Cut off the leg of most animals and you get a stump, a scar and a permanent disability. Cut off the leg of an axolotl and, within weeks, it grows a new one — bones, muscle, nerves and skin, perfectly formed, as if nothing had happened. Do it again and it will regrow the limb again. This pink, perpetually smiling Mexican salamander is one of the most remarkable regenerators in the animal kingdom, and it is also, biologically, a creature that never quite finishes being born.
The animal that stays a baby
Most amphibians live a double life. They begin in water as larvae — think of a tadpole, with gills and a tail fin — and then undergo metamorphosis, reabsorbing the gills, growing lungs, and crawling onto land as adults. The axolotl skips the second act. It reaches full size and becomes sexually mature while still in its larval body: it keeps its feathery external gills, which fan out from its head like a fringe, keeps its tail fin, and stays in the water its entire life. This condition — becoming a reproducing adult while retaining juvenile features — is called neoteny, and the axolotl is its most famous example.
The trigger for metamorphosis in amphibians is the thyroid hormone. Axolotls have the machinery but do not normally release the hormonal surge that would drive the change. Curiously, if given thyroid hormone artificially, an axolotl can be forced to metamorphose into a land-dwelling salamander resembling its close relatives, the tiger salamanders — losing its gills and its charm in the process. Left to itself, it never does.
Regrowing the irreplaceable
It is the regeneration that makes biologists lean in. Axolotls can regrow entire limbs, their tail, portions of the jaw, sections of the spinal cord, and even parts of the heart and brain — regenerating complex structures that in mammals would simply scar over. When an axolotl loses a leg, cells near the wound revert to a more flexible, immature state, forming a mass called a blastema that then rebuilds exactly the tissues required, in the right order and proportion. Perhaps most striking of all, they do this without the scarring that in mammals blocks true regrowth, and they are notably resistant to cancer despite all that furious cell division.
Understanding how could be transformative for medicine, which is why axolotls are among the most intensively studied animals in laboratories worldwide. In 2018 scientists sequenced the axolotl genome and found it colossal — roughly ten times the size of the human genome, one of the largest ever assembled at the time — which is part of why the work took so long. Somewhere in that vast genome lie the instructions for rebuilding a limb from scratch, instructions our own bodies seem to have switched off far back in evolutionary history.
Extinct where it belongs
There is a bitter irony at the heart of the axolotl's story. This creature, bred by the tens of thousands in laboratories and kept as a pet in aquariums around the world, is critically endangered in the wild and may be functionally extinct there. Axolotls are native to just one place on Earth: the network of lakes and canals around Mexico City, principally the ancient waterways of Xochimilco, remnants of the vast lake system on which the Aztec capital of Tenochtitlan was built. Urban sprawl, drained and polluted water, and introduced predatory fish such as carp and tilapia have devastated the wild population, which surveys suggest has collapsed to a tiny fraction of its former numbers.
So the animal thrives everywhere except home. It even carries an echo of the culture that once surrounded it: the name axolotl derives from the Aztec god Xolotl, associated with fire, lightning and the underworld, who in myth was said to have transformed himself into a salamander to escape death — a fitting legend for a creature that cheats bodily damage so readily.
The axolotl is a bundle of paradoxes: an eternal infant that reaches adulthood without growing up, a master of self-repair that cannot repair its own habitat, a wild rarity that is nonetheless one of the most abundant amphibians in captivity on Earth. It stares out of the tank with that permanent, guileless smile, waving its feathery gills, apparently untroubled by the fact that it can regrow almost anything except the lake it came from.
Quiz nuggets
- The axolotl can regrow entire limbs, its tail, parts of the spinal cord, jaw, heart and brain, rebuilding tissue via a structure called a blastema.
- It exhibits neoteny — reaching sexual maturity while keeping juvenile features such as external gills and staying aquatic for life.
- Given thyroid hormone, an axolotl can be forced to metamorphose into a land salamander resembling a tiger salamander.
- Its genome, sequenced in 2018, is roughly ten times the size of the human genome.
- Axolotls are native only to the lakes and canals around Mexico City, especially Xochimilco, and are critically endangered in the wild; the name comes from the Aztec god Xolotl.