Why do fence lizards bask in the sun? Turns out, it’s not just to warm up!

Dane A. Conley, Matthew S. Lattanzio

This is a plain language summary of a Functional Ecology research article which can be found here.

In nature, many lizards spend the day moving between sun and shade, behaviours we always thought were driven just by temperature. However, sunlight provides more than just warmth, it also provides exposure to ultraviolet (UV) light. As humans, we often relate UV exposure to sunburn risk, but UV exposure is also important for production of vitamin D. For day-active lizards, vitamin D production is also crucial for proper organ function, bone development, and even egg production. At the same time, too much UV light can also have negative health impacts. For these important reasons, lizards should regulate their UV exposure too! So… is this the case?

Here we conduct the first direct study of UV regulation in a common lizard species, the fence lizard. We captured fence lizards from the wild, measured their body temperature and UV exposure, and then brought them into our lab. At the lab, we then used a homemade Arduino electronic system to record UV and temperature exposures of lizards in an experimental arena over three treatments (temperature only, UV only, and a treatment combining UV and temperature). Each treatment consisted of a gradient of condition levels, from low to high, allowing us to see what specific conditions each individual lizard prefers, an important physiological consideration.

Adult female eastern fence lizard (Sceloporus undulatus), observed basking in the sun during a mark-recapture study in 2018 (credit: Matthew S. Lattanzio)

Overall, lizards did regulate their UV exposure, and preferred higher values than those suggested by captive care guidelines. Thus, fence lizards may experience a boost in health in captivity if given access to higher UV. In contrast, fence lizards preferred similar temperatures to those reported in prior studies. However, those preferences may be overridden by UV demands: in our combination treatment, lizards kept their preferred UV at the cost of exposing themselves to higher temperatures! Normally, we consider exposure to higher temperatures risky, and for good reason: high temperatures can be lethal. Our study however reveals at least one potential benefit of such exposure, access to preferred UV levels, that may partly offset this risk. So, next time you see a lizard lounging in the sun… it may not be trying to warm up, it may be basking for UV!