The fascinating axolotl, known for its external feathery gills and unique smile, often prompts a common question: do these amphibians eventually transform into lizards? This question arises from a misunderstanding of their biology and development.
Understanding the Axolotl
The axolotl, scientifically known as Ambystoma mexicanum, is a type of salamander and an amphibian. These aquatic creatures are native to the freshwater lakes and canals of Mexico City, particularly Lake Xochimilco. They possess distinctive physical characteristics, including three pairs of feathery external gills that protrude from their heads, a long tail, a flat head, and underdeveloped limbs. Axolotls reach an adult length of 15 to 45 cm, with 23 cm being common.
A defining feature of the axolotl is its state of neoteny, also known as paedomorphosis. This means they retain their larval features and remain fully aquatic throughout their entire lives, even as they reach sexual maturity and become capable of reproduction. While most amphibians undergo a metamorphosis from an aquatic larval stage to a terrestrial adult form, axolotls naturally bypass this transformation. They continue to breathe primarily through their gills, supplemented by their skin and rudimentary lungs.
Axolotl Metamorphosis: Fact vs. Fiction
Axolotls do not naturally transform into lizards. This means the axolotl remains an aquatic salamander, differing significantly from a lizard in its fundamental biology. The idea of them becoming lizards is a common misconception.
Under very rare and artificial circumstances, an axolotl can be induced to metamorphose. This process is triggered by exposure to specific hormones, such as thyroxine, or by severe environmental stress. For instance, experiments have shown that injecting thyroxine can reliably induce metamorphosis in laboratory settings. However, such induced metamorphosis is not a natural part of the axolotl’s life cycle and can be detrimental to its health in captivity.
When an axolotl does undergo induced metamorphosis, it transforms into a terrestrial salamander, not a lizard. During this transformation, the axolotl loses its external gills, its tail fin reduces, its skin thickens, and its limbs strengthen to support movement on land. The eyes also develop eyelids and become more prominent. The resulting terrestrial form resembles other land-dwelling salamanders, such as the tiger salamander, which shares a genus with the axolotl.
Amphibians, Reptiles, and the Lizard Question
The distinction between amphibians and reptiles is rooted in fundamental biological differences, clarifying why an axolotl, even one that has metamorphosed, cannot become a lizard. Amphibians, a class that includes salamanders, frogs, and toads, begin their lives in water as larvae and undergo metamorphosis to adapt to a more terrestrial adult life. Their skin is moist, permeable, and lacks scales, allowing for cutaneous respiration. Amphibians lay soft, unshelled eggs in water, and fertilization is external.
Reptiles, a distinct class that includes lizards, snakes, turtles, and crocodiles, have evolved to live predominantly on land. They possess dry, scaly skin made of keratin, which helps them retain moisture and protects them from the environment. Unlike amphibians, reptiles breathe exclusively with lungs throughout their lives and do not have an aquatic larval stage. Their eggs are hard-shelled or leathery, laid on land, and fertilization is internal. These biological classifications demonstrate that a salamander, despite any changes in its life cycle, remains an amphibian and cannot transform into a reptile like a lizard.