Axolotl

The smiling salamander that never 'grows up'

The axolotl always looks like it's smiling! It's a salamander, but it never grows up — it stays a 'baby' all its life, with pink gills on its head, living underwater. Most amazing: if it loses a leg, a new one grows back! It can even regrow parts of its heart and brain. In the wild it survives in just one lake near Mexico City.

The axolotl (Ambystoma mexicanum) is a textbook case of neoteny (breeding without maturing): it skips metamorphosis and stays aquatic with external gills; given iodine it can turn into a land salamander. It is the champion of regeneration — regrowing legs, tail, jaw, spinal cord and parts of the heart and brain without scars; its genome is 10 times the size of ours (32 billion base pairs), sequenced in 2018 — a key for medicine. In the wild it survives only in the Xochimilco canals of Mexico City: 6,000 per km² in 1998, 35 in 2014, due to pollution and introduced fish. Millions live in labs and aquariums (mostly descended from 34 axolotls from 1863). To the Aztecs it embodied the god Xolotl; it appears on the 50-peso banknote.

Fun facts

• An axolotl regrows a lost leg — and even part of its brain.
• An axolotl's genome is 10 times bigger than a human's.
• 35 left in the wild, millions in aquariums — a strange fate.