How Do Western Rattlesnakes Reproduce?

The Western rattlesnake (Crotalus viridis) is one of four venomous snakes in North America. Various subspecies of the Western rattlesnake thrive in all contiguous states with the highest concentrations in the southwestern United States and Canada. Western rattlesnakes live in dry, rocky climates with minimal vegetation, such as deserts, hills, scrub brush, inland prairies and mountains greater than 10,000 feet. They mate near their dens between August and early September, hibernate in the winter and give birth to live young between August and October. Their average life span is 25 years.

  1. Reproduction

    • Male Western rattlesnakes reach sexual maturity at three to four years of age and females between six and nine. During mating the female becomes passive, and the male climbs on top of her. He presses the rear portion of his body beneath her tail and performs quick thrusting movements while flicking his tongue. The mating process continues for several hours. After mating, the females stop eating and enter their winter dens for 13-14 months. Females store the sperm over the winter, and ovulation and fertilization of the eggs waits until the following spring. After fertilization, gestation occurs for approximately 90 days. Although females give birth to live young, she hatches the eggs inside her body --- referred to as an ovoviviparous reproduction system. The females give birth to between five and 20 young, measuring approximately 10-15 inches long. The litter size increases with the female's body size.

    Hibernation

    • Immediately following birth, the females re-enter hibernation without eating. The young remain near the den for approximately two weeks until they shed their skins, follow the female's scent trail and enter the den with their mother. Western rattlesnake dens are the focus of the reptile's activity and behavioral characteristics. Their seasonal movements begin and end at their community dens, with future generations oftentimes using the same one for more than 100 years. During the summer, they repeatedly travel within a common, one-mile radius for hunting and basking among the hot, rocky ledges and crevices. In the fall, they return to their family dens to hibernate from September to April, avoiding the cold temperatures of the approaching winter.

    Female Behavior

    • Pregnant females remain near the entrance to their dens throughout the summer to lie in the sun or retreat to the shade to regulate their body temperatures for their developing embryos. They don't feed during gestation or while hibernating for the winter following the birth of their young, resulting in a total fasting period of 19 months or more during the year in which they reproduce. Female Western rattlesnakes must double their body weight after reproducing and before beginning to breed again.

    Juvenile Western Rattlesnakes

    • Shortly after birth, young Western rattlesnakes are independent of their mothers and go out on their own to hunt for food. They're born with a "pre-button" at the end of their tails, which grows into their first rattle after seven to 10 days. Adults characteristically have a tan, brown, olive or gray body with a layer of large, dark-brown marks and a white or cream-colored ring. Juveniles, however, are lighter or vibrant in color and express a more notable difference between their spots. The venom of some species of juvenile Western rattlesnakes is more toxic than the adult's, making them dangerous from birth, given their sharp fangs and ability to strike repeatedly when disturbed. They grow quickly in the first few years, shed their skin and grow an extra rattle three times annually. Therefore, the number of rattles a Western rattlesnake has is not a reliable indicator of age.