A salamander the Aztecs believed was a god in disguise, now surviving almost nowhere but a shrinking set of canals in Mexico City — and possibly holding a key to regenerative medicine.
The axolotl is a salamander native only to the canals of Xochimilco in Mexico City. The Mexica believed it was the god Xolotl in disguise, hiding from sacrifice. It's since become a Mexican cultural icon, appearing on the back of the 2021 fifty-peso note, voted the world's best banknote design that year. Scientists study it for its ability to regenerate entire limbs, its spinal cord, and parts of its heart and brain without scarring, and for its unusually long, slow-aging life. It's also critically endangered: its wild population crashed by roughly 99 percent between 1998 and 2014, and fewer than 1,000 are now believed to survive in the canals where the Mexica first found it.
According to the priest Bernardino de Sahagún, who recorded Mexica belief in the Florentine Codex, the gods once decided that one of their own had to die so the newly made sun would have the strength to move across the sky. The god chosen was Xolotl, the twin brother of Quetzalcoatl, patron of fire, lightning, and the dead. Xolotl didn’t want to die. He hid first by turning himself into a double stalk of maize; found out, he turned into a two-part maguey plant. Found out again, he made his last attempt: he slipped into the water and became a salamander, the axolotl (ax-uh-LOT-ul in English; most Mexicans call it an ajolote, ah-ho-LOH-teh) — a creature named directly for him, āxōlōtl, roughly “water monster” or “water twin.”
The lake Xolotl supposedly hid in is long gone, but a fragment of it survives in the chinampa canals of Xochimilco, in the south of Mexico City, the only place on Earth the axolotl still occurs naturally. In 2021, the Bank of Mexico put one on the back of a redesigned fifty-peso note, alongside the canals and floating gardens it calls home; the front carries a scene from Diego Rivera’s mural of Tenochtitlan and the eagle-on-a-cactus symbol from the city’s own founding myth. The International Bank Note Society named it Bank Note of the Year, beating out entries from nineteen other countries. Mexicans have reportedly taken to holding onto the bill rather than spending it.
Long before the banknote, biologists had their own reasons to be fascinated. An axolotl can regrow an amputated limb, a severed piece of its spinal cord, or parts of its heart, jaw, and brain, and do it with no scarring, rebuilding the same tissue in the same arrangement it had before, sometimes more than once. Researchers finished sequencing its genome in 2018 and found it was the largest animal genome ever sequenced, roughly ten times the size of a human one. Despite regenerating tissue through the kind of rapid cell growth that would raise a red flag for cancer in almost any other animal, axolotls rarely develop tumors at all. They also barely age in any conventional sense: an axolotl never leaves its juvenile, aquatic form, a trait called neoteny, and it can live two decades without most of the physical decline that comes with getting older in other animals. Between the regeneration, the cancer resistance, and the neoteny, the axolotl has become one of biology’s most closely studied animals for anyone researching how to heal tissue, or simply how to age more slowly.
The species that scientists now study for its resilience is, in the wild, close to gone. Pollution, drainage, and invasive fish introduced into Xochimilco’s canals over the last few decades collapsed the wild axolotl population by roughly 99 percent between 1998 and 2014; recent surveys have put the number of wild individuals left at under 1,000, and some counts have found none at all in the canals proper, only in protected refuges. Researchers at UNAM have built small, filtered chinampa refuges within Xochimilco to keep a population alive and, in 2025, released a group of captive-bred axolotls into restored wetland habitat; all of them were still alive forty days later, a small but genuine milestone. Although this salamander is running out of water in which to hide, the axolotl still refuses the death assigned by the gods.
Sources: Bernardino de Sahagún’s Florentine Codex, as the primary account of the Xolotl myth; the International Bank Note Society’s 2021 Bank Note of the Year announcement; genomic and regeneration research on Ambystoma mexicanum; UNAM and IUCN reporting on wild axolotl population decline and chinampa refuge conservation efforts.
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