The Timber rattlesnake once ＿＿（36）＿＿ throughout the eastern United States, is now on the ＿＿（37）＿＿ species list and is extinct in two eastern states in which it once ＿＿（38）＿＿. Compared to its Western cousins, the Timer rattlesnake may be especially ＿＿（39）＿＿ because of certain behavior adapted for coping with the cold climates in which it lives. Rattlesnakes are generally found in warm climates because, like all ＿＿（40）＿＿, they cannot generate or ＿＿（41）＿＿ their own body temperature internally and must rely on the sun’s warmth of heat.