In some viperid snakes, natural selection has shaped coloration and patterning to match local habitats, and for males to show greater warning coloration than females due to a less sedentary lifestyle. The Northern Pacific Rattlesnake (Crotalus oreganus) is likely the most abundant ambush predator in eastern Washington, yet little is known about its color variation, how color patterns interact with their habitats, and if male and female snakes show differences in patterning or color. To explore how snake color might interact with the colors of their environment, I used standardized digital photography to record both snake and substrate coloration. I sampled 127 rattlesnakes from overwintering hibernacula and compared their coloration to that of the surrounding habitat. I also used satellite imagery and land cover classification to investigate possible relationships between snake coloration and habitat at a broader landscape level. The hypothesis that C. oreganus shows sexual dichromatism (inter-sexual differences in color and/or pattern) was also tested by comparing both overall snake color, and the strength of contrast in tail bands between the sexes. I found that C. oreganus varies in color significantly, both locally between hibernacula, and regionally. Snake coloration was not associated with the degree of forested area within a 0.5 km radius of the hibernacula, and no significant differences in color were observed between the sexes. Despite this, the color of male and female snakes interacted differently with the habitat surrounding the hibernacula, where male snakes showed increased Red/Blue color ratios in high forested areas, while females showed lower ratios. Importantly, males showed significantly greater contrast in their tail bands than did females, suggesting increased selection for warning patterns and coloration in male snakes. Sexual dimorphism is usually attributed to sexual selection. In contrast, the sexually dichromatic warning coloration observed in this study is an example of “Ecologically Caused Sexual Dimorphism”, which has now been observed in at least two species of viper. My results suggest that other viper species with similar natural history traits to C. oreganus may also exhibit sexual dichromatism, where males have exaggerated warning colors in comparison to females
