Crotalus oreganus concolor is a venomous pitviper subspecies found in the western United States. It is a small subspecies known for its faded color pattern.
Grows to a maximum length of 750 mm (29 1⁄2 in). The smallest gravid female measured was 522 mm (20 1⁄2 in).
The color pattern consists of a pinkish, pale brown, yellow-brown, straw-colored, reddish or yellow-brown ground color, overlaid with a series of brown elliptical or rectangular dorsal blotches. However, most specimens are gray or silvery. In juveniles the pattern is distinct, but becomes faded in adults, almost to the point where it is indistinguishable from the ground color.
Found in the United States in the Colorado and Green River basins. This area covers southwestern Wyoming, Utah east of long. 111° West (excluding the southeastern corner) and extreme west-central Colorado. The type locality given is “King’s Ranch, Garfield Co., at the base of the Henry Mts [Utah].”
This subspecies possesses the most toxic venom of the C. oreganus / C. viridis group, although there is apparently considerable variability among local populations (Glenn and Straight, 1977, 1978). It is even one of the most potent venoms found in North America (Glenn and Straight, 1977), and according to LD50 studies the venom is many times more potent than that of an Asiatic Cobra. It is characterized by the presence of a presynaptic neurotoxin, referred to as concolor toxin, the amount of which varies in individual snakes (Glenn and Straight, 1977, 1990; Wetstein et al., 1985).