to the point that he eventually transforms into one. With their "pink, Aztec faces," "eyes of gold" and "rosy little bodies, translucent ... ending in a fish's tail of extraordinary delicacy," the narrator observes an "absolute lack of similarity between axolotls and human beings." In considering these feathery, pinkish salamanders, "It would seem easy, almost obvious, to fall into mythology."

Cortázar's narrator is, on the face of things, correct in his estimation of the axolotl (Ambystoma mexicanum) or Mexican walking fish. For starters, the axolotl fell into mythology centuries ago — the Aztecs believed that the first axolotl appeared in the lake system around modern day Mexico City when the powerful underworld god Xolotl transformed himself into a small, feathery amphibian to escape capture.


In ancient Mesoamerican culture, these close relatives of the tiger salamander species were considered a food source supplied by Lake Xochimilco for the good of humanity.

And Cortázar was right about the axolotl's similarity to humans — our last common ancestor probably roamed Earth around 360 million years ago, and at first (or even 50th) glance, they're excessively different from us.

Among the first modern zoo animals, 34 axolotls were brought from Mexico (along with three deer and three wild dogs) to the Jardin zoologique d'acclimatation in Paris in 1864. And although they weren't as interesting to 19th-century zoo goers as the larger, more charismatic animals, scientists quickly realized these unassuming little animals were strange — almost mythological, in fact.


Axolotls Live Underwater

Axolotls are a large threatened species of salamander that live in fresh water — they're not actually fish at all, as their common name suggests. In the wild they are most often dark brown or black with speckles, but leucitic or albino, variants are common, and you often see them as pets. With their round heads and permanently smiling faces, wild axolotls are cute — it's one of the reasons they do well with pet owners in the pet trade. One reason they're cute is because they display neoteny, which means they retain a lot of their juvenile features throughout their adult life.

For instance, although adult axolotls also have functional lungs like other salamander species and can breathe through their skin, they have big, fluffy external feathery gills — something most amphibians don't keep after babyhood. They have small, delicate webbed feet and a long tadpole-like tail crested with a translucent fin because they don't have to rely on their webbed feet and legs for land travel, but they have to be able to move through the water like a big tadpole.


Scientists think they stay baby-like throughout their lifespans because, unlike other salamander species, the wild axolotl population evolved in very stable habitats. Most other salamander species, such as the tiger salamander, live in wetlands that dry up during certain parts of the year, so they have to get rid of their feathery gills and breathe through functional lungs and through their skin. Wild axolotls evolved in a habitat with year-round water and with very few aquatic predators, so they didn't need to spend energy changing their bodies to suit their changing circumstances.

An axolotl's life span is about 15 years in captivity but a wild axolotl probably lives only five or six years. They reach sexual maturity at one year, and though they're solitary creatures for the most part, in February, breeding season begins and wild axolotl males begin finding females using pheromones. When they get together, he does a courtship dance in which he shakes his tail in her direction. After the female acquiesces to his attentions, she pokes him with her nose and he deposits a sperm packet on the lake floor, which she picks up and uses to fertilize her eggs.

The female wild axolotl will lay hundreds of eggs in the weeds or around some rocks and then leave them to fend for themselves — baby axolotls receive zero parental care. In fact, baby axolotls, hungry after hatching from their eggs, have been observed gnawing on their siblings' legs and tails for sustenance. As you'll see, this is totally fine, because the legs will just grow back later.

In their home ecosystem, axolotls are — or at least used to be — top predators around the lakes, wetlands and canals of central Mexico. They're unusual among amphibians because they remain underwater for their entire lives, breathing through gills, while most other salamander species walk around on land and breathe with lungs during the adult stage of life. Although they appear unassuming, they're actually ruthless carnivores, feasting on worms, mollusks, insects and insect larvae, and even small fish in the wild.

Part of the Aztec mythology of the axolotl centers around the fact that, like a powerful god, they are difficult to kill. If an axolotl loses virtually any part of its body it can regenerate it, no problem. While some lizards can grow back a tail, bisected flatworms can grow back their other half and starfish can regrow a limb, an axolotl can regrow practically any part of its body in a few weeks.

An axolotl, such as this speckled leucistic axolotl, can live very happily in captivity for as long as 15 years, but for only about five or six years in the wild.

"Of the animals that are closest to us — the vertebrates — salamanders are the only ones that can regenerate in this way, and can heal without scars," said David Gardiner, a professor in the School of Biological Sciences at the University of California, Irvine, when we talked to him in 2019. "Other salamanders can regenerate, but axolotls do it best."

When the Europeans got wind of axolotl regeneration, axolotls went from being a sort of boring exhibit in the zoo to one of the most important and longest self-sustaining lab animals in history. Georges Cuvier, popularly considered the Father of Paleontology, studied axolotls in an attempt to figure out whether Carl Linnaeus was correct in categorizing the classes Amphibia and Reptilia separately — it was a big question in those days, and Cuvier concluded that axolotls, because they breathe through gills their entire lives, must be some sort of lizard that existed as a perpetual larva — in the words of paleontologist Stephen Jay Gould, a "sexually mature tadpole." (Cuvier was correct sometimes, but not in this case.)

Because axolotls did incredibly well in laboratory and aquarium settings, 19th-century zoologist Auguste Duméril took it upon himself to provide every lab in Europe with a supply of axolotls, which resulted in some truly horrific studies in which scientists chopped up lab axolotls just to test the limits of their regenerative powers.


The Marvels of Axolotl Regeneration

"These days, axolotls are hugely important model systems for our studies about regeneration," said Gardiner. "We've known for decades — centuries, even — that we can remove parts of a developing embryonic structure and the cells that are left behind will fill in, repair and regenerate that structure. But in most animals — mammals, for instance — the system sort of shuts down at the end of embryonic development. Axolotls and other salamanders seem to be able to revert back to that embryonic-like state, re-accessing the developmental program that's already there. Humans have the program, we just stop being able to access it when we're no longer an embryo. You could say we, like axolotls, have evolved the ability to regenerate just fine, but we've also evolved a mechanism that inhibits that."

Scientists are interested in axolotls because they hope to figure out how to one day apply its miraculous new limb regeneration abilities to the human body. Axolotls can regenerate new limbs, heart tissue, eyes and even its spinal cord and parts of its brain, and make new neurons throughout their lives, which human brains do, too, although not as readily.


It's possible to force an axolotl to metamorphose into an adult salamander without gills by injecting it with iodine or thyroxine, or by feeding it foods that are rich in iodine. However, scientists have found that after they have metamoprphosed they don't easily regenerate cells.


Wild Axolotls Are Critically Endangered

Wild axolotls might be god-like in their evolved ability to re-access embryonic instructions to regenerate organs and limbs — captive axolotls might even be able to endure living in a nasty 19th-century aquarium or lab, cut up into little pieces — but what they're not able to endure is their home ecosystem being overrun with introduced predators, environmental toxins and habitat degradation. The lakes in their home around ultra-urbanized Mexico City have become not only polluted by aging wastewater systems, but overrun by introduced tilapia and perch, both of which view axolotls as a delicious snack.

In 1998, scientists counted around 6,000 axolotls per square kilometer in Lake Xochimilco, but these days that number is perilously close to zero. Since 2006, the International Union for Conservation of Nature and Natural Resources (IUCN) has classified the axolotl as a critically endangered species, and in 2019 a population assessment concluded that there are probably fewer than 1,000 individuals left in the wild.


The Mexican government and many axolotl conservation groups all over Mexico are doing their best to save the threatened salamander species by restoring the lakes and natural habitats they live in. One strategy is to make stationary floating island habitats for them called chinampas — rafts made of aquatic vegetation, mud and wood that were used in ancient Mesoamerican culture as floating gardens. Hundreds of years ago when the Aztec capital of Tenochtitlan stood in the place of Mexico City, the Aztecs built and farmed on a vast network of chinampas for miles around the capital city. This system of agriculture created canals that were shallow and sheltered, and where the axolotl population thrived. When the European conquistadors arrived in 1519, they destroyed the Aztec civilization, removed the chinampas and drained the canals and lakes — something that continues to happen today.

Today the natural habitat of the wild axolotl population is limited to Lake Xochimilco in the southern part of Mexico City. People are working to remove the invasive fishes that eat the axolotls and begin using chinampas-based agriculture in Lake Xochimilco because the aquatic vegetation of chinampas not only provides habitat for the axolotls, it filter toxins out of the lake water. Ecotourism of these chinampas has assisted in funding axolotl conservation efforts.

While the wild axolotl population is not doing well, captive populations are doing great — they're the most widely distributed amphibian in the world. Because scientists desperately want to figure out how to help you regenerate a new set of toes, millions of them live in labs around the globe — millions more, in fact, than live in the wild. And while axolotl research is important in science, captive populations of pet axolotls are also popular — especially in Japan, a country where you can also get axolotls as a deep-fried snack in some restaurants.

It's not legal everywhere for pet owners to keep a pet axolotl, so it's important to check your local exotic pet laws before you get one. Because they're completely aquatic, it's important to fill the tank completely with water — a 15 to 20-gallon (57 to 151 liter) tank is best. Much like a pet fish, you never pick up or handle an axolotl, and you should never house them with another pet — a fish or another axolotl — because they won't get along with anybody. However, if you follow those few basic rules, you'll find them relatively hardy and easy to care for.