Fact check: You can’t tell a venomous snake by the way it swims

A widely shared post on social media makes the claim that venomous snakes tend to move on the surface of water, while common water snakes dive beneath the surface. The post alleges that this difference is generally a good indicator of whether a snake is dangerous or not. This claim contains a mixture of accurate and inaccurate information.

Reuters Fact Check. REUTERS/Axel Schmidt

The post shows a what appears to be a copperhead snake moving on the surface of water. It is visible here .

Reuters contacted a few herpetologists, or reptile and amphibian experts, to address the veracity of this claim.

John Maerz, Professor of Vertebrate Ecology at the University of Georgia, told Reuters that all snakes can swim, and most swim below the water, or partially submerged. “Snakes may swim under water when fleeing a predator or to hunt,” Maerz wrote, “and species like cottonmouths do eat fish and frogs just like water snakes.”

In his book “Secrets of Snakes”, David Steen, Reptile and Amphibian Research Leader of the Fish and Wildlife Research Institute in St Petersburg, Florida, also writes that distinguishing between venomous and non-venomous snakes by the way they swim might not be a foolproof strategy. Steen points to the example of the diamond-backed rattlesnake, which is venomous and dangerous to humans. This rattlesnake is known to increase its buoyancy to cross water with most of its body staying dry. He notes that cottonmouth snakes, which are venomous and dangerous to humans, are also capable of doing this, despite often swimming underwater ( ).

Harry Greene, Emeritus Professor in the Department of Ecology and Evolutionary Biology at Cornell University, told Reuters via email that he would not want to generalize for the more than 3,500 snake species worldwide, nor even for all the venomous snakes in the world.

But sticking to the southeastern United States and focusing on the cottonmouth and its close relative the copperhead, “both of those species tend to float with full body on the surface”, Greene said, as do rattlesnakes.

Greene told Reuters that non-venomous water snakes “generally swim and float at the surface with only their head (maybe also neck) above the water,” with the rest of their bodies at least at a slight angle below the surface. “I wouldn’t grab a snake or not [though] based just on that criterion!” Greene wrote.


Partly false. While some snakes behave in the way described in the post, experts do not recommend that as a definitive test of whether a snake is venomous or not.

This article was produced by the Reuters Fact Check team. Read more about our work to fact-check social media posts here