The axolotl is sometimes called the Mexican walking fish, though it is not a fish at all. It is an amphibian and a type of salamander, and its closest relative is the tiger salamander. It lives in the water for its whole life. Axolotls are neotenic, which means they keep juvenile characteristics into adulthood. They never go through metamorphosis and grow up to become adult salamanders. In experiments, treating axolotls with the thyroid hormones T4 or T3 can trigger complete metamorphosis. Adding iodine to their water causes the release of hormones that start the development of land-based traits. One idea is that axolotls stay in the water because their native lakes never dry up. So they never had to trade water traits for land ones.
Lazzaro Spallanzani (1729-1799) investigated salamander regeneration by amputating salamander tails and limbs. The word axolotl comes from Nahuatl, a language of the Aztecs. In 1864, 34 living axolotls were brought from Mexico to Paris. In 1867, Auguste Duméril, a French herpetologist at the natural history museum in Paris, published the first scientific papers on regeneration in the Mexican axolotl. Those first living axolotls were the founder generation of a population of hundreds of thousands of axolotls bred in laboratories and zoos. Most axolotls in research labs around the world are descendants of the group brought to Paris in the 1860s. The axolotl is easy to breed in the laboratory. It is the most commonly used salamander model organism in regeneration research.
Salamanders can grow back limbs, tails, retina and spinal cord during any stage of their life cycle. They can also grow back some sections of the heart and brain. Within hours after an amputation, a wound epithelium covers the severed edge of the limb. Then a mass of cells called a blastema forms. It can regrow the missing part. An axolotl can regrow a lost leg in a few weeks to months. When it does, every tissue is replaced, including skin, bone, cartilage and muscle. In axolotls, wound healing eventually restores normal skin architecture rather than forming a scar. A tiger salamander grows a limb back in 155 to 180 days. A juvenile axolotl takes approximately 40 to 50 days.
When immune cells called macrophages were removed, salamanders could no longer grow back a limb, and the salamanders formed scar tissue instead. Wound healing in a mammal leads to formation of scar tissue and does not progress to a blastema. As a salamander gets older, it grows back parts less well. But old salamanders still grow back missing or damaged tissues. Scarring prevents tissue from growing back, so finding out how and why axolotls do not scar could unlock human tissue regeneration. Genes that direct the growth of organs and limbs during development are silenced in adult people. In axolotls, these genes may be activated again after an injury.
In the wild, axolotls come from lakes and wetlands in southern Mexico City, in Mexico. They once lived in a series of lakes in the Mexican Central Valley. The Xochimilco-Chalco basin was once about 200 square kilometers of marshes, swamps and lagoons. It was greatly reduced when drainage projects were completed. Wild axolotls are mostly dark in color: black, brown or gray. They hide in mud and plants during the day and are active at night. They can reach about 12 inches long and live 10 to 15 years in the wild. They eat worms, insect larvae and small fish. They catch food by sucking it in. A female lays her eggs on plants or rocks.
As of November 2026, the IUCN Red List lists the axolotl as Critically Endangered. An IUCN assessment found only 50 to 1,000 axolotls left in the wild. The number of wild axolotls is decreasing. Urbanization and pollution of their lakes are the biggest threats. Fish put into the waterways, such as tilapia and carp, compete with axolotls for food and eat their young. In 2025, scientists released 18 captive-bred axolotls into artificial wetlands near Mexico City. Farmers are increasingly using traditional farming methods, free of pesticides and fertilizers, to help conserve axolotls.
Words to know
- neotenic: keeping young traits
- metamorphosis: big body change
- blastema: mass of cells
- macrophages: immune cells
- epithelium: covering layer
