Social lives of rattlesnakes: because the snakes bask, breed, and hibernate together, recognizing their relatives is a key advantage, especially for females

Natural History, March, 2005 by Rulon W. Clark

So, what good are they anyway?"

I sigh as I hear this question, yet again, about the animals I study: rattlesnakes. I suppose anyone who spends taxpayer dollars studying animal behavior has to deal with that kind of skepticism at some point along the way. Answering such a question gets even trickier if what you study is small and seemingly insignificant, like a cricket. And getting a sympathetic ear for such a feared, hated, misunderstood, and potentially dangerous species as a rattlesnake is nearly impossible.

On this occasion, a retired truck driver has queried me over breakfast at the counter of a rural Pennsylvania diner. As it happens--though he doesn't know it--we are just a few miles from one of the largest concentrations of timber rattlesnake dens in the country. Rattlers, usually thought of as solitary tailshakers, actually breed in groups, making them an easy, albeit elusive, target for hunters--not to mention the people who take part in the dreadful "rattlesnake roundups," in the mistaken belief that killing snakes improves the woods. The group behavior of rattlesnakes is the focus of my research. Yet it is becoming increasingly difficult to study large groups of the snakes, because their numbers are dwindling. Pennsylvania continues to be one of the few places in the United States where timber rattlesnakes (Crotalus horridus) are still common.

Immediately I'm on the defensive--cautious about revealing too much about where I'm going to study the snakes, yet eager to respond. Being flip, I nearly say, "Well, what good are you?" Instead, I settle on an easy, utilitarian answer: they help control rodents.

But the timber rattlesnake is more than a simple rodent-eater. As top-level predators, rattlesnakes are an integral part of the ecosystem, contributing to the overall health of the forests. The snakes influence prey populations and perform vital ecosystem functions through their natural effect on the dynamics of the food web, helping to maintain balances between herbivore populations, plants, and predators. The loss of any one species from a community may not be catastrophic, but the ongoing decline of many populations in an ecosystem erodes the balances that make natural systems persist.

I was in Pennsylvania to observe the spring emergence of the timber rattlesnakes. Several years ago an expert on local rattler populations, Curt E. Brennan, who works for the Pennsylvania Department of Conservation and Natural Resources, first showed the populous rattler den to me. My return trek in the spring took me to the secluded site up along a steep, forested bluff. At the summit of the bluff is a small clearing with exposed slate bedrock, fractured by cracks and crevices. Such exposed openings are few and far between in northeastern forests; whenever possible, the snakes use them as dens. Timber rattlesnakes are ectothermic, or cold-blooded, animals; come winter, they take refuge in the cracks beneath the frost line. By early May, though, temperatures become warm enough for the rattlers to move safely out into the woods. They don't return to their dens for hibernation until mid-October, just before the nighttime frosts begin.

I had arrived in the middle of May, the peak of the spring emergence period. The day was cool and slightly overcast--perfect for viewing snakes. They would be out in the open, trying to soak up what little warmth penetrated the haze. It took a moment of scanning to see them. A dozen or so large, thick-bodied, velvety timber rattlesnakes lay loosely piled and wrapped around each other just under the edge of a large stone slab. They basked contentedly in the sun, gently shifting their muscular coils every so often. If they were aware of my presence, they seemed unconcerned about it. I felt I was looking through a small window onto an alien world.

Everyone I have ever taken into the field--my mother, a businessman, a local redneck, a college student, a six-year-old child--has been equally moved by the sight of timber rattlesnakes in the wild. After such an experience, no one seems to have any further doubts about the value of these animals. Perhaps the overhyped fear and loathing whipped up about animals such as rattlesnakes arises from surprise encounters that people have with snakes, in which the snakes are perceived as intruders. Yet a visit to the same animal in its natural environment can have the opposite effect, fostering a sense of value and appreciation.

Timber rattlers are striking in such large aggregations. They come in two colors: some are almost solid black, whereas others are bright yellow, marked with splotches of brown. These two "color morphs" can even occur in the same litter. As I watched the jumble of snakes in May, I noticed one large, yellowish-brown male start to make his way slowly but steadily up the slope and into the woods. He was probably beginning his yearly migration through the forests in search of food. Males, juveniles, and nonpregnant females all leave the denning areas over the course of a few weeks. After a winter without food, the animals laced to seek out the small mammals that constitute their diet.