Lakes, streams and aquifers across North America are crawling with salamanders of all shapes and sizes. For most people, though, finding a 2-foot-long amphibian in the eastern United States might sound like a tall tale. 

But giant salamanders are more common than you might think. The eastern hellbender (Cryptobranchus alleganiensis), which can grow up to 29 inches long, is the largest salamander native to North America, and the fourth largest in the world. And unlike most salamanders, it spends its entire life in the water. 

Yet how did these giant salamanders evolve to grow several feet long when most of their kin only reach 4 to 6 inches in length? This question has long fascinated scientists — and the answer could lie in enormous amphibians’ evolutionary history.

While they share many genetic similarities with other amphibians, giant salamanders like the hellbender have developed unique genetic adaptations that allow them to be primarily aquatic. Some experts say these adaptations are what led them to grow as gigantic as they are today. 

“You have these large-bodied animals that live in permanent aquatic stream-like environments and do quite well there,” says Dave Weisrock, a biology professor at the University of Kentucky who has spent years studying hellbenders in their natural habitat. “But it’s difficult for us to know exactly how and why that evolved.” 

Giant Salamanders Don’t Fully Metamorphose 

Most amphibians have biphasic life cycles, meaning they start out as water-dwelling larvae and eventually undergo metamorphosis, swapping their gills and tailfins out for legs and lungs. (Imagine, for instance, a tadpole growing into a frog.)  

But giant salamanders like hellbenders don’t undergo a full metamorphosis. They’re known as paedomorphs — amphibians that maintain many of their aquatic survival features from the larval stage into adulthood. This means that all three species of giant salamander are primarily water-dwelling. 

Paedomorphs still undergo some changes that mark a transition from the larval stage into adulthood. However, the hormones that normally regulate metamorphosis in most amphibians are expressed differently in paedomorphs, leading these types of amphibians to grow in unexpected ways. 

“There’s all of these traits, these genes, that they would normally use for metamorphosing, and all that stuff can either be lost, or it can be used for making them into a better aquatic organism,” says Ron Bonett, a biology professor at the University of Tulsa who studies evolution in salamanders. “So we get all these really weird shapes that pop up in paedomorphic salamanders that we don’t see normally in other types of salamanders.” 

Several studies have confirmed an association between paedomorphism and “unusually large body size.”  While the trend isn’t seen in all paedomorphs, the four biggest salamander species — hellbenders, Chinese giant salamanders, South China giant salamanders and Japanese giant salamanders — all follow this developmental pattern.  

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Adapting to Thrive in Oxygenated Streams 

Instead of a full metamorphosis, paedomorphic salamanders undergo partial changes as they mature from larvae into adults, which Bonett and his team dubbed post-embryonic transitions in a 2021 study. Much like metamorphosis, these changes are spurred by spikes in key growth hormones.  

Hellbenders are a prime example of how post-embryonic transitions play out. During the first two years of life, hellbenders develop short, stubby limbs, and their gills gradually shrink into two small indentations that serve as gill openings, allowing them to remain in their underwater habitats into adulthood. 

They also develop a unique method of respiration: hellbenders absorb oxygen from the water around them through capillaries. Adults have folds in their skin to maximize surface area for oxygen absorption.  

Because of this, hellbenders need cold freshwater environments with highly oxygenated water. The two species of hellbender, Eastern and Ozark, live in freshwater streams in the Appalachian and Ozark mountains, respectively. They can often be found hiding beneath large, flat rocks on the riverbed in order to avoid predators, hence their distinctive flat head and body shape. 

“Hellbenders — they’re big. They’re probably a top predator within the system themselves,” Weisrock says. “And so that large size, at least as an adult, it’s a good form of predator deterrence.” 

Read More: 6 Unusual Traits of Animal Evolution

How Did Giant Aquatic Salamanders Evolve? 

Avoiding predators, though, is just one hypothesis about why hellbenders evolved into the marine mammoths that we see today. Experts are still exploring how some salamanders have evolved into paedomorphs — and how paedomorphism has given rise to so many giant salamander species. 

According to Bonett, metamorphosis can expend a lot of energy. For some amphibians, it may have been more advantageous to focus this energy on surviving in an aquatic environment, especially if they lived in regions with plenty of waterways. 

“Aquatic environments tend to be more productive, so stable, productive environments — that kind of drives the switch,” he says. 

Resource availability can also be a big factor for paedomorphs. Some salamanders may become paedomorphic because there are more resources available in aquatic environments, while others evolve this way out of necessity due to resource scarcity on land. 

Read More: Evolutionary Insight: Inside the Brains of Reptiles and Amphibians

Paedomorphism and Rapid Evolution in Salamanders

Paedomorphism, Weisrock says, can evolve fairly quickly based on specific, hyperlocal environmental pressures that salamander populations face. Even today, the genetic makeup of hellbenders is constantly changing. Weisrock’s team recently discovered that there could be four or more distinct species of hellbender living in North America, rather than just two. 

But there’s still no singular answer to this question. For now, these answers remain buried deep within the evolutionary history of these slimy stream-dwellers. 

“We’re really trying to understand the balance between isolation and gene flow, and how that has played out within the system, so that we can better understand basic questions about (hellbender) species diversity,” says Weisrock. 

Read More: Reptiles And Amphibians Could Hold The Secret To Longevity

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