Neoteny
Axolotls exhibit a property called neoteny, meaning that they reach sexual maturity without undergoing metamorphosis. Many species within the axolotl's genus are either entirely neotenic or have neotenic populations. In the axolotl, metamorphic failure is caused by a lack of thyroid stimulating hormone, which is used to induce the thyroid to produce thyroxine in transforming salamanders. The genes responsible for neoteny in laboratory animals may have been identified; however, they are not linked in wild populations, suggesting artificial selection is the cause of complete neoteny in laboratory and pet axolotls.
Neoteny has been observed in all salamander families in which it seems to be a survival mechanism, in aquatic environments only of mountain and hill, with little food and, in particular, with little iodine. In this way, salamanders can reproduce and survive in the form of smaller larval stage, which is aquatic and vegetarian and requires a lower quality and quantity of food compared to the big adult, which is a terrestrial carnivore. If the salamander larvae ingest a sufficient amount of iodine, directly or indirectly through cannibalism, quickly begin the metamorphosis and transform into bigger terrestrial adults, with higher dietary requirements.. In fact, in some high mountain lakes also live dwarf forms of salmonids, caused by deficiency of food and of iodine, in particular, which causes cretinism and dwarfism due to hypothyroidism, as it does in humans.
Unlike some other neotenic salamanders (Sirens and Necturus), axolotls can be induced to metamorphose by an injection of iodine (used in the production of thyroid hormones) or by shots of thyroxine hormone. Another method for inducing transformation, though one that is very rarely successful, involves removing an axolotl in good condition to a shallow tank in a vivarium and slowly reducing the water level so that the axolotl has difficulty submerging. It will then, over a period of weeks, slowly metamorphose into an adult salamander. During transformation, the air in the vivarium must remain moist, and the maturing axolotl sprayed with a fine mist of pure water. The odds of the animal being able to metamorphose via this method are extremely small, and most attempts at inducing metamorphosis lead to death. This is likely due to the strong genetic basis for neoteny in laboratory and pet axolotls, which means that few captive animals have the ability to metamorphose on their own. Artificial metamorphosis also dramatically shortens the axolotl's lifespan if it survives the process. A neotenic axolotl will live an average of 10–15 years (though an individual in Paris is credited with achieving 25 years), while a metamorphosed specimen will scarcely live past the age of five. The adult form resembles a terrestrial Mexican Tiger Salamander, but has several differences, such as longer toes, which support its status as a separate species.
Read more about this topic: Axolotl