Rattlesnakes are more social than we thought

A new discovery suggests reptiles may be far more social than we thought.

Place a rattlesnake in a bucket and shake it, and the result will be predictable: The stressed-out reptile will begin rattling its tail to warn off the enemies outside. 

But place two rattlesnakes in a bucket and something entirely different happens, according to findings published Thursday in the journal Frontiers in Ethology.

In the presence of a comrade — even a stranger — the jostled rattlesnakes’ heart rates stayed lower than when the snakes were alone.

That finding is potentially revolutionary, as it represents the first peer-reviewed discovery of reptiles that draw emotional support from each other’s presence.

The phenomenon the researchers were studying is called “social buffering,” and it has been widely documented across the bird and mammal families — as well as being a nearly universal human experience.

Anyone who has felt a stressful situation eased by the presence of a friend has firsthand experience of what this is like.

But until this study, scientists had never found evidence of the behavior in any reptile species — even amid a broader reevaluation of the class’s social lives.

The findings call further into question scientists’ long held — and rapidly eroding — beliefs that the scaly vertebrates are less social than their furred and feathered cousins.

“In the past, historically, people have had a very negative perception of reptiles and they thought of them as being kind of not very complex robots,” said lead author Chelsea Martin of Loma Linda University School of Medicine.

That deep-seated bias is an unintended result of snakes’ appearance, Martin argued. “They’re not fluffy. They don’t have facial expressions. It can be really difficult for us to perceive what they’re feeling or what’s going on with them. And so I think historically, people have thought, oh, there’s nothing happening.”

The study was born from a chance observation by William Hayes, Martin’s adviser. 

As a recognized local snake expert, Hayes occasionally finds himself called into the mountains to pick up rattlesnakes trapped by anxious homeowners — animals that would otherwise likely be killed.

This procedure involves transporting a snake that has been trapped in a bucket, which can be a noisy experience, Hayes said.

The reptiles “get scared really easily,” Hayes said. “Rattlesnakes aren’t angry — they’re not trying to hurt us. They’re just afraid of us. So they’re in the buckets and they often rattle.”

But in some cases, homeowners had captured two snakes a few days apart — which Hayes brought out in the same bucket.

“And it seemed to be that when there were two of them in there, they weren’t rattling as much,” he said.

Hayes’s offhand observation came amid a broader rethinking of sociality in snakes — and of vipers like rattlesnakes in particular.

“When I was in graduate school, the general paradigm was that animals don’t even think, they just follow simple stimulus responses,” he said. 

As a graduate student, he was lead author on a paper that found rattlesnakes could precisely control how much venom they injected when they struck — and could choose not to inject venom at all.

After that paper, a leading snakebite expert wrote him to complain. “He says ‘They don’t have the mental capacity to make these decisions,’” Hayes recalled — even though that behavior had already been established in insects, which would have been assumed to be simpler.

But while snakes lack the expressive faces, fur or feathers that accompany sociality in birds and mammals, they have their own means of sensing and communication.

Some of these are all but invisible to humans, because of our vastly different sensory tools. 

Snakes take in information — and potentially transmit it — via channels of scent, heat and vibration, according to the book “Rattlesnakes of Arizona.”

But with the weight of decades of studies, scientists now know that despite their unsavory reputation — their Latin name is the evocative Crotalus horridus — rattlesnakes are among the most social reptile species, and many of their behaviors echo those among mammals.

For example, it had long been known that rattlesnake mothers give birth to live babies — hatched from eggs that remain within their bodies.

But a big breakthrough came in 2002, when a landmark study found that pit viper mothers — the broader grouping that rattlesnakes are a part of — cared for their newborns for several days after birth. 

In fact, the scientists suggested that the heat-sensing facial “pits” that give the vipers their name  — which allow them to strike prey in total darkness — evolved in tandem with their rattling tails and habit of maternal care.

Added together, the scientists suggested that this complex of behaviors means a watchful mother can see a threat coming for her newborns and ostentatiously warn them off. 

Subsequent studies found that females of many species spend the winter coiled together in communal dens with each other and their offspring — with an influential 2004 study finding that female rattlesnakes tended to sleep dramatically closer to their sisters than to unrelated females.

This finding was particularly remarkable, given that the snakes had been separated at birth and raised in isolation, the scientists noted.

This 2004 finding was strong evidence that the snakes “may lead much richer social lives than previously thought,” said lead author Rulon Clark of Cornell University at the time.

And since then, a slow accretion of evidence has built up, tilting the scientific scales toward a growing acceptance of social behavior in snakes.

A 2011 study showed that black rattlesnakes actively cared for their newborns, including protecting them against predators.

After their mother leaves them, newborn timber rattlesnakes also tend to follow her scent — a small study found that of six newborns, five spent the winter in the same den as their mother, and the remaining one in the den of a related female. 

These behaviors appear widespread among the pit vipers. A 2016 study found that 94 species in the group live communally at least part time. And in at least 18 species, these communities are “stable,” drawing in snakes from diverse ranges and sometimes lasting for years. 

And sociality seems to go beyond the viper family. According to a 2016 study in Nature, young water snakes hatched from eggs that incubated alone — instead of in direct contact with their siblings — grew up to be markedly less social.

This social behavior can be a devastating liability where humans are concerned. Since the 1950s, communities across the West have carried out “rattlesnake roundups,” in which gasoline is poured into dens of rattlesnakes, forcing them to the surface — where they are captured to be killed en-masse in carnivalesque public gatherings.

In the world’s largest such display, which happens each March in Sweetwater, Texas, thousands of snakes are milked of venom and forced to show their fangs — before being beheaded and skinned for leather.

These sorts of events are only possible because people don’t consider snakes to be “sentient,” Hayes said. 

“People would go to jail if they treated dogs and cats that way. It’s just horrible,” he said.

To test whether snakes drew comfort from each other’s presence, Martin and Hayes created a deceptively simple experiment. 

First, they maneuvered the front half of an unsedated rattlesnake into a specially made clear plastic tube — which allowed them to safely attach heart rate-tracking electrodes to “the other half,” Martin said.

They then jostled the wired-up snakes in buckets — either alone, with a snake-sized length of rope, or with another rattlesnake of the same species.

Even though the snakes were strangers to each other — unrelated members of the same species — the heart rates of the snakes who had been given a cellmate stayed lower.

Martin said that she hoped the study would help people to see snakes as kindred creatures — and therefore to treat them better.

“When people start to see that they’re more similar to us, and that they maybe have more complex things that are going on with them — then that might change their perception,” she said.

“And that’s the goal. So again, you know, knowing that ‘Hey, humans socially buffer and so do snakes,’ or ‘Hey, female rattlesnakes take care of their babies” — it might make people say that, well, we’re not that different from them. It might make them a little friendlier and less fearful.”