The dark variation has a heavy speckling of black or very dark brown that hides much of the lighter pigment. Timber rattlesnakes range from southern New Hampshire, west through the Champlain Valley, south through the Appalachian and Blue Ridge Mountains, into northeastern Texas and north along the Mississippi River to Wisconsin. Timber rattlesnakes breed in either the spring or fall and give birth to an average of 9 young in August to late September. The young snakes are born live in a membranous sac which they open with a sharp egg tooth. Timber rattlesnakes reach sexual maturity between 7 to 10 years of age.
Females breed every third or fourth year.
The species has been eliminated from many parts of its historic range. Disturbance by humans and lack of suitable den sites appear to be the major limiting factors for rattlesnakes in Connecticut. In the past, some Connecticut towns had bounties on rattlesnakes, and many of the dens were repeatedly decimated. Once documented in over 20 towns in Connecticut, this snake is now limited to isolated populations in 10 towns. These pits are sensitive to radiant heat and help the snake detect warm-blooded prey in the darkness.