The Amazon rainforest hosts the largest and heaviest (although not longest) snake species on Earth: the green anaconda. However, what was thought to be a single species covering a vast area has now been revealed as two, with a little help from Will Smith and an accompanying documentary crew.

Professor Bryan Fry of the University of Queensland did not set out to untangle the anaconda family tree. Instead, he is undertaking a long-term study of the effects of oil drilling in the Amazon in collaboration with the Indigenous Waorani people.

“When you look at the male and female anacondas you wouldn’t think they were the same animal, let alone the same species,” Fry told IFLScience. The legendary beasts that grow beyond 5 meters (16.4 feet) long (with unreliable rumors of much larger) are all females. The males are half the length and slimmer (no wonder some females can't be bothered). Not surprisingly, the two sexes have very different diets, with the females feeding on grazers like deer and capybara, while the males eat waders.

“As top predators, anacondas are especially vulnerable to habitat degradation,” Fry and colleagues write in a new paper. “Not only do they suffer from the damage to the habitat, they are also heavily impacted by the damage to their prey base.” The team reasoned that by comparing concentrations of heavy metals between the sexes, they could detect the consequences of oil spills in the ecosystem.

They also had some celebrity help in the process. Fry was scientific team leader for the Amazonian part of the National Geographic documentary Pole to Pole with Will Smith. This brought resources to the anaconda research that Fry told IFLScience was far above what he was used to – including assistance from Smith himself, who had no issues with wading into the water and helping capture anacondas.

The team learned that the males have 1,000 percent more lead and cadmium in their bodies than the females, Fry told IFLScience, reflecting the way these elements are infiltrating the aquatic ecosystem and making their way up the food chain.

In the process of the most extensive anaconda sampling ever conducted, they noticed the green anacondas of Ecuador are larger than those in Brazil. “We found one that was 6.3 meters (21 feet), which is close to the official record,” Fry told IFLScience. 

An even larger beast got away, after sinking its fangs into one of the team and trying to drag him into the water. The Waorani man who was bitten, and those who helped free him, estimated its length at 7.5 meters (24.6 feet). Based on the scars Fry confirms its jaws, and probably the whole snake, were much larger than any of those they captured. Despite greatly exaggerated rumors, across most of their range, even the largest females don't get far past 5 meters (16.4 feet).

The Ecuadorian snakes don’t otherwise look or behave differently, having almost identical markings. However, when the team took genetic samples, they found these were definitely not the same species – there is a 5.5 percent difference in DNA between the two. “To put it in perspective, humans differ from chimpanzees by only about 2 percent,” Fry noted in a statement. 

Further investigation revealed the larger species is also found in Colombia and Venezuela, earning the name northern green anaconda (Eunectes akayima) while the existing species E. murinus is to be renamed the southern green anaconda. Although there is some overlap in their territories, the two live in different river basins, following a geographical division that began 10 million years ago that is reflected in species divisions among many other Amazonian animals.

What science gives, it sometimes also takes away. In the paper announcing their findings Fry and co-authors also report that what were considered to be three distinct species of (smaller) yellow anacondas are local variations of the same species. 

Fry admitted to IFLScience that it was sad to be removing two species from the records, but added. “We have to be dispassionate and let the genetics tell the story […] we have to be consistent to maintain the credibility of the science.” 

[This is] the biggest achievement of my career. The only way it could be bigger is if I [scientifically] described a xenomorph that had just jumped out of Taylor Swift’s chest.

Prof. Bryan Fry

That credibility is important because, while the southern green anaconda is so widespread it is in no danger, the same may not be true for the northern version. Not only is their range smaller, but most of it is affected by oil drilling, along with widespread logging and the devastating drought the region is suffering. Fry is hoping to get back to the area to investigate if oil pollution is affecting male fertility, on top of everything else. 

Such a discovery would be terrible news for the health of the ecosystem, but might at least focus attention on the threat oil drilling poses to the Amazon. If so, it might partially repay the Waorani, who welcomed the expeditions into their land and shared their knowledge on anaconda capturing, with two Waorani collaborators becoming authors on the new paper. The Waorani are acutely aware of oil drilling’s devastating consequences to their lands; last year their long campaign succeeded in getting a national referendum passed banning drilling in parts of it, but implementation remains a challenge.

Contrary to reputation, neither anaconda is known to eat people – but Fry said they will sometimes kill those they perceive as a threat, so catching specimens is not a job to do alone.

The discovery represents one of the largest new species identified this century, although overshadowed by proof of a second African elephant species. “Usually if you want to discover a new species you go small,” Fry told IFLScience.

The combination of the size of the find, celebrity link, and environmental significance makes it “The biggest achievement of my career,” Fry said. That’s quite a statement, given Fry has found fish with opiates for venom, shown dragon venom could prevent strokes that could transform the fight against many diseases, and even shown humanity’s slow reaction time is a side-effect of evolving resistance to cobra venom.  “The only way it could be bigger,” he added to IFLScience, “is if I [scientifically] described a xenomorph that had just jumped out of Taylor Swift’s chest.”

The study is published open access in the journal MDPI Diversity

This article has been updated to include an amendment clarifying the sighting of the larger snake that got away.