Shell colouration is associated with sex, boldness, and innate immunity in wild adult eastern box turtles (Terrapene carolina carolina)

Abstract Animal colouration often plays a role in intraspecific communication. The badges of lizards and ornaments of birds are frequent examples of honest signals regarding physiological condition and behaviour. Turtles share evolved characteristics with both non-avian and avian reptiles and many express colour markings on the shell and head, yet the significance of colouration in turtles remains unclear. We assessed aspects of shell colouration (carapace spot hue, saturation, and brightness and percent melanism of the carapace and plastron) in adult eastern box turtles ( Terrapene carolina carolina ) from a wetland population and explored their association with age, sex, boldness, haemolysis capabilities, plasma triglycerides, plasma corticosterone, and body condition. We found evidence of varying bold temperaments based on high among- and low within- individual variation across several behaviours during and after handling. Findings further suggested that separate aspects of shell colouration were associated with sex, boldness temperament, and innate immunity. Specifically, males exhibited redder carapace spots than females, bolder turtles exhibited brighter carapace spots and less melanic (more spotted) plastrons, and turtles with higher in vitro haemolysis scores tended to have more saturated carapace spots. These associations suggest potential roles for shell colouration to signal aspects of behaviour and immune function, however, we discuss limitations arising from our methods such as the use of a single blood sample for physiological metrics. Further studies on these associations are needed to determine their stability, underlying mechanisms, and ecological implications.