The salamander that never grows up

With its feathery gills, wide head and permanent half-smile, the axolotl has become one of the most recognisable amphibians in the world. It stars in aquariums, cartoons and video games. But in the place where it actually comes from, it is close to disappearing.

A salamander that stays a larva

The axolotl, Ambystoma mexicanum, is a salamander. Most salamanders, like frogs, go through metamorphosis: the aquatic larva loses its gills and changes into an adult that can live on land.

The axolotl skips that step. As the Natural History Museum in London explains, it keeps its juvenile features and remains aquatic for its whole life — a condition called neoteny, or paedomorphism. It keeps its feathery external gills instead of switching over to breathing air, and it can also take up oxygen partly through its skin. Even so, it becomes sexually mature and breeds, at about one year old.

Axolotls grow up to 30 centimetres long and weigh around 50 to 200 grams. In the wild they live 10 to 15 years.

Home: the canals of Mexico City

Axolotls are native to lakes and wetlands in the south of what is now Mexico City, especially Lake Xochimilco and Lake Chalco. Both lakes have since been drained. Wild axolotls now survive in the remaining canals, and possibly in Lake Chapultepec.

In their habitat, they are top predators. They eat molluscs, worms, insect larvae, crustaceans and some small fish.

In the wild, axolotls are brown-grey. The pink and pale forms people know from pet shops are mostly bred in captivity. Albino axolotls lack the brown pigments and look pinkish-white, a colour that is popular in the pet trade.

Regrowing a body

The axolotl’s most famous ability is regeneration. If it loses a limb, it can grow a new one. According to the Natural History Museum, axolotls can regrow limbs, eyes and even parts of their brains.

That makes them important animals for science. Researchers are trying to understand which genes make this possible, and how these may switch back on after an injury. Humans can heal wounds but cannot regrow a lost arm — understanding why the axolotl can is one of the big questions in biology.

Many in tanks, few in the wild

Here lies the paradox. Axolotls are bred in large numbers for laboratories and as pets. Yet in the wild, the Natural History Museum notes, only between 50 and 1,000 adults are thought to remain. The IUCN Red List classifies the species as Critically Endangered.

The threats are clear. The canals suffer from pollution and habitat loss, and from disturbance by people. Introduced fish such as tilapia and carp compete with axolotls for food and eat their young.

Captive axolotls cannot simply replace the wild population. Many of them are highly inbred, which makes them vulnerable to disease. The wild animals, adapted to their own canals, are a different resource altogether.

Why it matters

The axolotl is a strange kind of famous: known all over the world, yet almost gone from the one place it belongs. Its future depends not on how many live in tanks, but on whether the canals of Xochimilco remain clean, connected and alive — a wetland in the middle of one of the largest cities on Earth.