Beyond thermoregulation: Active behavioural regulation of ultraviolet light exposure by a free-ranging lizard

Reptile behavioural ecology and regulatory behaviours have long been interpreted primarily through the lens of thermoregulation. However, basking and shuttling also expose animals to ultraviolet (UV) radiation necessary for vitamin D₃ synthesis and other fitness-relevant processes. Here, I adapt a standard thermoregulation framework to UV data to test whether lizards actively regulate UV exposure in nature. Using field observations, operative models, laboratory preference trials and custom-built UV data loggers, I quantified the effectiveness of UV regulation and thermoregulation by ornate tree lizards (Urosaurus ornatus) across contrasting microhabitats (trees vs. snags). Lizards consistently preferred low UV exposures despite high environmental variation, particularly on snags. While U. ornatus thermoregulated equally well in both microhabitats, lizards were far more effective at UV regulation in trees than in snags. This difference likely reflects structural contrasts, as trees offer finer mosaics of sun and shade, whereas snags often impose unavoidable full-sun exposure during basking. My results provide field evidence that wild lizards actively regulate their UV exposure. They further suggest that, for U. ornatus at least, the physiological consequences of microhabitat selection depend more strongly on UV conditions than on thermal conditions. Recognising UV regulation as a fundamental component of reptile homeostasis alongside thermoregulation, together with the trade-offs between these regulatory demands, will broaden our understanding of reptile physiology and ecology and provide a stronger basis for predicting their ecological and evolutionary responses to environmental change.