The Axolotl is a fascinating creature for a number of reasons, including its grotesque appearance, its ability to regenerate, and primarily the fact that it exhibits the phenomenon known as neoteny. Ordinarily, amphibians undergo metamorphosis from egg to larva (the tadpole of a frog is a larva), and finally to adult form. The Axolotl, along with a number of other amphibians, remains in its larval form throughout its life. This means that it retains its gills and fins, and it doesn’t develop the protruding eyes, eyelids and characteristics of other adult salamanders. It grows much larger than a normal larval salamander, and it reaches sexual maturity in this larval stage. Another term to describe this state is “perennibranchiate”. The animal is completely aquatic, and although it does possess rudimentary lungs, it breathes primarily through its gills and to a lesser extent, its skin.
It is generally accepted that neoteny is a “backward” step in evolution, because the Axolotl is descended from what were once terrestrial salamanders, like the closely related species, the Tiger Salamander, Ambystoma tigrinum and Ambystoma mavortium spp. (in fact, one likely theory suggests that the Axolotl is in fact a Tiger salamander off-shoot, as it can interbreed with that species with some success). Through some quirk of nature, a neotenous form developed and, probably due to environmental conditions, prospered. Neoteny is sometimes found in other amphibians, but tends to be caused by low levels of iodine (an essential element for animals to make thyroxine hormones, necessary for growth and development), or possibly by random genetic mutation. Research has also shown that very low temperatures can suppress the production of these hormones, thus also inducing neoteny.
In the Axolotl, neoteny is now totally genetic (click for more information on the Axolotl’s genetics). When treated with hormones, the axolotl will usually begin to metamorphose, but in very rare cases it will metamorphose spontaneously, such as the metamorphosed wild type axolotl pictured here. The metamorphosed wild type axolotl bears a close resemblance to the Mexican race of the Tiger Salamander, Ambystoma velasci.