A new study, recently published in Ecology and Evolution, uses network analysis to understand the social lives of rattlesnakes, a group previously considered to be asocial animals.
Co-author of the study, Mark Davis, a conservation biologist at the Illinois Natural History Survey at the Prairie Research Institute, said this was the first time social network analysis tools have been used to study the behavior of snakes.
“For the longest time, rattlesnakes and other pit vipers have been viewed as very solitary animals that are sporadically encountered across the landscape doing things that snakes do, like scaring people and eating rodents,” Davis said, noting that the researchers were interested in finding out if this perception was accurate.
“It is increasingly apparent that their social structure may be a lot more complicated than we give them credit for. And so, using a hard-won set of long-term data, we used modern network analyses to begin to understand the social structure of what historically were thought to be largely asocial animals.”
The study used a dataset that tracked a population of western diamondback rattlesnakes in Arizona’s Suizo Mountains over 15 years. Past field research has suggested most snakes form distinct clusters of individuals, but those observations had not been quantitatively tested.
The researchers found that the population of rattlesnakes formed social groups, called nodes in network theory, that they frequently interacted within. Despite having overlapping home ranges, they rarely if ever interacted with snakes outside of their social groups. The groups were not formed based solely on geography or family relationships, but rather appear to be based on the time of year, as they leave and return to their winter homes. A phenomenon known as fission/fusion.
“They’re all coming back to these same communal dens to spend the cold winters together,” Davis said. “They’re interacting around these dens, and as they’re dispersing out or coming back they’re encountering and engaging with one another.”
The analysis showed high site fidelity as individual snakes returned to the same dens — often crevices in rocky granite outcroppings — year after year. However, there was an exception to that pattern that the team is interested in exploring.
Occasionally individual females, who were previously members of a node, will spend the winter alone. Davis said the preliminary data suggests that this happens after difficult years when the individual snake is in poor body condition. Because they encounter males in the communal space, these females may avoid getting pregnant the next year because their resources are depleted.
“They’re actively altering their social environment to respond to conditions on the ground, which I think shows a really interesting phenomenon: They’re able to adjust and move away from these ritualized behaviors when it is in their best interests,” he said. “Ultimately the goal of these snakes and why they do these things is to increase their contribution to the next generation.”
Now that there is quantitative evidence of social behavior under natural conditions, Davis said future research will explore the details and patterns of their behavior, as well as how it evolved. Next, Davis and his collaborators hope to analyze a population of rattlesnakes in New York state — and start to answer questions about whether the behavior evolved independently over time for survival in areas with limited resources or whether it is consistent across rattlesnake populations with varying geographic, ecological and life history factors.
“This is something that’s novel in pitvipers — the subfamily of snakes that includes rattlesnakes — it’s new, we’re just learning it,” he said. “There are an unbelievable number of questions that could be asked and answered as we explore these things with these new social network tools.”
Davis explained that there is a limited understanding of snake behavior stemming from the fact that they are difficult to observe, as well as cultural perceptions of snakes. Snakes are cryptic, meaning they blend into their surroundings, and some spend most of their time underground or only come out at night during certain seasons. Additionally, they can live for 30 years or more, making it challenging to gather data over their lifecycle.
And negative perceptions of snakes haven’t helped, Davis added. As humans evolved in places with venomous and constricting snakes, a fear of them became encoded in the human genome. Human cultures also have viewed snakes in a negative light, such as the serpent in the biblical story of the Garden of Eden.
“Snakes in general, pitvipers and rattlesnakes specifically, provide tremendous value to human populations. We are deriving incredible medicines from their venoms that allow medical professionals to save lives. Without those animals out in the wild and in nature, we can’t harness that information,” Davis said. “So, they’re really important for a number of reasons, but all of those negative associations I think are what have made it difficult for us to be able to study them effectively, to get funding for it and to be able to explore all of the fascinating things that they’re doing out in the world.”
Davis hopes the team’s research will help people see that snakes are more complex than they are often given credit for.
“They’re unique creatures that have these social relationships with one another and are interacting in nuanced ways, just like we are,” he said. “Keep a healthy distance when you see a snake in nature, but maybe watch it for a little while and see what it does, because they are doing really interesting things out there.”