Predation and the Evolution of Color Polymorphism in the Mottled Rock Rattlesnake (Crotalus lepidus lepidus)
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Questions regarding the origin and maintenance of biodiversity are key topics of evolutionary biology, including in particular, polymorphism. The mottled rock rattlesnake (Crotalus l. lepidus) is a montane species that exhibits striking levels of color polymorphism associated with two distinct substrate types separated by lowland desert, a habitat not used by Crotalus l. lepidus. I hypothesized that selective predation on high contrast color and blotching patterns maintains the distinct races (phenotypes).
To test this hypothesis I performed a predation experiment using model snakes made of urethane foam, at 12 sites in the west Texas portion of the species range. The sites were split between the two regions in which the contrasting races are found including the eastern region composed primarily of light colored limestone and the western region composed primarily of dark volcanic rocks. Two color treatments were used; one mimicking the light colored eastern race and one the dark colored western race. Additionally, these models varied in their blotching pattern. Half had no blotching to simulate the eastern race which has high proportions of anterior blotch fading. The other half of the models were painted with complete blotching to simulate the western race resulting in four total treatments. Forty models were placed at each site (N=10 per treatment). Photographs were taken of all models when they were placed at their respective sites as well as when the models were retrieved two weeks after placement. The number and location of non-predator disturbances and avian attacks of models were recorded. Attack and disturbance data were analyzed with a general linear model under a Poisson distribution with a reciprocal link function with number of avian attacks and nonpredator disturbances as the response variable. Attack location data was analyzed with a one-way ANOVA comparing distance from head (computed as a percentage of total length) as the response variable and type of damage (Avian attack or non-predator disturbance) as the independent variable. I found that high contrast color models were attacked significantly more often; however there was no difference in the frequency of attacks on the different blotching treatments. Additionally, there were no significant differences in non-predator disturbances between the different color or blotching patterns. Predation attempts occurred significantly closer to the anterior end of models than did non-predator disturbances. These results are consistent with the hypothesis that color pattern has been maintained by selective predation.