Potential adaptive significance of color change in Crotalus helleri

Streit, M. Benjamin

mstreit@students.llu.edu

Gren, Eric C. K

Martin, Chelsea E.

Pikstein, Rachel

Hayes, William K.

Department of Earth and Biological Sciences

Loma Linda University

Loma Linda, California USA

Coloration in animals represents an adaptive trait, with important implications for the animal’s ecology. Colors and color patterns confer adaptive advantages, and therefore vary widely across taxa. Functions of adaptive coloration include thermoregulation, reducing detectability o prey or predators, and inter- and intraspecific signaling. Individual animals may change color over time in response to changes in the environment through changes in pigmentation, either via synthesis or degradation of pigments in the dermal tissues or through active reconfiguration of pigment-containing cells. Metachrosis comprises the ability of certain animal species to change color through this latter method, often over brief time periods. Among tetrapods, metachrosis has been extensively studied in amphibians and lizards, and has been reported in some snakes, including several rattlesnake species; however, the adaptive significance of metachrosis in snakes remains poorly understood. Reports of environmental factors affecting rattlesnake coloration remain speculative or anecdotal. We therefore conducted three studies on the environmental stimuli that influence pigmentation in Southern Pacific Rattlesnakes (Crotalus helleri). First, we assessed skin pigmentation in preserved museum specimens across the species’ California range to associate pigmentation with environmental parameters. Second, we evaluated photographs of captures and recent road-kills across an elevation gradient of the San Jacinto Mountains (from desert to coniferous forest) for the same purpose. Lastly, we subjected captive rattlesnakes to differing temperature and stress conditions in the laboratory while actively monitoring color change over a three-hour time scale to determine if metachrosis in C. helleri occurs as a thermoregulatory response to ambient temperature or a physiological response to acute stress. Collectively, our results should provide insights on the selective pressures on coloration faced by rattlesnakes in their natural habitat and the adaptive function of metachrosis, thereby furthering our understanding of these animals’ ecology.