Western Rattlesnake {Crotalus viridis). 



Description: Rattlesnakes belong to the pit-viper family, Crotalidae, which have a heat-sensing 

 pit located between the nostril and the eye. The fangs are hollow and hinged, allowing them 

 to be folded back against the roof of the mouth. The head is triangular in shape, the front of 

 which is blunt-nosed. The eyes are slightly elevated. There are several white lines which run 

 along the side of the head. Adult Western Rattlesnakes have a narrow neck but a stout body 

 with total length ranging from 37-1 12 cm. The dorsal backgroimd color varies from pale 

 green to brovra with a series of brown or black blotches edged with a dark and then light line 

 extending the length of the body. The blotches often merge into rings on the tail. There are 

 also blotches on the sides of the body. The venfral side is pale yellow to white and without 

 blotches. The scales are keeled. The tail ends in a rattle which helps to warn potential 

 predators of the snake's presence. Females give birth to 4-21 yoimg during the summer; the 

 young have the same color pattern as the adults (Hammerson 1 982a) 



Habitat and Habits: The Western Rattlesnake is an inhabitant of more open and arid country but 

 it is also found in Ponderosa pine stands or mixed grass-coniferous forests. It is more likely 

 to be encountered on south- facing slopes and areas of rock outcrops. Western Rattlesnakes 

 may den in large numbers, moving up to 10 miles out from the dens during the summer 

 (Peterson, pers. comm.) Dates of encounters on the BNF range from 1 July to 1 1 August. 

 Undoubtedly the period of surface activity on the Forest is greater than this. In Wyoming, it 

 is found at elevations of over 8500' (Baxter and Stone 1985), but BNF records are from 

 below 5000'. Western Rattlesnakes prey on a variety of animals including mice, ground 

 squirrels, rabbits, amphibians, and other snakes. 



Surveying: Walk-through surveys on warm sunny days, including rolling over rocks and logs is 

 probably the best method for determining relative numbers. Mark-recapture methods can be 

 done to determine more precise numbers. Roads can be driven slowly at night while looking 

 for basking or roadkilled individuals. 



Status: The Western Rattlesnake was not encountered during the 1995 survey of the BNF. There 

 are but five records for the BNF, all from the Darby and Sula districts low in the Sapphire 

 Mountains. However, the records span 1932-1994, and the species is probably more 

 abundant and widespread than indicated by the paucity of records. It is feared and often 

 needlessly killed due to its poisonous bite. The habit of denning at fraditional sites in large 

 numbers makes rattlesnakes vulnerable to commercial collecting or simply killing by fearful 

 people. Any den sites should be carefully documented and protected. Encounters with this 

 species should also be documented, reported to the Heritage Program, and kept on file. 



Montana Natural Heritage Program Rank: G5 S4. 



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