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ABSTRACT Animal coloration is a complex phenotype that may be affected by genetics, evolution, ecology, and environment. Disentangling the impact of environment on phenotype can often be done in laboratory studies, but the results do not necessarily correspond to the natural variation present in the wild. Painted turtles are a brightly colored freshwater species that inhabit a variety of environments in North America. There is known to be plasticity in the melanin coloration of the shell of painted turtles in a lab setting, but this has not been measured in the wild. The bright skin coloration that gives painted turtles their name is caused by carotenoids, which can only be obtained from an organism's diet in vertebrates. Though the availability of carotenoids likely varies between environments, and there is evidence that some of the carotenoid‐based coloration in this species is a visual signal, it is unknown if or how environmental variation impacts coloration in the wild. To address this, we measured the effect of the environment on turtle coloration by assessing multiple populations of painted turtles in northern Wisconsin. We measured water clarity and aquatic plant density at each site where turtles were caught. We found that females had brighter carapaces than males, and that plastron brightness varied with water clarity and plant density, despite its ventral orientation. We also found that neither water clarity nor plant density predicted carotenoid chroma, despite reason to believe that light environment and carotenoid availability should impact a visual signal. These findings suggest that colorful phenotypic traits in this turtle species are complex and their potential role as visual signals requires more research. It is crucial to understand the different phenotypes of painted turtles since coloration may influence fitness in this species, and since laboratory studies are unable to represent natural variation. 

Color signals in animals are often honest, containing information about the individual for potential mates or predators. Both males and females may have honest color signals, though female signals are less often assessed for honesty. Different colorimetric traits, such as overall brightness, hue, ultraviolet brightness, or carotenoid chroma, may be assessed by receivers for information. Painted Turtles, Chrysemys picta (Schneider, 1783), are a brightly colored and widely distributed freshwater turtle species with no visible sexual dichromatism, but the function of their coloration is unknown. We assess two populations of Painted Turtles to compare colorimetric traits across and within populations, and to determine whether any color traits correlate with innate immune function. We find that there is greater carotenoid chroma on areas of the shell not typically associated with courtship than on the neck stripes that are thought to have a role in mate choice. We also find that only one measure of coloration in one color patch, the carotenoid chroma of the neck stripes, is correlated with bactericidal capacity, and in only one population. This system provides an example of a common species having vibrant but understudied coloration that may provide insights into unknown functions of color or uncommon sexual selection variation.