Colour-driven activity adjustments mediate thermoregulation in sand lizards

Animal colouration serves multiple functions, including camouflage, communication, and thermoregulation, and is shaped by the trade-offs between these roles. In ectotherms, which primarily regulate body temperature through behaviour, colour can influence how much heat they absorb from the environment. The thermal melanism hypothesis predicts that darker individuals heat up faster than lighter ones, particularly in basking species, but the role of colour in thermoregulation is complex and context-dependent. Although ectotherms rely mainly on behaviour for thermoregulation, colour may modulate heat absorption. Here, we tested how environmental and biotic factors influence body temperature and activity timing in sand lizards (Lacerta agilis). We collected body temperature data and dorsal colouration measurements from two populations in Czechia and used structural equation modelling to disentangle direct and indirect effects. Environmental temperature was the strongest predictor of body temperature, showing a positive effect, whereas body size and site had negative effects. Colouration did not directly affect body temperature but significantly influenced activity timing, with darker and more saturated individuals emerging earlier in the day. This result suggests that colour-mediated thermoregulation in sand lizards is primarily behavioural, shaping basking opportunities rather than passively dictating heat gain. Our findings demonstrate how environmental, morphological, and behavioural factors collectively shape lizard thermoregulation, with colour primarily influencing activity timing rather than directly affecting body temperature.