Head coloration reflects health state in the red-eared slider Trachemys scripta elegans

Sexual signals can be evolutionarily stable if they are honest and condition-dependent or costly to the signaler. One possible cost is the existence of a trade-off between maintaining physiological health and elaboration of ornaments, such that only healthier individuals may afford to produce more elaborate sexual displays. We analyzed the relationship between head coloration and health state of Trachemys scripta elegans turtles. Results showed that turtles with a higher immune response and with a higher body condition had postorbital red patches with brighter coloration with higher values of long-wavelength reflectance (i.e., more reddish). Similarly, turtles with a higher immune response and with a lower heterophil/lymphocyte (H/L) ratio had chin yellow stripes with darker coloration with higher values of medium wavelengths (i.e., more yellowish). These relationships suggest that the health state of T. scripta elegans turtles is reflected by the colorful skin patches and stripes of the head. Characteristics of coloration did not differ between sexes, suggesting that this visual signal may be used by both sexes in intrasexual and intersexual communication. Because many other turtle species have similar colorful patches, it is likely that coloration may have a still unexplored significant role in sexual selection in many turtles.