Crotalus atrox reaches sexual maturity at 3 years. Mating occurs in the spring following emergence from hibernation. Females are passive during courtship, while the male crawls in jerks on top of the female, all the while flicking his tongue. Vigorously jerking the hind portion of his body, he presses his tail beneath that of his partner, who in turn lifts her tail. Their cloacas make contact, and the male then inserts his hemipenis, which is deeply forked. Copulation lasts for hours, with several interspersed resting periods.
The gestation period lasts for 167 days. The birthing process may last for 3 to 5 hours and produce 10 to 20 young. This species is ovoviviparous; the young pierce their thin egg membranes immediately before birth and are born live. The young only stay with the mother for a couple of hours, for a day at the most. Then they scatter in search for food and potential winter refuge. The young population declines drastically after the first winter due to lack of food, freezing temperatures, and vulnerability to predators.
Average number of offspring: 14.
Average age at sexual or reproductive maturity (male)
Sex: male: 1095 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 1095 days.