Virtual Nature Trail

Scientific name: Croatus horridus
Common name: 
Timber Rattlesnake

(Information in this Species Page was compiled by Amanda Zenuh in Biology 220W, Spring 2003, at Penn State New Kensington)

The timber rattlesnake (Croatus horridus) is the largest of the three species of poisonous snakes found in Pennsylvania (the other two poisonous species are the northern copperhead and the endangered Massasauga rattlesnake). Adult timber rattlesnakes are typically 36 to 48 inches long with a small number reaching up to 72 inches in length. Individuals of this species have the distinctively broad, flat, triangularly shaped heads and vertically slit pupils that are characteristics shared by all of the poisonous snake species in Pennsylvania. Rattlesnakes retain and accumulate the dried, shed segments of their integument on their tails. These “horny segments” form the distinctive rattle which is used by the snakes in a variety of warning and defensive displays. The timber rattlesnake has two basic colorations: a light stage (which consists of a light yellow or gray background highlighted by V-shaped cross bands of dark brown or black) and a dark stage (in which a yellow background is almost completely covered by thick brown or black cross bands. The snake’s tail, though, regardless of color stage, is always black.

Croatus horridus has a broad geographic distribution throughout the eastern United States. Its northern boundary runs from southern Maine to southeastern Minnesota, and its southern boundary runs from northern Florida to central Texas. Timber rattlesnakes have been collected in all of the counties in and around our campus’s Nature Trail. We have never, however, collected a timber rattlesnake on the actual trail.

In northern portions of their distribution, timber rattlesnakes are most commonly found in mountainous areas in which there are numerous rocks and rock crevices or in dense, thick mixed forest sites. In the southern portions of their distribution, timber rattlesnakes are often found in swampy, marshy habitats. These various habitats all provide not only sufficient prey densities to support a breeding population of snakes, but also open, sunny areas (like rocks and logs) for basking and heating and crevices for hiding, shedding, hibernating, and cooling.

Hunting Behavior and Prey
Timber rattlesnakes are primarily nocturnal. Peak activity is seen during the very dark nights associated with the new moon. Prey of this snake is determined by size and opportunity. Small mammals (like mice, moles, chipmunks, gray squirrels, rabbits, and weasels) make up most (over 90%) of their diet. Birds (small song birds, grouse, baby turkeys, baby ducks, etc), other reptiles (garter snakes and possibly other rattlesnakes) and amphibians (frogs and toads) make up the remaining ten percent of their probable prey. The timber rattlesnake is an ambush predator that conceals itself, coiled up, under rocks or behind logs until the body heat and chemical scent signature of a prey species is sensed. Its excellent sense of vibration (centered in its lower jaw and connecting skull bones) also gives it data on the size of the approaching organism. These excellent sensory modalities enable the snake to quickly retreat if the approaching species is too large or is dangerous. If the individual, though, is of suitable size, the snake will lunge at it as soon as it enters the snake’s strike radius and dig its fangs into the animal injecting it with a dose of venom. The snake then releases the envenomed prey and withdraws. The venom takes only a few minutes to take effect. During this time the prey individual may have moved some distance away. The snake follows the prey’s scent trail and rapidly locates it and determines that it is dead. The snake then swallows the prey whole and retreats to a basking site to raise its body temperature to the optimal 80 to 85 degrees F for maximally efficient digestion. Along with numerous efficient digestive secretions, the venom itself also accelerates the breakdown of the prey’s body. Generally, a healthy rattlesnake will consume three times its body weight per year in prey and drink the equivalent of its body weight in water.

Predators and Mortality
Timber rattlesnakes are eaten by a variety of predators including coyotes, foxes, raccoons, opossums, domesticated and feral cats, eagles, hawks, owls, turkeys, black snakes and king snakes. Young snakes, of course, are the most vulnerable to predators. Humans also hunt and kill timber rattlesnakes for sport, out of ignorance, and, only very rarely, for food or for their skins. Human destruction of the snake’s habitats is a major factor in the declining numbers of these snakes throughout their geographic range.

Timber rattlesnakes hibernate from early October to late April in dens that typically extend below frost line. On warm days during the hibernation period, the rattlesnakes may emerge to bask and warm their bodies. They will retreat back into their dens, though, as daytime temperatures fall. Optimally located dens may contain a large number of often related, timber rattlesnake individuals. There may also be individuals of other snake species (including copperheads, black snakes, and garter snakes).

Mating and Reproduction
Timber rattlesnakes mate most actively between July and August. Males engage in dominance wrestling matches for the right to mate with receptive females. The females are capable of storing the sperm for many months, and, thus, typically delay fertilization of their ova until June of the following year. The 6 to 14 young are born live, encased in transparent membranes, in August or September. These newborn snakes are 8 to 10 inches long and have fully functional fangs and venom glands. Females are capable of reproducing only once every 3 to 4 years. Males may reach sexual maturity by age four, while females do not become sexually mature before ages seven to eleven. Timber rattlesnakes have life expectancies in the wild of 20 years or so. Females are thus only able to reproduce two or three times in their lifetimes. Populations of timber rattlesnakes, then, are only capable of very slow replication and growth. When the female is in the latter stages of gestation, she does not feed and is active only to precisely regulate her body temperature to meet the physiological needs demanded by her developing young. During this time period, gravid females are often in their dens and are thus quite vulnerable to capture and killing especially by humans. Human predation, then, damages the most vulnerable portions of the timber rattlesnake’s population and has huge impacts on the viability and continuance of this species in the wild.

Bite of a Timber Rattlesnake
A bite from a timber rattlesnake is a serious medical event. Fortunately, the docile nature of this species, their keen ability to distinguish between prey and non-prey organisms, and their tendency to retreat quickly from non-prey species make human encounters and bites extremely uncommon events. If you are ever bitten by a timber rattlesnake, though, remain calm. Keep the bitten body part below the level of your heart and immediately seek medical attention. There is a very good chance that no venom was injected into the snake’s warning bite, but any bite must be treated carefully.

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