Why Don't Axolotls Turn Into Salamanders? Neoteny Explained

What is neoteny?

Most amphibians have two very different life stages. A frog starts as a tadpole and a salamander as a gilled larva, both living in water. Then metamorphosis rebuilds the body for life on land: gills disappear, lungs take over, the tail fin shrinks and the skin and eyes change.

The axolotl stops partway. It grows to adult size and becomes able to breed, but it keeps its feathery external gills, its tail fin and its thin, water-loving skin. Keeping juvenile features into adulthood is called neoteny, or more broadly paedomorphosis (“child form”). An adult axolotl is not a baby; it is a fully mature animal in a larval-looking body.

Why does the axolotl stay in larval form?

In normal salamanders, a gland in the brain called the pituitary releases thyroid-stimulating hormone. This tells the thyroid gland in the neck to release thyroid hormones, mainly thyroxine, which switch on the genes that remodel the body.

In the axolotl, this chain of signals is too weak. Young axolotls make some thyroid hormone, but adults have almost no detectable levels. The axolotl’s tissues can still respond to the hormone, as experiments show, but the body never sends the signal strongly enough. Genetic studies have linked this to a small region of the axolotl’s DNA, and the trait is thought to have become fixed over many generations.

What changes as an axolotl matures?

Even without metamorphosis, an axolotl changes a great deal between hatching and adulthood. A newly hatched larva is only about 1 cm (0.4 in) long, has no legs and lives on tiny water animals. Its front legs appear within a few weeks, followed by the back legs, and it gradually grows into the familiar adult shape.

As it matures, usually at about one to two years old, its lungs become larger and more useful, and its reproductive organs develop so that it can breed. Males develop a swollen cloaca in the breeding season, while females fill with eggs. The gills, tail fin and lidless eyes, however, stay exactly as they were in the larva. The axolotl also keeps growing slowly throughout its life instead of stopping at a fixed adult size.

Why would staying in the water help?

Neoteny is common in mole salamanders that live in cold, high-altitude lakes surrounded by harsh land. The axolotl’s home, the lakes of the Valley of Mexico at about 2,240 m (7,350 ft), fits this pattern.

For an axolotl, the lake offered steady temperatures, plenty of food and few predators, while the surrounding land was dry and cold for much of the year. An animal that could breed in the water without the risky, energy-hungry change to a land form had an advantage. Over time, natural selection favored axolotls that never left the water.

Can an axolotl ever metamorphose?

Yes, but almost only when people make it happen. The first recorded case was a surprise: in 1865, French zoologist Auguste Duméril found that some of the axolotls sent from Mexico to Paris had changed into land salamanders. Biologists of the time had thought the axolotl and the land form were two different animals.

In 1913, Czech scientist Vilém Laufberger showed that thyroid treatment could trigger the change, and English biologist Julian Huxley did the same a few years later. Today laboratories can reliably induce metamorphosis with thyroid hormones. The axolotl loses its gills and fins, grows eyelids and thicker skin, and starts breathing only with its lungs.

Metamorphosis is a heavy strain on the body, and axolotls that go through it usually live shorter lives and regenerate injuries much less well. That is one reason scientists study how the axolotl regenerates mainly in normal, unchanged animals.

Which other salamanders never grow up?

The axolotl is the best-known example, but not the only one:

The difference is that mudpuppies, sirens and the olm cannot be made to metamorphose with thyroid hormone. The axolotl and other mole salamanders still can, because their bodies have kept the machinery for change.

How is the axolotl related to the tiger salamander?

The axolotl’s closest relatives are the tiger salamanders, which belong to the same genus, Ambystoma. Their larvae look so similar that tiger salamander larvae are often wrongly sold or described as “axolotls”. A metamorphosed axolotl looks much like an adult tiger salamander, though with longer toes.

The two can even interbreed in captivity. Tiger salamander genes have been crossed into some laboratory axolotl lines in the past, which is one reason captive axolotls differ from wild ones. To learn how salamanders differ from lookalikes, see newt vs salamander.