Crotalus viridis
(Western Rattlesnake)

General Description:
Western Rattlesnakes are readily identified with their large triangular head offset by their narrow neck, the deep pit located between their nostrils and their upper labial scales, and of course, their rattle.  Neonates lack a well developed rattle, having instead a "button" that will gain new segments each time the snake sheds its skin.   The pit in the face is used to detect infrared (heat) signals from potential prey and gives this group of snakes the common name "pit vipers".  Western Rattlesnakes have highly keeled scales,and their eyes have vertical pupils.  This pupil configuration helps distinguish Western Rattlesnakes from Gopher Snakes who have round pupils.  Western Rattlesnakes vary in coloration among subspecies, but generally have a light dorsal coloration (green, tan or gray) that is marked with contrasting dark saddles, which are sometimes edged with white.   There is usually a lateral series of blotches as well.  The ventral scales are generally a solid cream or yellow color. 

There are three subspecies of Western Rattlesnake that occur in Idaho, and they are different enough in coloration to warrant additional description.  The subspecies with the smallest range in Idaho is the the Prairie Rattlesnake (Crotalus viridis viridis).  Prairie Rattlesnakes are only found in east-central Idaho, in Valley and Lemhi counties (Nussbaum et al. 1983).  The ground color of these snakes can be greenish-gray, yellow or brownish gray (Cobb 1996).  Prairie Rattlesnakes have wide eyestripes that are bordered on either side by light stripes, and these stripes angle towards the jaw.  The blotches on Prairie Rattlesnakes are straight edged, and are bordered by light-colored scales (Nussbaum et al. 1983).

The subspecies found in the southwestern portion of the state is the Great Basin Rattlesnake (Crotalus viridis lutosus).  These snakes have less distinct markings on the head, and less prominent body blotches that are irregular in shape and may have light colored centers (Nussbaum et al. 1983).  However, I have encountered juvenile Great Basin Rattlesnakes that have light colored outlines around their blotches and on either side of the dark eye stripe.  Great Basin Rattlesnakes have a range of ground colors that varies among individuals.  The ground color can be buff-gray, yellowish gray or light brown (Cobb 1996).

The subspecies of Western Rattlesnake found in the remaining extent of the species range in Idaho is the Northern Pacific Rattlesnake (Crotalus viridis oreganus).   This subspecies is found in west-central, Idaho, in association with the Salmon, Clearwater and Snake River drainages.  Northern Pacific Rattlesnakes have head markings similar to C. v. lutosus, but they have dark brown, dark gray or even black ground colors (Nussbaum et al. 1983).  Like the Great Basin Rattlesnake, Northern Pacific Rattlesnakes generally lack a light outline around the dorsal markings.

Although not the longest snake species occurring in Idaho, Western Rattlesnakes are the most heavy-bodied.  They can reach sizes of up to 163 cm (~65 in.) in total length (Cobb 1996) and males are generally larger than females (Storm and Leonard 1995).

Western Rattlesnakes differ from many Idaho snakes because they can mate at any time during active season, giving birth to 3-12 young, which are born in August or September (Storm and Leonard 1995).  The young resemble adults morphologically, except for the prebutton or button that they have before a multi-segmented rattle develops.  Juveniles may be more vividly colored than adults.

Habitat:
Western Rattlesnakes are usually found in drier regions with sparse vegetation (Storm and Leonard 1995).  The area usually has a rocky component (or at least has one near enough to be utilized as a hibernaculum during the winter).  Hibernacula are generally on south facing slopes and are not shaded by vegetation (Nussbaum et al. 1983).

Idaho Distribution:
In Idaho, Western Rattlesnakes are found in all but the northern portion of the state, and at high elevations.  From southwestern Saskatchewan, west to southern British Columbia, south to central Baja California and north-central Mexico, and across U.S. from Pacific Coast to western Iowa and central Kansas.

Diet:
Eats mainly small mammals, but may also consume birds, lizards, and amphibians (rarely). In some regions, juveniles prey mostly on lizards rather than on small mammals. In southwestern Idaho, adults prefer mice, wood rats, ground squirrels and rabbits. British Columbia study found that most feeding occurs from June through August.

Ecology:
Primarily terrestrial. Mostly diurnal in cool weather, and nocturnal/crepuscular in hot, summer weather (active in morning and late afternoon in far northern range). Hibernates/aestivates. Remains in mammal burrows, crevices, or caves when inactive; individuals may congregate at hibernation dens, at times with other snake species. Active from about April to November over most of range, and from late March to October in southern British Columbia and northern Idaho. Gravid females may or may not feed, depending on area. Idaho's only dangerously venomous snake species. Preyed upon by raptors such as Red-tailed Hawks.

Reproduction:
Young are born August-October (August-early October in Idaho). Litter size increases with female size. Adult female may not give birth every year; British Columbia study found 2-yr (or more) interval between litters. Northern Idaho study identified females that reproduce in consecutive years; annual versus biannual reproduction was linked to level of fat reserves in body. In areas with short growing seasons, adults require several years to reach sexual maturity (4-6 yr in Idaho; 5-7 yr for females in British Columbia).

Conservation:

Status:	Unprotected nongame species
Global Rank:	G5
State Rank:	S5

Important State References:
Diller, L.V. and R.L. Wallace. 1984. Reproductive biology of the northern Pacific rattlesnake (Crotalus viridis oreganus) in northern Idaho. Herpetologica 40:182-193.