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) Appearance
The timber rattlesnake (Croatus horridus) is the largest of the three
species of venomous snakes found in Pennsylvania (the other two venomous
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 venomous 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. 
Habitat
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.
Hibernation
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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