The world’s heaviest snake has been hiding a big secret

The world’s heaviest snake has been hiding a big secret.

According to a study published today in the open-access journal MDPI Diversity, the animal known as the green anaconda, or Eunectes murinus, is actually two genetically distinct species. This despite each species looking so similar that even experts can’t tell them apart.

“Genetically, the differences are massive,” says Bryan Fry, a National Geographic Explorer, biologist at the University of Queensland in Australia, and coauthor of the new study.

“They’re five-and-a-half percent different, genetically. Now, to put that into context, we’re about two percent different from chimps,” he says.

To make the shocking discovery, Fry and his coauthors collected blood and tissue samples from green anacondas in Ecuador, Venezuela, and Brazil, a process documented exclusively by National Geographic for their upcoming Disney+ series, Pole to Pole With Will Smith. The study authors also examined each animal closely to count scales and look for other physical traits that could signal an evolutionary divergence.

After running the genetic data, they found a clear divide between anacondas sampled in the northern part of the range as opposed to those in the south. And based on those findings, they propose renaming the snakes found in the north as the northern green anaconda (Eunectes akayima), while E. murinus will continue to refer to southern green anacondas.

Fry says his jaw dropped when the analyses completed.

“I didn’t expect that level of divergence,” he says. “It’s just mind-blowing. We were all of us very much doing happy dances.”

While it may seem like splitting hairs to re-classify two populations of snakes that look identical to one another, Fry emphasizes how important such delineations can be for understanding the threats posed to these creatures. Right now, the International Union for Conservation of Nature classifies the green anaconda as a species of least concern when it comes to extinction risk—but this classification is based, in part, on how widely distributed a species is.

“It’s important, because the newly described northern green anaconda has a much smaller range than the southern, and so that means it’s much more vulnerable,” says Fry.

Up close and personal with giants

Working with giant serpents isn’t easy, but perhaps for different reasons than you’d think.

At weights of more than 500 pounds and recorded lengths of more than 29 feet, the largest anacondas are probably capable of killing and eating a human. However, such incidents have only been reliably documented in Asia, and with snakes known as reticulated pythons.

(Read more about a python that swallowed a woman whole in a rare attack.)

Still, there are other occupational hazards.

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While counting scales for his research, Fry says that, sometimes, the most informative areas are the ones closes to the snake’s nether regions. At the same time, anacondas are wont to release their bowels while being handled.

“When you’ve got a big anaconda, it can put about a liter and a half of funk all over you,” he laughs. “But you’re living the dream!”

Of course, the fact that anacondas are massive, powerful predators is just one reason the animals haven’t been more closely studied. They also spend most of their lives submerged in the murky waters of swamps, marshes, streams, and rivers.

However, more work of this kind may be needed to understand how the northern and southern green anacondas embarked on separate evolutionary tracks. After all, the two species appear to coexist in French Guiana, even as close as being found on opposite riverbanks, says Fry. And yet, there’s no evidence of interbreeding in their genetics.

Next, Fry would like to know if there are differences in the snakes’ genitals. This is because many snake species evolve structures that fit into each other like a lock and key. And when two species can no longer functionally mate with each other, they are more likely to continue diversifying.

“For every question you answer, seven more interesting ones pop up,” says Fry. “To me, that’s a sign of a good study, is that it raises as many new questions as it answers the old.”

Okay, now do yellow anacondas

“This is a very thorough study, and I full trust the genetic results,” says Wolfgang Böhme, an honorary staff member and senior herpetologist at the Museum Koenig in Germany in an email. “The discovery of the deep genetic split within the green anacondas is an important discovery.”

Interestingly, Böhme was less certain about another finding in the study—specifically that the three species of yellow anaconda (E. notaeus, E. deschauenseei, and E. beniensis) should be combined into one (E. notaeus).

This finding was based on the same sort of genetic work done with the green anacondas. However, in this case, Fry and his coauthors argued that the genetic differences between the three current species of yellow anaconda are not great enough to support separate species status.

The question of whether to lump or split is, to some extent, philosophical, says Fry.

“I’m very conservative, and I’m a lumper at heart,” he admits. “So for me, it fits with lumping the yellows together.”

Böhme remains unconvinced and thinks the move to combine yellow anaconda species is “premature”. However, he also has good reason to believe the discovery of E. beniensis as its own species, which only just happened in 2022, remains important in its own right.

“I admit that I am also a bit biased,” he says, “because I was much involved in the discovery of E. beniensis.”