Metachrosis, or color change, in reptiles is used for thermoregulation, crypsis, and many other purposes. The mechanism and function of metachrosis remain unknown for many species, however, especially snakes. Anecdotal observations suggest that some snake species, including rattlesnakes, undergo varying degrees of color change when captured and confined. A possible explanation for this color change is the increase in plasma levels of the primary stress hormone, corticosterone (CORT). In this study, we implanted Southern Pacific Rattlesnakes (Crotalus helleri) with either CORT or sham silastic implants and photographed them under standardized lighting in a curtained box at the time of implant and 2 and 4 wk postimplant. We quantified light value (brightness or darkness) of the dark and light bands of the subjects' tails and examined the relationships of these variables to baseline CORT levels (CORT level at time of capture) as well as CORT levels after 1 h of acute confinement stress. CORT-treated snakes had higher baseline CORT than control snakes, but treatment had no direct effect on color. Regardless of treatment group, baseline CORT was positively correlated with lighter light bands, but had no relationship with the dark bands. Additionally, the magnitude of the CORT increase during acute stress was related to greater increase in contrast between light and dark bands. Defensive behavior was negatively correlated with contrast. We discuss potential reasons for the relationship between stress, defensive behavior, and color change.

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