Kade Skelton, Kimberley A Day, Chava L Weitzman, Christine Schlesinger, Craig Moritz , Keith Allen Christian 

Maintaining stable hydric and thermal states are dual challenges for reptiles that inhabit terrestrial environments with variable conditions across time and space. Under some conditions, reptiles face a conundrum where both physiological parameters cannot be simultaneously maintained at optimal states by behavioural or physiological means. Prioritisation of behavioural regulation of hydric or thermal state, and at which point this prioritisation changes, was tested for nine species of congeneric tropical geckos by assessing their use of microhabitats with distinct thermal and hydric conditions in a controlled environment. Gehyra geckos were presented with two crevices of contrasting humidity levels, and time spent in either crevice was recorded across three ambient temperature treatments of 32 °C, 27 °C, and 22 °C. Temperatures in the humid crevice matched treatment temperatures, while temperatures in the dry crevice remained at 32 °C. In these trials, all species showed a strong preference for the dry (and warm) crevice in the 27 °C and 22 °C treatments, while preference for the humid or dry crevice was variable in the 32 °C treatment. Thus, Gehyra geckos prioritised thermoregulation and maintained thermal state through behavioural responses, and maintenance of hydric state was not optimised even when it did not compromise the animal’s thermal state. Although body temperature was optimised in the short-term, this does not preclude the possibility that hydric state can be regulated on a seasonal time-scale.