Heat and water loss versus shelter: A dilemma in thermoregulatory decision making for a retreat-dwelling nocturnal gecko

Abstract

Understanding the interaction between upper voluntary thermal limit (VTmax) and water loss may aid in predicting responses of ectotherms to increasing temperatures within microhabitats. However, the temperature at which climate heating will force cool-climate nocturnal lizards to abandon daytime retreats remains poorly understood. Here, we developed a new laboratory protocol for determining VTmaxin the retreat-dwelling, viviparous Woodworthia 'Otago/Southland' gecko, based on escape behaviour (abandonment of heated retreat). We compared the body temperature (Tb) at VTmax, and duration of heating, between two source groups with different thermal histories, and among three reproductive groups. We also examined continuous changes in Tb(via an attached biologger) and total evaporative water loss (EWL) during heating. In the field, we measured Tband microhabitat thermal profiles to establish whether geckos reach VTmaxin nature. We found that VTmaxand duration of heating varied between source groups (and thus potentially with prior thermal experience), but not among reproductive groups. Moreover, geckos reached a peak temperature slightly higher than VTmaxbefore abandoning the retreat. Total EWL increased with increasing VTmaxand with the duration of heating. In the field, pregnant geckos with attached biologgers reached VTmaxtemperature, and temperatures of some separately monitored microhabitats exceeded VTmaxin hot weather implying that some retreats must be abandoned to avoid overheating. Our results suggest that cool-climate nocturnal lizards that inhabit daytime retreats may abandon retreats more frequently if climate warming persists, implying a trade-off between retention of originally occupied shelter and ongoing water loss due to overheating.