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

<p>Understanding the interaction between upper voluntary thermal limit (VT_max) and water loss may aid in predicting responses of ectotherms to increasing temperatures within microhabitats. However, the temperature (VT_max) at which climate heating will force cool-climate, nocturnal lizards to abandon daytime retreats remains poorly known. Here, we developed a new laboratory protocol for determining VT_max in the retreat-dwelling, viviparous <i>Woodworthia</i> Otago/Southland gecko, based on escape behaviour (abandonment of heated retreat). We compared the body temperature (T_b) at VT_max, and duration of heating, between two source groups with different thermal histories, and among three reproductive groups. We also examined continuous changes in T_b (via an attached biologger) and total evaporative water loss (EWL) during heating. In the field, we measured T_b and microhabitat thermal profiles to establish whether geckos reach VT_max in nature. We found that VT_max and 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 VT_max before abandoning the retreat. Total EWL increased with increasing VT_max and with the duration of heating. In the field, pregnant geckos with attached biologgers reached VT_max temperature, and temperatures of some separately monitored microhabitats exceeded VT_max in 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.</p>