Sistrurus tergeminus

Sistrurus tergeminus, also known as the western massasauga, is a rattlesnake found in the southwestern plains of the United States and northern Mexico. Like all rattlesnakes, it is a pit viper and is venomous.

Taxonomy
It was once considered a subspecies of the eastern massasauga (Sistrurus catenatus). It contains two subspecies: the nominate S. t. tergeminus, or plains massasauga, found in the Great Plains, and S. t. edwardsii, or desert massasauga, found in the deserts of the Southwestern United States and northern Mexico. The latter is considered "Vulnerable" by NatureServe. According to Campbell and Lamar (2004), a population also exists in southeastern Colorado that is morphologically somewhat intermediate between S. t. tergeminus and S. t. edwardsii.

Description
Adults range in size from 35 to 91 cm. The standard length for 43 male and 63 female adult specimens was 68 cm. Conant (1975) mentions an average length of 46 - 66 cm, with a maximum of 88.3 cm.

The color pattern is similar to that of S. catenatus, but paler: the dark brown blotches contrast strongly with the tan-gray or light gray ground color. The venter (belly) is light with a few dark markings.

Common names
Western massasauga, ground rattlesnake, Gulf Coast massasauga, víbora de cascabel (Mexico), Edward's massasauga, large ground rattlesnake, Say's false rattlesnake, Sonora ground rattlesnake, Texas massasauga, three-spotted shield rattler, triple-spotted rattlesnake, prairie massasauga.

Distribution
It is found in the United States in the southwestern plains from southeastern Colorado to extreme southeastern Nebraska and northwestern Missouri, southwest through east-central Kansas and west-central Oklahoma into northern and central Texas about as far southwest as the Colorado River. It is also found in Mexico, in the states of Tamaulipas, southern Nuevo León, north-central Coahuila, and Samalayuca, Chihuahua. The type locality given is "between the Mississippi River and the Rocky Mountains".

Behavior
They are primarily found in grassland areas, on the edge of open woodland, or on rocky hillsides, and often make use of the burrows of other animals for shelter. They primarily eat rodents, but may also eat lizards and frogs, having for the purpose a set of enlarged adrenal glands. Their rattles are significantly higher pitched than those of larger species of rattlesnake, sometimes giving them the nickname "buzztail", and as such should not be relied upon for warning. Primarily nocturnal, the species is considered mild-mannered and sluggish especially during the summer, when the weather is too hot for them to be active and they are sometimes found out sunning themselves. Though they are most often found by water or immediately after rain, they prefer arid or heavily-covered ground.

Venom
Drop for drop, massasauga venom is more potent than that of many larger species of rattlesnakes, but due to the lower yield (the amount it is capable of delivering in a single bite), its potential for harm is greatly reduced. The venom is a powerful hemotoxin which can cause swelling, necrosis, and severe pain. Despite its smaller size and less severe bite, envenomation can still be fatal if untreated, and treatment should be sought immediately for any venomous snake bite. The antivenin CroFab, while not type-specific, can be used to treat severe envenomations from massasaugas.