Stress can put anyone on edge, and venomous snakes are no exception.

New research suggests that habitat loss, climate change, and other human-driven environmental stressors prompt cottonmouths to attack people more often than they otherwise would—a finding that turns a longstanding depiction of the snake as malicious aggressors on its head.  

Cottonmouths, also called water moccasins, have a fearsome reputation due in part to their propensity to stand their ground and flash the white insides of their mouths when threatened. Though rare, their attacks can be deadly—a man was killed in Missouri in 2015 after being bitten on both legs while wading through a river. “One of the things people often say is that if you threaten a snake, it’s going to be more likely to bite you,” says Tracy Langkilde, the head of the biology department at Pennsylvania State University and a coauthor of a study recently published online in Genetics and Comparative Endocrinology.

Langkilde and her colleagues wanted to see if this belief was true, so her coauthors set out in snake country in Georgia on a sting mission to aggravate cottonmouths. The researchers approached within a yard of 32 of the snakes then grabbed the middle of their bodies with special tongs, noting whether the animals tried to strike or threaten them. They then put the vipers into buckets and grabbed them again with the tongs after half an hour, noting their behavior once more.

For a precise read on the snakes’ physiological response, the researchers took two blood samples from the animals during this process. The first was done immediately after they were caught, before their blood had time to register any change from the acute stress of a human encounter. The second came after the reptiles had spent some time in the bucket, to determine whether their stress levels had changed.

Analysis of the blood samples showed that the snakes with a higher level of the stress hormone corticosterone in their blood upon first encounter with the researchers were the ones that more often struck the snake tongs or threatened the researchers with their bright white mouths. “Whether they had been handled or not did not affect whether or not they were more or less likely to strike,” Langkilde says. “The only thing that mattered was how stressed they were when they were hanging out in the field when we first found them.”

The immediate shock of being pestered by a human, in other words, did nothing to provoke the vipers. It was whether or not they were in a sour mood, caught in the middle of some rough patch, that determined if they lashed out.

Which isn’t to say humans aren’t at fault. More likely, in fact, is that rather than the unlucky occasional victims of vicious attacks, people are the ones stressing the snakes out on a grander scale in the first place. Gordon Schuett, a snake expert at Georgia State University who was not involved with Langkilde’s study, says the paper shows that the more that humans encroach on snake habitats, the more angry they might become—and, in turn, the more predisposed they might be to lunge at unsuspecting passersby.

Climate-change related problems like droughts or heat snaps could create high baseline stress levels in cottonmouths, too, Langkilde says. A study published last year found something similar in a related species of pit viper in Costa Rica, where hospital records showed that the amount of attacks on humans from the fer-de-lance, one of the most dangerous of vipers, increased in extreme weather.

Schuett wants to see more similar work done on snakes in areas undergoing climate-related changes or habitat loss to confirm whether this causes higher stress levels. “When it comes to snakes and other reptiles, we’re only beginning to see the effects of climate change on populations and behavior,” he says.

Research he’s done on western diamondbacks shows that the snakes are more aggressive during breeding seasons—another factor that could also affect cottonmouth stress or temperament. “There are times of the year during reproduction when male [testosterone] levels are real high, and they’re much more dangerous during those high-T levels,” he says. Other work on copperheads, another pit viper, demonstrates that when they fight, the loser will have higher stress levels for several days.

According to Langkilde, snakes are a lot like humans in the way they respond to ongoing problems. Shrinking habitat could lead to more cottonmouths living in concentrated spaces, bringing all the fights and stress that cramped living conditions might entail.

If this is the true, her research makes the case that cottonmouths’ malicious reputation is unjustified, at least to a degree. While many of the snakes threatened the researchers, a very low percentage actually tried to attack. But the problems humans are causing to cottonmouth habitats, whether intentionally or indirectly through climate change, might be coming back to bite us.