Western Massasauga (Sistrurus tergeminus)

Western Massasauga  (Sistrurus tergeminus)

by Jeff LeClere

A western massasauga, Sistrurus tergeminus, from Mills County, Iowa.


PROTECTED and Species of Greatest Conservation Need. It is illegal to kill or collect this species by law in Iowa. Reports are needed for parts of western Iowa. Please report any sightings to us or the DNR.


Dangerously venomous.This is a small rattlesnake that measures 17 – 39 1/2 inches in length. There is a row of black or dark brown mid dorsal blotches on a lighter brown or gray background. There are two or three rows of small spots on the sides. All blotches and spots may be outlined in white. The blotches usually turn into rings on the tail. It has an overall more gray dusky or smoky appearance than the eastern massasauga. There is a rather broad dark stripe from the eye past the angle of the jaw. The belly is mostly black with irregular white or yellowish marks.

Western massasaugas are stout bodied snakes with a triangular shaped head. The pupils are vertically elliptical and there is a heat sensitive pit located between the eye and nostril. This pit is larger and positioned lower on the face than the nostril. There is a rattle at the end of the tail. The rattle is comprised of a series of interlocking keratinous segments that make a buzzing noise when the tail is vibrated. The scales are keeled and the anal plate is single.

This snake may be quite difficult to distinguish from the harmless fox and water snakes. One may look for a rattle at the end of the tail, but massasaugas usually have small, dark rattles that are not always distinctive. Western massasaugas will rattle with their tails off the ground (though not as high as other rattlesnake species). Harmless snakes that vibrate their tails must hold their tails close to the ground to produce noise. The best thing to do is leave it be if you are unsure of what species it is. Use the range to separate western massasaugas from eastern massasaugas in Iowa.


One subspecies, the prairie massasauga, Sistrurus tergeminus tergeminus, is found in Iowa.


Western massasaugas are extremely rare in Iowa. There were larger populations in southwestern Iowa, but today these snakes are reduced to a very small area in Iowa. The records for the western massasauga are found in extreme southwestern Iowa and records for the eastern massasauga are in the eastern half of the state.

If you see a western massasauga in Iowa, do not kill or harm it in any way. Also, do not attempt to catch or move it. It may be a useful record, however. If you wish to report it, try to take a good picture of it. If the specimen is found dead, a good photo of a dead snake will suffice! Most importantly, give clear and exact locality documentation, and report it to the DNR, or us!

Range map for the massasauga in Iowa
Range map for the massasauga in Iowa



Most authors seem to agree that western massasaugas in this portion of their range prefer to overwinter in low, swampy areas close to marshes, lakes, and rivers. They then travel to prairies, open grasslands, meadows, or dry woodlands. Bailey (1944) found specimens well away from water when they had a more expansive range in the state.


Some references state that western massasaugas are slow, sluggish snakes that are quite passive and slow to rattle or strike. They rely on their coloration for camouflage to avoid detection. One may almost step on this snake before evoking a reaction from it. I have found in Kansas and Nebraska that they are diverse in their behavior as some will rattle at the least provocation and strike repeatedly. The massasauga’s rattle is small, and sometimes can only be heard at relatively close distances.

This snake is generally diurnal except during hot weather. They seem to enjoy warm overcast weather. They take shelter under rocks, logs, pieces of bark, under piles of brush, muskrat houses, or in crayfish burrows. They breed in the spring and fall. They may only breed every other year or less and are sexually mature at the age of three or four. They give birth to living young (ovoviviparous). The young number from 5 – 24 and are about 5 – 7 inches in length. The young are equipped with fangs and are venomous as soon as they are born. They are usually more brightly colored than the adults. Instead of overwintering communally, like many species of snakes, they overwinter alone in mammal, or, more commonly, crayfish burrows and other areas where they can gauge the water line. It is reported that they emerge with the spring flooding and basically follow the water line during this time.


The diet of western massasaugas is comprised largely of small mammals, but small birds, lizards, frogs, toads, and other snakes are also consumed. While adults usually feed upon rodents, small snakes are an important food item for young massasaugas. It is interesting to note that this snake will bite and release adult rodents then search them out and swallow them after they are dead. When birds, lizards, or snakes are to be consumed, they are held in the snake’s jaws until the venom takes effect or are eaten immediately.