Thermal heterogeneity of selected retreats in cool-temperate viviparous lizards suggests a potential benefit of future climate warming.

Rocky retreats are limited and geologically constrained resources for rock-dwelling nocturnal lizards. Such lizards should seek retreats that offer thermoregulatory benefits without the risk of overheating during the day, and that protect from predation. For cold-adapted species where air temperature is frequently lower than optimum temperature for performance, factors influencing retreat-site selection and whether future warmer conditions will force superficial rock slabs to be abandoned on hot days remain poorly known. Here, we predicted that retreats selected by a nocturnally foraging, cool-temperate gecko from southern New Zealand would be thermally heterogeneous and that future warmer temperature will force lizards to abandon daytime retreats on hot days. We sampled loose rock slabs (potential retreats) in a tussock-grassland site in all seasons. We measured seasonal rock temperature profiles and field body temperature (T) of captured geckos using thermography and quantified the physical characteristics of each potential retreat. We found that both physical characteristics and rock temperatures determine choice of retreats. Field T of lizards positively correlated with retreat and air temperatures. Also, retreat temperatures, including those of the substrate below the rock slabs, showed complex heterogeneity enabling lizards to choose microsites within retreats to achieve preferred body temperatures intermittently. Observed seasonal shifts in characteristics of occupied rocks imply that lizards choose retreats to maximise warmth in spring, minimise risk of overheating (remain below voluntary thermal maximum, VT) in summer and avoid freezing over winter. Our study demonstrates the importance of microclimatic conditions in influencing retreat-site selection. Climate warming might lead to seasonal changes in use of rock slabs and possibly be beneficial initially, but longer-term implications need to be examined.