Color Variation, Tail Banding, and Sexual Dichromatism in Washington Populations of Northern Pacific Rattlesnakes, Crotalus oreganus

Abstract: Northern Pacific Rattlesnakes (Crotalus oreganus) are ambush-hunting predators that rely on crypsis to forage and to avoid encounters with predators, yet little is known about color variation in this ubiquitous rattlesnake species. This study addressed how coloration and contrast in tail banding in C. oreganus vary among habitat types in Washington State. We also explored whether C. oreganus exhibit sexual dimorphism in coloration or tail band contrast (sexual dichromatism). We sampled 127 rattlesnakes from seven different populations across northern and central Washington state during spring emergence from overwintering hibernacula in 2017 and 2018. We characterized snake coloration as red/blue color ratios from standardized photographs, and used GIS supervised classification schemes of satellite imagery to characterize habitat. We used generalized linear models to assess relationships among snake color and tail bands, habitat, and sex. We found that coloration (red/blue color ratios) in C. oreganus varied greatly across the landscape, both within and among populations, likely a reflection of their variable and heterogenous shrub-steppe and forest ecotone habitats. In 20% of 21 pairwise comparisons, populations differed in body color. Rattlesnake coloration was not associated with habitat (amount of forested land within 0.5 km of the snake den), but male and female rattlesnakes showed different associations between color and percent forested habitat. Male rattlesnakes did not differ in body coloration from females, but males showed greater contrast than females in the black and white banding present on the tail. We discuss several, nonmutually exclusive, hypotheses for sexual dichromatism in tail band contrast, including the possibility that tail banding constitutes warning coloration in rattlesnakes. Our results suggest that sexual dichromatism, and the role of tail banding in rattlesnake ecology, are topics worthy of further investigation.