Prairie Rattlesnake. Crotalus viridis

Description: A medium-sized, thick and heavy-bodied snake that attains a total length of from 370 to 1,400 mm. The head is broad and triangular, and much wider than the neck. Facial pits are present behind and below the nostrils, one on each side of the head. The eyes have vertical pupils. Elongate, paired, retractile fangs are present in the upper jaw. The dorsal scales are keeled and the anal scale is single. The tail is short and bears a lobate rattle at the distal end. The general background colour is tan to brownish, with a repeating pattern of dark blotches or cross bands along the back. Smaller blotches are present along the sides. The tail usually has alternating light and dark rings. There is often a light stripe running from behind the eye to the corner of the mouth. The venter is a yellowy-cream.

Variation: Males are larger than females.

Natural history: This species is found mostly in the drier areas of grassland and sage brush. It may also be found in farmlands and fields. It is typically found in areas of sandy soil, frequently in association with rock piles or boulders. It dens in mammal burrows, rock crevices or caves, and may den communally. In these dens, body temperature is maintained only a few degrees above ambient, but they may move to warmer parts of the den during the period of "hibernation." Winter hibernacula may be a long distance from the summer foraging sites. Emergence from the dens will not occur until air temperature is about 10°C. It preys upon small mammals, birds, reptiles and amphibians.

The heat-sensing facial pits are used to locate warm-blooded prey. Individuals may communicate with one another by way of an alarm pheromone secreted from the cloacal sacs. Crotalus usually enters hibernation in late September and emerges in late April or early May. It may remain active after the first frosts. It is generally crepuscular, but may be active at any time. When molested, it will coil, raise the head, shake the rattle, and strike. It may strike repeatedly. It is venomous and dangerous, but rarely fatal. It is capable of swimming. This species may live for up to fifteen years.

Reproduction: Sexual maturity is reached at five to seven years. Females may have their first litters in six to eight years. In the Okanagan Valley of British Columbia, mating occurs in late summer and parturition takes place in the following September or early October. Mating may take place in aggregations of several individuals. Multiple females may use a single rookery with preferred thermal conditions. The prairie rattlesnake is live-bearing and usually gives birth to four to twelve large young. They have a total length of about 280 mm at birth and weigh about 18 g. The female may give birth in or near the overwintering dens and may not feed during the period of gestation. Females probably breed only every second to fourth year. Males may engage in combat bouts near the dens during the breeding period.

Distribution: In Alberta, this species is found in the short-grass prairie regions of the south east. Older records suggest a range extending along the Red Deer River almost to the level of Red Deer, and along the Bow River valley almost to the level of Calgary. More recent collections and sightings, however, are confined to the areas east of 112°53' W and south of 51°22' N. On the range map accompanying this description, the current range of the prairie rattlesnake is indicated by solid circles. Solid squares represent pre-1978 reports that are now disjunct from the current distribution and probably represent extirpated populations. Extralimitally, this species occurs from south central B.C. and southern Saskatchewan south to Baja California and north central Mexico. Its range extends from the Pacific coast to eastern Nebraska. It is absent from Vancouver Island and much of its range is highly fragmented.

Conservation status: The range of the prairie rattlesnake has become reduced in recent decades and anecdotal reports have intimated that this animal is not as common as it once was in the southeastern corner of the province. Numbers still appear to be healthy, however, and a monitoring project is underway to attempt to determine if land use patterns have a direct effect on rattlesnake density. Special attention will be paid to such effects on den sites.

Remarks: Eight subspecies are recognized. Crotalus viridis viridis is the Alberta subspecies. The range of this subspecies essentially encompasses the range of the entire species east of the Rockies. It is of considerable economic value in controlling pests. In Alberta, the conversion of rangeland to cultivation may be responsible for a shrinking distribution.

Significant references: Cookl965; Cowles and Phelan 1958; Diller and Wallace 1996; Dobie 1965; Duvall 1986; Duvall et al. 1985;Ernst, 1992; Fitch 1985; Gannon 1978; Gloyd 1940; Graves and Duvall 1988, 1993; Hennessy and Owings 1988; Jacob and Painter 1980; Klauber 1936,1937,1972; Lewin 1963a; Logier 1932; Ludwig and Rahn 1943; Lynn, W.G. 1931; Macartney 1985; Macartney and Gregory 1988; Macartney and Weichel 1989; Marion and Sexton 1984; Melcer and Chiszar 1989; Melcer et al. 1988; Milner 1979a; Murphy and Crabtree 1988; Parker and Brown 1974; Pendlebury 1976a, 1977; Rahn 1942; Rodgers and Jellison 1942; Schaeffer et al. 1972, 1978; Sexton and Marion 1981; Stark 1984, 1985, 1986a, b; Watson and Russell 1997; White and Lasiewski 1971; Williams 1946; Zimmerman 1948