Active regulation of ultraviolet light exposure overrides thermal preference behaviour in eastern fence lizards

Abstract

Over a century of ecophysiological studies on lizards have perpetuated the assumption that basking and shuttling movements between sun and shade function solely for temperature regulation. However, these behaviours also modulate exposure to ultraviolet (UV) wavelengths that are essential for maintaining physiological homoeostasis as well as ensuring proper growth and development and enhancing long-term fitness. An alternative hypothesis is that lizards also actively regulate their UV exposure. In this scenario, UV needs may even override temperature needs (or vice versa), generating asymmetries in the ability of a lizard to regulate both conditions equally. We test this hypothesis using field and laboratory data collected on adult Sceloporus undulatus. We found that S. undulatus actively regulate UV exposure and prioritize UV over temperature, favouring body temperatures much higher than preferred values to sustain preferred UV exposure. In stark contrast, temperature had no reciprocal impact on UV regulation behaviour. Our field data support these patterns, suggesting that lizards may even seek out hotter environments despite thermal costs to enhance UV exposure. We conclude that S. undulatus actively regulate for UV as well as temperature. Unfortunately, outside of zoos and private hobbyists, appreciation of the importance of UV for ectotherm survival and reproductive success has been minimal. Addressing this deficit will therefore be vital to improve our understanding of the factors shaping the evolution of ectotherm photoregulation behaviour in nature. Read the free Plain Language Summary for this article on the Journal blog.