Axolotls can regrow lost legs, and even parts of their heart and brain.

When an axolotl loses a leg, cells at the wound form a small bud called a blastema. These cells behave somewhat like embryonic cells, multiplying and then becoming the right tissues, including bone, muscle, skin and nerves, arranged in the correct order. Over weeks to months, a complete, working leg grows back, usually without a scar.

Axolotls can repeat this many times. They can also repair damage to their spinal cord, regrow parts of their heart muscle and even regenerate sections of their brain. Most adult mammals, including humans, heal wounds with scar tissue instead, so axolotls have become a major subject of regeneration research.

They also keep many juvenile features into adulthood, including their feathery external gills.

Axolotls come from a small set of lakes and canals around Mexico City, especially Lake Xochimilco. The name comes from the Aztec language Nahuatl and is often linked to Xolotl, a god associated with transformation. In 1864, a group of axolotls was sent to Paris, and from them laboratory colonies spread across Europe, making the species a classic research animal.

In the wild, axolotls are critically endangered. Urban growth, pollution and introduced fish such as tilapia and carp have devastated their habitat. Conservation projects in Xochimilco work with local farmers to restore clean water channels. Ironically, while wild axolotls are rare, they are common as pets and in laboratories around the world.

Regrown limbs come back at the right size and shape, with fingers in the right order. Cells in the blastema carry positional memory, knowing whether they came from the shoulder or the wrist.

Researchers have identified chemical signals involved in this memory, which could explain how axolotls rebuild exactly what was lost.

Regeneration is widespread in the animal world, but rare in mammals. Starfish can regrow arms, flatworms can regrow whole bodies, and lizards can regrow tails, though lizard tails are simpler copies. Axolotls stand out because they can rebuild complex body parts almost perfectly.

Humans have some regenerative ability too. The liver can regrow after partial removal, fingertips in young children can sometimes regrow, and skin and the lining of the gut constantly renew themselves. But for large wounds, our bodies favour fast scarring.

Scientists think scarring may have evolved as a trade off. It closes wounds quickly and reduces infection, which matters for warm blooded animals, but it blocks the slower process of rebuilding tissue.