Adults usually grow to total length of 91–152 cm (36–60 in). It was found in Pennsylvania that the smallest size females could produce viable eggs was 72.2 cm (28.4 in). Most adult timber rattlesnakes found measure less than 100 to 115 cm (39 to 45 in) in total length and weigh on average between 500 and 1,500 g (1.1 and 3.3 lb), often being towards the lower end of that weight range. The maximum reported total length is 189.2 cm (74.5 in) (Klauber, 1956). Holt (1924) mentions a large specimen caught in Montgomery County, Alabama, which had a total length of 159 cm (62.5 in) and weighed 2.5 kg (5.5 lb). Large specimens can reportedly weigh as much as 4.5 kg (9.9 lb).
The dorsal scales are keeled and arranged in 21–26 scale rows at midbody (usually 25 rows in the southern part of its geographic range, and 23 rows in the northern part). The ventral scales number 158-177 in males and 163–183 in females. Males have 20–30 subcaudal scales, while females have 15–26. The rostral scale is normally a little higher than it is wide. In the internasal-prefrontal area there are 4–22 scales that include 2 large, triangular internasal scales that border the rostral, followed by 2 large, quadrangular prefrontal scales (anterior canthals) that may contact each other along the midline, or may be separated by many small scales. Between the supraocular and internasal, only a single canthal scale is present. There are 5–7 intersupraocular scales. The number of prefoveal scales varies between 2 and 8. Usually the first supralabial scale is in broad contact with the prenasal scale, although slightly to moderately separated along its posteroventral margin by the most anterior prefoveals.
Dorsally, they have a pattern of dark brown or black crossbands on a yellowish brown or grayish background. The crossbands have irregular zig-zag edges, and may be V-shaped or M-shaped. Often a rust-colored vertebral stripe is present. Ventrally they are yellowish, uniform or marked with black. Melanism is common, and some individuals are very dark, almost solid black.
Found in the eastern United States from southern Minnesota and southern New Hampshire, south to east Texas and north Florida.
Its historic range includes southern Ontario and southern Quebec in Canada, but in May 2001, the Canadian Species at Risk Act listed it as extirpated in Canada. A Canadian government sponsored recovery strategy is currently under study to support the reintroducing of this predator of many pests to its former Canadian habitat.
Although several experts disagree, many were found in some of the thick forest areas of central and southeastern Iowa, mostly within the Mississippi, Skunk, Iowa, and Des Moines River valleys, in several places in these areas; bites from timber rattlesnakes have been widespread, especially in a localized area of Geode State Park, in southeastern Henry County, along Credit Island Park, in southern Scott County, and in the forested areas of southern Clinton County.
In Pennsylvania, it is not found west of Chestnut Ridge, which is in the Laurel Highlands, nor is it found in the southeastern corner of the state. Thus, its range does not include the areas of Philadelphia and Pittsburgh, the two largest cities in Pennsylvania. C. horridus is extinct in Maine and Rhode Island and is almost extinct in New Hampshire.
In Massachusetts, the snakes are active from mid-May to mid-October. Early settlers were afraid of the snake, as its population was widespread throughout the state. Since that time their habitat has been reduced to the Blue Hills south of Boston, The Berkshires in Western Massachusetts as well as parts of the Connecticut River Valley, notably in the area of the Holyoke Range. The snake is so rare in the state that it is rarely encountered by people and is considered endangered, making it illegal to harass, kill, collect, or possess.
Generally, this species is found in deciduous forests in rugged terrain. During the summer, gravid (pregnant) females seem to prefer open, rocky ledges where the temperatures are higher, while males and nongravid females tend to spend more time in cooler, denser woodland with more closed forest canopy.
Female timber rattlers often bask in the sun before giving birth, in open rocky areas known as "basking knolls".
During the winter, timber rattlesnakes brumate in dens, in limestone crevices, often together with copperheads and black rat snakes.
Their prey are mainly small mammals, but may include small birds, frogs, mice, other small animals, or other snakes. Although capable of consuming other rattlesnakes, the most common snakes they eat are garter snakes. Like most rattlesnakes, timber rattlesnakes are known to utilize chemical cues to find sites to ambush their prey and will often strike their prey and track them until they can be consumed. Timber rattlesnakes are known to use fallen logs as a waiting site for prey to pass by, giving them an elevated perch from which to effectively strike their prey, which is almost entirely terrestrial rather than arboreal (even arboreal prey such as squirrels tends to be caught when they come to the ground). The primary foods by genera of timber rattlesnakes were as follows: Peromyscus (33.3%), Microtus (10.9%), Tamias (qv) (10.6%), Sylvilagus (10.4%), Sigmodon (5.3%) and Sciurus (4.2%). Based on examination of the snout-to-vent length, it was found that juvenile timber rattlesnakes differed slightly in dietary preferences from adult rattlesnakes, being more likely to consume smaller prey such as shrews (averaging 8 g (0.28 oz) and unable to attack subadult eastern cottontail rabbits (averaging 500–1,000 g (1.1–2.2 lb) but Peromyscus was the number one prey item for both young and adult rattlesnakes. Several birds, although always secondary to mammals, are also known to be hunted, mainly ground-dwelling species such as bobwhites, but also a surprising number of passerines.
Potentially, this is one of North America's most dangerous snakes, due to its long fangs, impressive size, and high venom yield. This is to some degree offset by its relatively mild disposition and long brumation period. Before striking, they often perform a good deal of preliminary rattling and feinting. Cist (1845) described how he lived in western Pennsylvania for many years, and the species was quite common there, but in all that time, he heard of only a single death resulting from its bite.
Considerable geographic and ontogenetic variation occurs regarding the toxicity of the venom, which can be said for many rattlesnake species. Four venom patterns have been described for this species: Type A is largely neurotoxic, and is found in various parts of the southern range. One effect of the toxin can be generalized myokymia. Type B is hemorrhagic and proteolytic, and is found consistently in the north and in parts of the southeast. Type A + B is found in areas where the aforementioned types apparently intergrade in southwestern Arkansas and northern Louisiana. Type C venom has none of the above components and is relatively weak.
The neurotoxic component of the type A venom is referred to as canebrake toxin, and is a phospholipase A2. It is analogous to the neurotoxins found in the venoms of several other rattlesnake species, and when present, contributes significantly to the overall toxicity. Other components found in the venom include a small basic peptide that works as a myotoxin, a fibrinogen-clotting enzyme that can produce defibrination syndrome, and a bradykinin-releasing enzyme.
CroFab antivenom, while not specific for C. horridus, is used to treat envenomations from this species.
The timber rattlesnake was designated the state reptile of West Virginia in 2008. That state's legislature praised "...a proud contribution by the eighth grade class at Romney Middle School, from West Virginia's oldest county, in West Virginia's oldest town, to have been instrumental in making the timber rattlesnake the state reptile..."
This snake became a prominent symbol of American anger and resolve during the American Revolution due to its fearsome reputation. In the 18th century, European-trained doctors and scientists had little firsthand experience with or information on timber rattlesnakes, and treatment of their bites was poorly effective. The motto Nemo me impune lacesset (with the verb in the future tense) appears above a Crotalus horridus on a 1778 $20 bill from Georgia as an early example of the colonial use of the coiled rattlesnake symbol, which later became famous on the Gadsden flag.
The subspecies C. h. atricaudatus (Latreille in Sonnini and Latreille, 1802), often referred to as the canebrake rattlesnake, is currently considered invalid. Previously, it was recognized by Gloyd (1936) and Klauber (1936). Based on an analysis of geographic variation, Pisani et al. (1972) concluded no subspecies should be recognized. This was rejected by Conant (1975), but followed by Collins and Knight (1980). Brown and Ernst (1986) found evidence for retaining the two subspecies, but state it is not possible to tell them apart without having more information than usual, including adult size, color pattern, the number of dorsal scale rows and the number of ventral scales. Dundee and Rossman (1989) recognized C. h. atricaudatus, but others take a more neutral point of view.
This species is classified as least concern on the IUCN Red List (assessed in 2007). Species are listed as such due to their wide distribution, presumed large population, or because they are unlikely to be declining fast enough to qualify for listing in a more threatened category.
The timber rattlesnake is listed as endangered in New Jersey, Vermont, Massachusetts (along with the copperhead viper), Virginia, New Hampshire, Indiana, and Ohio, and it is threatened in New York, Connecticut, Illinois, Minnesota, and Texas.
Timber Rattlesnakes have already gone extinct in Maine and Rhode Island and only one population remains in New Hampshire. They are protected in many of the Appalachian states but their populations continue to decline.