Like Peter Pan, the alpine newt sometimes refuses to grow up.
In their juvenile form, these alpine newts slurp up plankton in the icy mountain pools they call home, as high as 8,000 feet above sea level in the European Alps. A distinctive frilly crown of gills, used to filter oxygen from the water, marks them as larvae, the newt equivalent of what tadpoles are to frogs.
Like other salamanders, the vast majority of alpine newts wriggle through the life stages like clockwork. They trade their frilly gill fronds for lungs, crawl on land, and graduate to a diet of insects before their first winter.
But every generation, a handful of alpine newts simply opt not to leave their watery cradle. Life is good in the pond, so it would seem, and they hold onto their aquatic adaptations and continue to grow, while very much remaining larvae. These big babies are called paedomorphs, a term meaning "child form," and they fend off metamorphosis for months, years, or even a lifetime—but they can still reproduce.
Newts that pursue this life strategy are in danger, though, threatened by invasive species and shifting precipitation patterns associated with climate change. A more thorough study of these animals could help shed light on the intricacies of metamorphosis and the development of amphibians—which are also globally in decline. Unlike Peter Pan, however, researchers do not have forever. (Related: An exotic fungus is killing off amphibians.)
When it comes to sexual maturation for alpine newts, it’s not about your age—it’s about how fat you are. Since a plump newt is a successful newt, these salamanders’ gonads kick into action when they get close to a certain weight-by-length threshold, a kind of "newt BMI" cutoff for parenthood. A salamander’s version of “the talk” would be a lot weirder than ours.
A Different Mode of Life
It’s tempting to think of metamorphosis as an analogue for puberty in humans. But for many species of salamanders, physical and sexual maturity are “totally decoupled,” explains Mathieu Denoël, a biologist at the University of Liège and the world expert on alpine newts.
“Metamorphosis just allows the animal to change habitat—to a different mode of feeding, a different mode of life,” says Denoël, who also has an appointment with Belgium’s National Fund for Scientific Research. An alpine newt can begin producing eggs before or after she grows into an adult body—and she may live in a juvenile’s body forever. (Related: See the prehistoric-looking beetle whose sex life baffled scientists for centuries.)
Instead of puberty, we can think of metamorphosis as a hyper-speed version of the first years of human life. As a toddler grows, her torso catches up to her massive cranium and her legs become proportionately longer as she shifts from crawling to walking.
It’s not just that humans get bigger, it’s that the fundamental schematic of the body changes. That’s the part paedomorphs skip.
In human terms, imagine the proportions of a human baby—the bulbous head, the giant eyes, the stubby limbs—on a crawling, 180-pound man who subsists entirely on applesauce. As long as he can reproduce, that’s a paedomorphic alpine newt.
An Aquatic Insurance Policy
It’s hard to imagine how a paedomorph’s stunted development can turn out to be an advantage. But according to Francesco Ficetola, zoologist at the University of Milan, it’s actually a brilliant adaptation to scarce resources and an unpredictable environment.
Since a meal is hard to find high in the Alps, paedomorphs have edged their way into a major vacancy in mountain ecology that metamorphs—their siblings that relent to growing up—can’t occupy.
“In an isolated pond, it’s not connected to a river or a lake, so you don’t have fish” in many cases, says Ficetola. That means no aquatic predators and a whole watery world of vacant real estate.
Scientists think paedomorphosis may have evolved to reduce intra-species competition—it’s better for the whole family if siblings aren’t fighting over the same resources, in other words. And keeping part of every generation in the water ensures they’re ordering from two different menus.
Paedomorphs also seem to crop up where the terrestrial habitat is particularly challenging—say, an abundance of predators or a lack of suitable food. (See also: Ground zero of amphibian 'apocalypse' finally found.)
But in times of extreme scarcity, when filling your belly is hard both on land and in water, staying aquatic may be the best way to play it safe.
Metamorphosis requires a huge energetic investment—they’re dissolving and growing entire organs, after all—so hungry newts may not be able to afford to proceed to the next stage of life and still invest in sexual maturation.
Prioritizing reproduction over metamorphosis means paedomorphs act as a kind of insurance policy for catastrophic events, Denoël explains. “Paedomorphs can reconstruct the population quite fast, and their genes spread to the next generation more quickly than [those of] metamorphs,” since these animals can take two or three years to waddle back to a breeding pond.
After all, paedomorphosis shouldn’t be much of a gamble for the newts—if the pond dries up or conditions on land improve a few years down the road, they can still sprout some lungs and hustle aground. “Even very late in life, even close to death”—which doesn’t come until 20 years of age for some newts in the Alps—“there’s still the possibility of metamorphosis,” says Denoël.
Though the adaptive benefits of paedomorphosis are clear, we’re not sure what triggers paedomorphosis—or rather, suppresses metamorphosis. Denoël hypothesizes that a confluence of factors lead to paedomorphosis, from temperature and water level to population genetics and the density of metamorphs in a given pond. It’s a delicate equilibrium that’s very environmentally attuned and very much under siege.
Ponds in Peril
Alpine newts thrive in ponds and lakes across Europe, but some new neighbors have scientists like Denoël and Ficetola worried about the future of paedomorphosis.
While paedomorphosis is common in that it’s found across species of salamanders and newts, only about 100 populations of alpine newts exhibit the phenomenon. That’s much less than one percent of the tens of thousands of alpine newt communities in Europe, according to Denoël.
Sport fishers are introducing salmonid fish, such as trout, to those pristine waters. And as a paedomorph facing invasive species, Ficetola says, “You lose against fish.” Every time.
Paedomorphs are fully aquatic, so they can’t escape as their metamorph cousins can. It takes days to metamorphose when life in the water sours, too long for most paedomorphs to elude their new predators. “In all the lakes where fish have been introduced, paedomorphosis has disappeared totally.” (Related: Baby frogs have a super-speedy way to escape snakes.)
Ficetola points to climate change as another existential threat. It’s “changed the rhythm of precipitation,” meaning once-deep ponds that supported aquatic populations for decades now dry up after a single season, effectively evicting paedomorphs.
All this means the big babies are in trouble, Ficetola says. “There are areas in the Balkans where they’ve lost more than 95 percent of the paedomorphic population.”
“In general, amphibians are declining,” Ficetola adds, “but the paedomorphs are often much more threatened.”
For newts, growing up is clearly not the only sound move. But if people could, it would probably benefit us all, human and amphibian alike.