In the canals of Xochimilco, on the southern edge of Mexico City, lives one of the strangest animals in the world. It is a salamander, but it never quite becomes one.

The axolotl spends its whole life looking like an oversized tadpole with legs. It has a wide, seemingly smiling mouth, tiny eyes and three pairs of feathery gills sprouting from the sides of its head like a headdress. Wild axolotls are usually dark, mottled brown or grey; pet ones are often pale pink.

Biologists have been fascinated by it for more than 150 years, because it can do something that we cannot: rebuild its own body.

The animal that stays young

Most salamanders begin life in water as larvae, with gills, and then go through metamorphosis, losing their gills and moving onto land. Axolotls usually don't. They reach adulthood and breed while keeping their larval body. This is called neoteny.

It isn't that they can't change. Their bodies don't respond strongly to the hormone signals that trigger metamorphosis in other salamanders. Given thyroid hormone in a laboratory, an axolotl can transform into a land-dwelling adult that looks much like its close relative, the tiger salamander.

Rebuilding a body

Cut off an axolotl's leg, and it will grow a new one — complete with bones, muscles, nerves and skin, correctly shaped. It can regenerate its tail, including the spinal cord inside it, and repair damage to parts of its heart, eyes and even its brain. It can do this many times, and it does so without the scarring that humans form.

How does it work? When a limb is lost, cells near the wound form a cluster called a blastema. These cells behave somewhat like the cells of a developing embryo, multiplying and organising themselves into the missing structures. Researchers are still working out how the cells "know" what to rebuild and where to stop.

In 2018, scientists published the axolotl's genome. It turned out to be enormous, about ten times larger than ours, full of long repeated stretches of DNA. Studying it may help identify which genes control regeneration, and whether any of the same tricks could ever be encouraged in human tissue.

A national celebrity

In recent years the axolotl has become something of a pop-culture star. It appears in video games and cartoons, and its smiling face sells soft toys and stickers around the world. In Mexico it has long been a cultural symbol; in 2021 the Bank of Mexico put an axolotl on a new 50-peso banknote, swimming among the plants of Xochimilco.

That fame has helped raise money and attention for conservation. It has also created a strange situation, where an animal can be one of the most recognisable in the world while almost disappearing from the only place it naturally lives.

A water monster in trouble

The axolotl's name comes from Nahuatl, the language of the Aztecs, and is usually linked to Xolotl, a god associated with fire, lightning and transformation, who in legend turned himself into an axolotl to escape being sacrificed.

Today, the animal is in danger. Its native lakes have mostly been drained to make way for Mexico City, leaving the canals of Xochimilco as its last stronghold. There, it faces polluted water, introduced fish such as carp and tilapia that eat its eggs and young, and the spread of the city. Surveys over recent decades have found steep declines, and the species is listed as critically endangered.