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The Prairie Rattlesnake is distributed across the western Great Plains and the lower elevations of the Rocky Mountains, from Canada into Montana and North Dakota, south to New Mexico, Texas, and Oklahoma. In Wyoming it is found in all of the counties east of the Continental Divide as well as areas in Carbon and Sweetwater counties west of the divide. It is widespread in the prairie and foothills life zones where it inhabits the grassland, sagebrush, and woodland communities. It is generally restricted to elevations below 7,000 feet but does occur up to 8,500 feet in a few areas where suitable denning sites can be found. This lie-in-wait predator is active from April into October, ambushing warm-blooded animals such as mice, ground squirrels, pocket gophers, chipmunks, and rabbits. It is primarily diurnal but may become nocturnal during hot weather.
These venomous snakes are well known for their rattle—a scale modification at the end of the tail where a new segment is added each time the snake sheds its skin. As members of the pit viper family, they are also of some renown for the heat-sensing pit located between the nostril and eye which is used to help it locate prey. Their hypodermic needle-like fangs are used to inject venom into and kill their prey.
When encountered in the field they may remain motionless, or they may attempt to slowly move away seeking shelter, or they may coil and assume a defensive striking posture and vibrate their rattle. If they strike it may be a false strike with no attempt to bite or make contact. If they do bite, they are able to regulate the amount of venom that is injected—if any. The reach of a strike is about ½ the length of the body.
They are large and heavy-bodied snakes with a total length (TL) in the range of 3-4 feet. The dorsal scales are keeled and overlapping; the anal plate is entire. The background color is pale brown or tan and may be a greenish brown in animals that have recently shed. They have about 40 dark brown, rectangular dorsal blotches with a narrow white border; these blotches become bands and rings near the end of the tail. The belly is white and unmarked. They have a narrow neck with a wide triangular shaped head and the eyes have vertically elliptical pupils. The head has two pair of white stripes that are about 1-1/2 scales wide: one extending from behind the eye to the rear of the jaw, the other running from between the nostril and eye to the middle of the jaw. They are distinguished from the Midget Faded Rattlesnake by having white rather than black borders around the dorsal blotches, by the narrower head stripes, and by their larger size.
These rattlesnakes give birth to 5-20 live young (viviparous) in August and September. Mating generally occurs away from the den in mid or late summer. The sperm remains viable overwinter with fertilization and birth occurring the year following copulation. Depending on age and health, females reproduce every 2nd or 3rd year. Gravid females move only a short distance from winter hibernacula and often congregate and give birth in communal areas known as rookeries. Sexual maturity is reached in the 3rd or 4th year.