In 1864, French zoologist August Duméril was baffled while investigating the axolotl. Unlike many other amphibians, which transform into terrestrial adults, axolotls retain their juvenile characteristics and never leave the water. In an attempt to induce metamorphosis, Duméril spent months removing their gills. But in most instances, the axolotls simply... grew them back. Indeed, axolotls are masters of regeneration: they can flawlessly regenerate body parts ranging from amputated limbs and crushed spines to parts of their eyes and brains. So how do they do it? And what other secrets are they keeping?
This extraordinary salamander is native to the wetlands in Mexico City. Ancient Aztec people considered it the incarnation of a God named Xolotl— hence the axolotl’s name, roughly meaning “water monster.”
Axolotls reach sexual maturity with gills and a tadpole-like dorsal fin. Scientists think their forever-young condition, called “neoteny,” evolved because of their stable habitat. For salamanders that develop in waters that dry up, efficiently transitioning to land is essential. But the lakes axolotls evolved in were unchanging year-round and didn't host many aquatic predators. So, scientists think it was advantageous for axolotls to forgo the demands of metamorphosis. However, they haven’t completely lost this ability. If exposed to certain substances, axolotls will turn into adults. But they’ll often experience shorter lifespans and lose some of their self-healing abilities.
These regenerative talents may seem like crazy superpowers to begin with, but axolotls have good use for them. As babies, they’re in direct competition. So, they snack on each other. This is usually not a huge problem thanks to how quickly they can regenerate body parts. When an axolotl loses a limb, tissues stimulate growth in the area. Skin cells divide and cover the wound. Then, progenitor cells, which can develop into various bodily tissues, form a mass at the site of injury and nearby nerves secrete growth-promoting proteins. Over the next few weeks, a new limb emerges as cells proliferate and differentiate in coordination. This process could potentially lead to uncontrolled growth and tumor formation. But axolotls are remarkably resistant to cancer. They have a system in place that tightly controls cellular proliferation.
To better understand the axolotl’s baffling biology, scientists sequenced its genome. They found it to be more than ten times longer than a human’s. Mutations can change the length of any animal’s genome. For whatever reason, salamanders have much more DNA than other vertebrates because they lose parts of it less frequently. Investigating the axolotl’s genome, scientists saw many repeated sequences, most of which don't code for proteins and have no known function. They also found genes that are key in regeneration. However, the biggest factor that sets axolotl regeneration apart may not be a set of unique genes, but how they regulate their genes.
It’s no wonder that axolotls are one of science’s most studied animals. But their population in the wild has plummeted. Hundreds of years ago, axolotls thrived under the Aztec capital. Within the surrounding lakes, Aztec people built islands called chinampas for growing crops. This highly productive form of agriculture created a vast system of canals, expanding the lake system’s shallow, sheltered habitat— the axolotls’ ideal environment. But when Spanish invaders arrived, they began draining the lakes. And even more water has been diverted in recent years. Today, the entire population of wild axolotls is found in just one place, Lake Xochimilco, where it’s threatened by pollution and invasive fishes.
People are working to regenerate the ecosystem and strengthen the 2,000 year old chinampa farming tradition. If interest grows, farmers could recover abandoned chinampas and support the local community— along with the axolotl. Eventually, the benefits of saving this salamander might be even greater. Scientists hope that one day we’ll be able to apply the axolotl’s masterful tumor suppression and regenerative abilities to the human body. Perhaps its secrets are the real reason for the slimy god monster’s smile.
