When the Aztecs settled the Valley of Mexico in the 13th century, they found a large salamander living in the lake surrounding the island where they built their capital, Tenochtitlán. They called the salamander "axolotl" after Xolotl, their god of fire and lightning. Xolotl was said to have transformed into a salamander, among other forms, to avoid being sacrificed so the sun and moon could move in the sky. He was eventually captured and killed.
In the same vein, axolotls were commonly killed for food by the Aztecs and are still eaten today in Mexico. They've also become one of the world's most popular pets, thanks to their easy care and charisma. The creatures' extraordinary regenerative abilities have made them an interesting study subject for scientists. But in their native home, the salamanders have almost disappeared.
Axolotls (Ambystoma mexicanum) are amphibians belonging to the single living genus of the family Ambystomatidae. There are more than 30 salamander species in the Ambystoma genus, known as the mole salamanders.
Axolotls can grow on average to a length of 9 inches (20 centimeters), but some have grown to more than 12 inches (30 cm) long. In captivity, the salamanders live on average for 5 to 6 years, but some have lived for up to 17 years, according to the University of Liverpool's The Animal Ageing and Longevity Database.
Where do axolotls live?
Wild axolotls live exclusively in the swampy remnants of Lake Xochimilco and the canals leading to it on the southern edge of Mexico City. Axolotls once also lived in Lake Chalco, another of Mexico City's five "great lakes" where the ancient Aztecs settled. But all of those lakes, except for Xochimilco, were drained by the 1970s to prevent flooding and allow urban expansion, NBC News reported.
Axolotls' carnivorous diet historically put them at the top of the food chain. They grab anything they can snatch: Mollusks, fish and arthropods like insects and spiders. They even eat each other. In the 1970s and 1980s, though, the United Nations Food and Agricultural Organization introduced tilapia and carp fish to the salamander's habitat to provide local people with more protein, according to a JSTOR Daily article. Those fish chow down on young axolotls and are an invasive threat to the salamanders.
The axolotl mating dance
Axolotl reproduction starts with dancing — literally. After a male and female nudge and stroke one another's urogenital opening, called the cloaca, the salamanders step in a circle in a sort of waltz, according to the University of Michigan's Animal Diversity Web. The male then struts away while shimmying his tail like a hula dancer, luring the female to follow. As the two dance partners step together, the male drops a small white capsule full of sperm called a spermatophore. With the female in tow, the male moves forward until the female just skirts over the spermatophore and picks it up with her cloaca.
Axolotls go through this courtship once a year, typically from March to June. With the courtship dancing behind her, the female axolotl will individually attach her 100-300 jelly-coated eggs on aquatic plants or rocks. Around 10 to 14 days later, the eggs hatch, and the young fend for themselves. It takes about a year for axolotls to become sexually mature.
Unlike most amphibians, axolotls never metamorphose into lung-breathing, terrestrial adults. American evolutionary biologist Stephen Jay Gould described the salamanders as "sexually mature tadpoles" because they forever retain their juvenile characteristics: A fully aquatic lifestyle, a finned tail and frilly gills. This evolutionary phenomenon of "everlasting youth" is called paedomorphosis, or neoteny. Scientists can force axolotls in the lab to metamorphose by injecting them with thyroid hormones, but axolotl metamorphosis rarely occurs in the wild.
Are axolotls endangered?
The International Union for Conservation of Nature and Natural Resources considers axolotls critically endangered and their population declining. Surveys in 1998 and 2008 found that the population density had dropped from about 6,000 individuals per square kilometer to 100 individuals per square kilometer. A more recent survey in 2015 found about 35 individuals per square kilometer.
Pollution has been particularly detrimental to the species. Poor waste regulations and increasing tourism in Mexico City mean that trash, plastics, heavy metals and high levels of ammonia spilled from waste-treatment plants clog the canals where the salamanders live.
A substantial captive population exists in research labs around the world, accounting for several thousand individuals. But these salamanders stem from 33 individuals shipped to Paris from Xochimilco, Mexico, so the population is highly inbred.
Axolotls in research
Among the axolotl's trademark talents is its ability to regrow almost any body part — feet, legs, arms, tails, even bits of the heart and brain. And they don't stop with regeneration of their own body parts. All sorts of organs, including eyes, can be transplanted between axolotls without rejection by the recipient body's immune system. In 1968, researchers showed that they could even transplant the head of one axolotl to another axolotl, and it functioned normally. The combination of these abilities make axolotls attractive model organisms for scientists.
In 2018, researchers discovered another oddity about axolotls: Their genome is enormous. At about 32 billion pairs of DNA nucleotides, the axolotl genome dwarfs the human genome, which is about 10 times smaller, and ranks as the largest animal genome sequenced from beginning to end so far. Researchers are wading through the genome to uncover the secrets behind the axolotl's regenerative abilities.