An Assessment of Thermal Preference of Two Species of Knob-tailed Geckos, Nephrurus levis and N. laevissimus, at Uluru Kata-Tjuta National Park

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Nocturnal lizards are often active at temperatures that are sub-optimal for physiological functioning, a phenomenon that has been referred to as the ‘nocturnal paradox’. The purpose of this study was to investigate the general habitat and thermal preference of two species of nocturnal gecko in the genus Nephrurus with a focus on differences between preferred body temperatures determined under laboratory conditions and those measured in animals at the time of field collection. The Smooth Knob-tailed gecko (Nephrurus levis) and the Pale Knob-tailed Gecko (N. laevissimus) inhabit the desert environment of Uluru Kata-Tjuta National Park in the Northern Territory of Australia. Habitat preferences were determined by documenting capture locations for these species while thermal preferences were determined using laboratory-based thermal gradients. Analysis of habitat use demonstrated a significant difference in habitat preference between the two species. Nephrurus levis was most often in Spinifex sandplain and mulga shrubland and N. laevissimus was most often found around sand dune habitats. Fieldactive body temperatures of both species ranged from 14.5 °C to 32.2 °C and were significantly correlated with air and ground temperature at the time of capture. The thermal preference (Tpref) of N. levis (mean day Tpref mid = 29.1 °C and mean night Tpref 24 = 28.2 °C, n = 19) and N. laevissimus (mean day Tpref mid = 29.5 °C and mean night Tpref 24 = 27.8 °C, n = 27) were not significantly different, although both species exhibited significantly higher daytime and night-time preferred body temperatures than body temperatures observed in the field. Body size did not affect thermal preference for either species. As such, the thermal preferences of these species support the concept of the nocturnal paradox. Additionally, it has been suggested that the characteristic swollen tail tip displayed by all Nephrurus species may play a role in assessing the thermal environment. While not investigated extensively here, combined data for both species demonstrated that individuals oriented their tail toward the heat source in thermal gradients significantly more than expected if orientation were random.