The grin has not been photoshopped onto this juvenile axolotl. What kind of monster, after all, would impose emotions on an innocent salamander? Photo: Jan-Peter Kasper/Corbis
BY MATT SIMON
In medieval bestiaries, the salamander was a mythical creature of great power, capable of crawling leisurely through fireand even extinguishing it with its body. Leonardo da Vinci, in fact, believed that it even ate flames, though this is unlikely. Everyone knows fire is low on vitamins.
Unbeknownst to these Europeans, on the other side of the world there lives a salamander with powers far stranger than resistance to fire. It’s the axolotl of Mexico, a bizarre amphibian capable of regrowing limbs and even its spine with ease. Scientists have for decades struggled to unlock its regenerative secrets, all while our environmental meddling has nearly driven it to extinction.
Mexico City’s expansion, with its pollution and overfishing and incursions of invasive species, has virtually destroyed the salamander’s native lakes, which now exist as scant canals. A recent three-month survey there turned up zero axolotls, though a new survey currently in progress may have more luck.
But where the axolotl is actually flourishing is in labs, like that of biologist Randal Voss at the University of Kentucky, who studies the critter’s genetics. In particular, Voss looks at the dynamics of the salamander’s neoteny: its ability to indefinitely postpone metamorphosis–the type of transformation that turns tadpoles into frogs or caterpillars into butterflies–yet still reach sexual maturity. Thus the axolotl is the world’s most awkward-looking teenager.
While the axolotl’s natural hue is quite dark, almost black, scientists can breed them into a range of colors. The one at center looks forlorn because it lost its pigment sometime around birth, and has been unable to find it.Photo: Courtesy of Lee Thomas
Whereas other salamanders ditch their frilly external gills in favor of lungs and head for land, the axolotl plays it safe with the life aquatic. It not only holds on to the gills, it also develops lungs and can absorb oxygen through its skin.
So, why cling to youth? “The one thing that neotenic species have as an advantage is that if you don’t undergo this metamorphosis, you’re more likely to reproduce sooner,” said Voss. “You’re already one step ahead.”
An axolotl pair begins such pursuits with a sort of dance, nudging and pushing while the male releases pheromones(think of them as really effective colognes, à la Sex Panther) from his cloaca, a kind of all-purpose digestive and reproductive opening. As the female kicks these up, the male drops a rather odd-looking spermatophore.
Because the axolotl never truly metamorphoses, it retains its frilly larval gills. It has lungs too. And can breathe through its skin. Scientists therefore reckon that the axolotl is hopelessly addicted to oxygen, more so than most critters. Photo: Courtesy of Lee Thomas
“It looks just like a Hershey kiss,” said Voss. “It has a sperm packet on the very tip, and after laying that down he’ll walk up the body length, stop, and she’ll find it and sit down on it and break off the sperm cap. And that’s how she picks up the spermatophore.” Once fertilized, the female can lay more than 1,000 eggs, which hatch into larvae that will, alas, inevitably eat each other. (They should consider themselves lucky, though: Sand tiger shark young actually eat each otherin the womb. I mean, at least give each other a chance.)
But the axolotls that emerge settle into life as adept bottom-dwelling predators. The axolotl hunts just like its distant cousins, the 6-foot giant salamanders of Asia, quickly firing open its maw to create a vacuum that pulls in small prey. And they can be quite voracious–in captivity axolotls feeding on clouds of brine shrimp have been known to nip off the limbs of their peers.
‘Tis But a Scratch
But like the Black Knight in Monty Python and the Holy Grail shrugging off the involuntary removal of his limbs, the axolotl can’t be bothered with mere flesh wounds. Incredibly, it can regrow entire arms, a tall order when you consider the complexity of such structures. Pretty magical, right?
Not quite. According to biologist David Gardiner, whose lab at the University of California, Irvine is working to unravel the axolotl’s regenerative secrets, this critter isn’t that special after all. There are no unique tissues or structures making this possible. We humans have the same signaling molecules, as do fish and chickens, because long, long ago we shared a common ancestor that passed down these genes.
“We’re talking about very fundamental biological processes,” said Gardiner. “It’s not that they have special genes, it’s how they use those genes. It’s how they regulate them. That’s what’s special. And that’s how they regenerate and we don’t.”
It’s important to keep in mind that we humans regenerate just fine. Get a sunburn and your skin cells flake off to reveal new ones. Develop a scab and peel it away, and there’s your brand new tissue. And of course our livers readily regenerate, as poor Prometheus would attest if he wasn’t so busy having his plucked out by an eagle day in and day out.
The axolotl, however, doesn’t develop scar tissue at all. Instead, its unremarkable genes turn on or off in a remarkable sequence of steps that lead to the total regeneration of, say, an arm. Human regeneration actually begins to follow the same series of steps, but something hangs up along the way. Gardiner isn’t sure yet how many steps there are exactly, it could be 15 or 20 or 50, but he notes that we’re obviously failing somewhere early in the regeneration process.
“So if we make a choice somewhere around step three or four,” he said, “then the next 15 steps are irrelevant, because we do not go down that pathway. And we don’t know whether we can do them or not.”
“My sense is that we probably can do almost everything you need to do to regenerate,” he added. “Because we already do. I mean, most of the tissues regenerate–blood vessels and nerves and epithelial cells and muscle. The tissue that doesn’t regenerate is connective tissue, and that’s where we make scars.”
These connective tissue cells are what Gardiner focuses on in his axolotl research, because they’re the ones that manage the behavior of all other cells. “I like to think about it as the orchestra and the conductor,” he said. “Everything is there, but if there’s no conductor, the orchestra can’t make music.”
Gardiner thinks that it’s almost a certainty that humans will eventually be able to regenerate complex structures like limbs. It’s a long ways off though. We’re nowhere near to understanding how exactly the axolotl regenerates, much less figuring out how to give humans such powers. But say we do get there one day. Gardiner argues that what matters most with something like an arm is function, not form.
Just because our limbs look the way they do, doesn’t mean that’s the best way to do it. Sure, millions of years of evolution have adapted us to our environments, and that can’t be wrong. But what if we can hack the regeneration process to produce upgraded limbs, just like scientists rebuilt the Six Million Dollar Man? Or what if our new and improved appendages looked radically different?
“As we begin to acquire the ability to replace body parts,” said Gardiner, “then we’re going to have to ask ourselves that question, about do we necessarily want to put them back the same way that they were. And that raises some moral, ethical questions. Keep in mind there are people who believe we are destined to look this way. I don’t share that belief.”
So it is that a creature we’ve damn near wiped off the planet could end up revealing more about us–both physically and philosophically–than we could have imagined. Give ‘em a hand, ladies and gentlemen. But don’t expect to grow it back just yet.