With lidless, beady eyes, pale pink skin, feathery gills branching from its neck like soft coral, and a seemingly fixed, disconcertingly human-like smile, the axolotl looks more like an alien than your typical salamander. Unsurprisingly, this strange Mexican amphibian has intrigued and fascinated people for centuries, from the ancient Aztecs to modern-day scientists.
Prior to the Spanish conquest of the Aztec Empire, axolotls were far more common than they are today. The builders of the original Mexican cities were sometimes paid in axolotls for they were considered a delicacy, supposedly tasting like eel. The fat of the amphibian was also used as a medicine similar to cod liver oil – and even today, when stirred into a syrup, axolotl is used as a folk remedy for respiratory ailments.
In the 16th century, Spanish naturalist Francisco Hernandez was the first European to write about the axolotl. In his published report on the natural history of Mexico, he called it the ‘edible tadpole’ and the ‘ludicrous fish’. But he also gave it, almost as an afterthought, an Aztec name, ‘axolotl’, which has remained in use, in Mexico and elsewhere, to the present day. There are two explanations for this name. One connects it to Xohotl, an Aztec god associated with fire, lightning and death, the twin brother of the famous feathered serpent god Quetzalóatl. Another theory says the name comes from atl and xolotl, the words for ‘water’ and ‘dog’ in the Aztec language. The two explanations are not as disparate as they first seem, for the god Xohotl was commonly depicted as a monstrous dog and his spirit animal forms were said to be the Mexican hairless dog and the axolotl itself.
Today, the axolotl is a highly favoured study vertebrate in the laboratory due to its amazing healing and regenerative abilities. It can regrow not only limbs, but also its jaw, spinal cord, heart, and even parts of the eye and brain – all without any evidence of scarring. It can receive transplanted organs from other individuals and accept them without rejection, and is a thousand times more resistant to cancer than most mammals. The axolotl has the added scientific attraction of having especially large embryos, making it easier to use for research purposes, especially in the field of developmental biology.
Probably the most unusual feature of the axolotl, however, is that it does not develop into its mature form. Instead of growing well-developed lungs and taking to the land like most salamanders do as they transition from tadpole to adult, the axolotl remains gilled and aquatic, never venturing out of water. It is the Peter Pan of the amphibian world.
The phenomenon whereby an animal is able to reproduce before it has fully transformed into an adult is known as neoteny. A few other salamanders are also capable of doing this. A close relative of the axolotl known as the tiger salamander will, under normal conditions, metamorphose from a gilled tadpole into a land-based adult in the usual fashion. But if there is a very wet season, if the lake in which it lives does not dry up, a tiger salamander larva may keep its feathery gills and remain in the water. Even though it stays in a juvenile-like form, this salamander can still become sexually mature and breed.
The axolotl, however, has eliminated its adult form almost entirely. The change from larva to adult is triggered by several hormones, particularly thyroxine, which is produced by the thyroid gland. Perhaps the conditions in the lakes in which the axolotl historically dwelled were such that this gland did not develop properly, so it never received the hormones needed for metamorphosis. When French naturalist Georges Cuvier examined axolotl specimens sent to him from Mexico in 1800, he decided that they were the larval form of an unknown air-breathing species of salamander. The truth that the axolotl is perfectly capable of reproducing despite its juvenile appearance wasn’t discovered for another 60 years.
Once upon a time, the neoteny of the axolotl may have been a survival strategy that allowed it to stay in the relative safety of the water for its entire life without ever having to step foot into a risky, predator-filled terrestrial environment. But now that neoteny is a huge disadvantage. The Mexican lakes that these salamanders once thrived in have been drained and polluted, and the axolotls, restricted as they are to a wholly aquatic lifestyle, cannot escape from them and colonise new habitats.
Today, over 99% of the world’s axolotl population exists in captivity. They are found primarily in laboratories and zoos, but because the species is relatively easy to keep and breed, it has also become a popular pet in recent years. Indeed, its captive leucistic and albino forms are much better known than its wild, rarer, fully pigmented variety (seen in the image above).
Most captive-bred individuals alive today are descended from just six individuals that were shipped from Mexico City to Paris in 1863. They still showcase neoteny, but if you add some thyroxine to a tank in which an axolotl is kept, it will lose its external gills, climb out of the water, and adopt a more terrestrial life. In this rarely-seen mature form, the axolotl greatly resembles an adult tiger salamander .
The axolotl may be ubiquitous in captivity today, but it is vanishingly rare in the wild. Now considered Critically Endangered, it lives only in Lake Xochimilco in Mexico City, which is but a pale shadow of its former self, comprised of just a few polluted canals and reservoirs. The loss of its habitat, coupled with the introduction of non-native fish such as carp and tilapia, which eat axolotl young and eggs, has driven this remarkable animal to the very edge. This means that despite its ‘smile’, the axolotl has a lot to be unhappy about.
 Another salamander, the mudpuppy, which lives further north than the axolotl, in the United States, has irrevocably reverted to its larval form. This species cannot be forced to mature; its body can detect the thyroid hormone, but for some reason the treatment does not affect the genes that control metamorphosis.