Forelimb stripe coloration signals age, but not physiological health, in painted turtles

Variation in aspects of carotenoid-based coloration reflects physiological health in many taxa. Painted turtles (Chrysemys picta) have brightly colored limb stripes produced by carotenoids, but the function of this color and its relation to physiological function are unknown. We tested the hypothesis that variation in painted turtle forelimb stripe coloration is associated with variation in physiological health - including glucocorticoids and both innate and adaptive immune function. We assessed adult painted turtles of both sexes across a range of sizes/ages from a long-studied population. We found that body size - a strong indicator of age in this species - was associated with changes in forelimb stripe color, such that larger turtles have brighter, more colorful stripes with increased contrast from the background forelimb color. We further found that a number of physiological health measures either changed with size, or differed between sexes, yet physiology measures were not associated with stripe color. Thus, forelimb stripe coloration is not a direct honest signal of quality. Secondarily, we provide a suite of protocols for future use to understand physiological variation of reptiles in the wild. We discuss our results in the context of signaling, physiological health, aging, and sex-specific life histories.

The online version contains supplementary material available at 10.1007/s00265-025-03679-0.