Timber rattlesnakes (Crotalus horridus), known as pit vipers due to the heat-sensing pit organs below each eye, have the widest geographic range of any North American pit viper. They are found from the forests of the northeastern United States to Louisiana. Photo by Scott A. Eckert.
With a scientific name like Crotalus horridus, you might expect the worst of timber rattlesnakes. The Greek word krotalon means “rattle” or “castanet,” a nod to their famous tail sound, while the Latin horridus translates to “rough,” “bristly” or “dreadful.” Yet, this ominous name doesn’t quite fit their true personality. Timber rattlesnakes are surprisingly shy and mild mannered, preferring to hide or slip away quietly rather than confront threats. Found throughout forests of the central and eastern United States and once ranging as far north as eastern Canada, timber rattlesnakes have the widest geographic range of any North American pit viper. Still, even experienced outdoor enthusiasts rarely see them. Their secretive nature is helped by their excellent camouflage: a base color ranging from golden-yellow to olive-brown, overlaid with dark wavy bands, and often a rusty stripe down the back, especially in the Midwest. This pattern blends perfectly into the leaf litter of Illinois’ oak-hickory forests and the maple-birch dominated forests of the northeast. In over a decade of studying timber rattlesnakes, we’ve witnessed just how well they remain hidden. On one occasion, a student unknowingly set her backpack right on top of a rattlesnake we were tracking—without the snake reacting or revealing its presence!
Despite their secretive nature, timber rattlesnakes, while not aggressive, are more than capable of defending themselves if they feel threatened. They can grow up to 2 meters (6 feet) long, be as thick as a baseball bat and are remarkably strong. Equipped with two large folding fangs up to 2.5 cm (1 inch) long, they deliver a potent venom. For snakes, venom is a vital adaptation. Without limbs or claws they rely on venom to quickly and safely subdue prey. A timber rattlesnake strikes, injects venom and retreats while its prey succumbs nearby, minimizing risk during feeding. Venom has evolved many times across the animal kingdom and appears not only in snakes, such as cobras and rattlesnakes, but also in some lizards. The venom of timber rattlesnakes is hemotoxic, meaning it damages blood cells and disrupts clotting. While it can be dangerous to humans, it acts relatively slowly and is fully treatable with modern antivenom. Fortunately, these snakes are reluctant to bite and prefer to avoid conflict, so bites in Illinois are extremely rare.
Over the past 10 years, we have closely studied these remarkable reptiles using radio telemetry and remote cameras to track their movements and behavior. Each spring, we safely capture snakes as they emerge from their underground dens called hibernacula. With help from veterinarians and herpetologists at the St. Louis Zoo, we implant a VHF radio transmitter inside each snake and return it to the exact spot where it was found.
From there, our diligent student research assistants use radio receivers and directional antennas to locate every snake, every day throughout the summer. Even with a signal guiding us to within 10 feet, finding a camouflaged rattlesnake in dense forest isn’t easy. Our team hikes through swarms of mosquitoes, thick underbrush, and steep ravines in heat, humidity, and rain—logging an average of 250 miles each between March and October. It’s tough work, not for the faint-hearted. Each time we locate a snake, we record its exact location, habitat and behavior. These efforts have given us a detailed view of the species’ annual cycle—where they hunt, where they overwinter and how their movements unfold over the year.
The annual cycle of a timber rattlesnake revolves around three main goals: finding food, reproducing and avoiding the cold of winter. Foraging occurs mostly in deep forests and along their edges, where snakes hunt using a sit-and-wait strategy—typically lying motionless with their chin on a log, waiting for prey to pass within striking range. Despite common myths, they can only strike about one-third of their body length—roughly 12 inches (30 cm) for a 3-foot snake—but the strike is lightning-fast, often under half a second. Only about half of strikes are successful, but when they are, the snake waits quietly before tracking the prey using two advanced sensory systems. The first is their forked tongue which gathers scent molecules and delivers them to the Jacobson’s organ in the roof of the mouth, which detects direction. The second is their heat-sensing pits located below the eyes, which pick up infrared radiation or body heat as precise as 0.003°C, aiding in both hunting and thermoregulation.
Our research shows that timber rattlesnakes return each year to the same hunting grounds and travel routes. Such site fidelity appears crucial for survival, as snakes rarely thrive if they are relocated to other areas, and should therefore never be moved out of their home range. In Illinois, males occupy home ranges of about 89 hectares (roughly 220 football fields!), while females use around 28 hectares, using the same foraging areas and hibernation sites with remarkable consistency .
Mating by timber rattlesnakes occurs between late June and early August each year. During this time, females remain within their usual foraging grounds, while males roam widely in search of mates. We’ve observed a dramatic increase in the movement of breeding-age males during this period—from about 25 meters per day to more than 100 meters per day4. When multiple males find the same receptive female, they often engage in a non-lethal wrestling match to establish dominance by rearing up and attempting to push each other off balance. The winner gets to mate with the female, which he may guard for several days before leaving to seek other mates. Both sexes may mate with multiple partners.
Females remain in their foraging areas until late summer, then return to underground hibernation sites for winter. Fertilization doesn’t occur until the following spring, when the female emerges from hibernation and begins developing embryos using sperm stored from the previous summer. She moves to a warm, sheltered area near the den and fasts throughout gestation, often losing up to 30 percent of her body weight. In Illinois, females give birth to 7 to 12 live young in August or September and stay with them for one to two weeks before feeding once more and returning to their winter dens. The newborn snakes may follow their mother via chemical cues to her foraging sites for their first meal and then back to the same den as winter approaches.
Across much of their range, timber rattlesnakes cannot survive above ground during winter. Being ectotherms, they have little ability to generate their own body heat and instead depend on external warmth to survive. Thus, when winter nears, they must find warm, stable refuges to endure the cold months. In Illinois and similar areas, these refuges are underground where temperatures remain steady at around 10°C (50°F). Our research shows that Illinois hibernacula are typically found on steep, south- to southwest-facing limestone outcrops near large forests. Such rocky formations contain cracks and crevices leading to deep underground passageways, with rubble piles covered by soil creating protected overwintering spaces. These sites are thermally stable and geologically reliable, allowing snakes to return annually. The sunny slopes offer basking spots on warmer winter days, and nearby forests provide spring foraging grounds. Individuals typically spend up to seven months each year inside these refugia and may share their overwintering quarters with black rat snakes, garter snakes, copperheads and racers. Deciding when to enter or leave underground refugia is a challenge for timber rattlesnakes. If they go underground too early, they lose feeding opportunities, but if they go underground too late, they may die of exposure. Our research indicates that snakes use a series of complex thermal cues in deciding when to enter or leave their hibernacula. Essentially, snakes do not emerge until outside temperatures (particularly at night) consistently exceed 11ºC (52ºF).
Timber rattlesnakes face the risk of extinction across much of their range. In several states—particularly in the Northeast and Midwest, including Illinois—their populations have declined sharply, and they have become rare or disappeared entirely from places such as Connecticut, New Hampshire and parts of Canada. Such declines result primarily from habitat loss caused by urban development and agriculture, as well as direct persecution driven by fear and misunderstanding of these snakes. Fragmented habitats further restrict their movement between essential hibernation sites and hunting grounds, which are vital for their survival. In Illinois, timber rattlesnakes have been listed as threatened since 1994 and designated a Species of Greatest Conservation Need, highlighting ongoing conservation challenges. Because timber rattlesnakes mature slowly and reproduce infrequently, their populations are especially vulnerable and recover only gradually from losses, making conservation efforts critical to their long-term survival.
Timber rattlesnakes don’t often get the credit they deserve—they’re rarely seen, often misunderstood and sometimes feared—but they’re key players in keeping forest ecosystems healthy. These fascinating creatures help control populations of small mammals and quietly go about this important role with great effectiveness. Thanks to their incredible camouflage, sharp senses and secretive habits most people don’t encounter them in the forest, which only adds to the mystery surrounding this rattlesnake.
Sadly, habitat loss and prejudice have caused their decline in many places, but we hope that with greater understanding of the ecological value of this shy snake and a recognition of its benign nature, that appreciation for this remarkable animal will lead to reduced persecution, and protection of its critical forest habitats.
[1] Clark, R.W. 2006. Post-Strike Behavior of Timber Rattlesnakes (Crotalus horridus) During Natural Predation Events. Ethology 112: 1089–1094
[2] Kardong, K. V. & Bels, V. L. 1998: Rattlesnake strike behavior: kinematics. J. Exp. Biol. 201: 837—850.
[3] Bakken, G.S. and A.R. Krochmal. 2007. The imaging properties and sensitivity of the facial pits of pitvipers as determined by optical and heat-transfer analysis J. of Exp. Biol. 210: 2801-2810
[4] Eckert, S.A. and A.C. Jesper. 2023. Home range, site fidelity, and movements of the Timber Rattlesnake (Crotalus horridus) in West-Central Illinois. Animal Biotelemetry. 2024 12:1 https://doi.org/10.1186/s40317-023-00357-8
[5] Jesper, A.C., S.A. Eckert, S.R. Ballard, J.A. Crawford and M.J. Dreslik. 2024. Distribution and drivers of critical hibernacula for the timber rattlesnake Crotalus horridus in Illinois, USA. Endangered Species Research 53: 467–480, https://doi.org/10.3354/esr01312
[6] Jesper, A.C., Eckert, S.A. , B.J. Bielema, S.R. Ballard, M.J. Dreslik. 2023. Phenology and predictors of spring emergence for the Timber Rattlesnake (Crotalus horridus). PeerJ 11:e16044 https://peerj.com/articles/16044/
Dr. Scott A. Eckert is the Kent Smith Chair of Math and Natural Sciences and Professor of Ecology and Ecosystem Management at Principia College. He has a long research career of more than 45 years with over 150 scientific publications primarily on sea turtles and other deep diving vertebrates. His long-standing interest in reptiles and in the recovery of endangered species, led him to undertake research close-to-home with his students in the Principia College forests on the Illinois Endangered timber rattlesnake (Crotalus horridus).
Dr. Andrew C. Jesper is a post-doctoral researcher at the National Great Rivers Research and Education Center in Alton, Illinois. Andrew works closely with the Center’s Terrestrial Wildlife Ecologist and other external and internal collaborators to conduct applied research on various aspects of reptile and amphibian ecology to inform management and conservation. He has studied Timber Rattlesnakes at Principia College for over a decade, publishing research that has directly informed conservation strategies for this declining species.
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