Heat seekers: A tropical nocturnal lizard uses behavioral thermoregulation to exploit rare microclimates at night.

Abstract

Environmental temperatures play a vital role in the physiological and behavioral activity of ectotherms. Behavioral thermoregulation allows animals to modify their body temperature to optimize functions critical to fitness, including digestion, growth, reproduction, and locomotor performance. Diurnal reptiles are a classic model system to answer questions related to thermal ecology, whereas behavioral thermoregulation in nocturnal species is thought to be strongly constrained by low environmental thermal heterogeneity at night. The few studies describing the thermal ecology of nocturnal reptiles indicate a majority of thermoregulatory behavior (if any) occurs during the day within diurnal retreats, but few examined this behavior throughout the night. In tropical systems, thermal heterogeneity may remain high, even at night, allowing nocturnal ectotherms to thermoregulate through conduction on surfaces that retain heat after sunset. We investigated the thermoregulatory behavior of a tropical nocturnal gecko (Australian house gecko, Gehyra dubia) by measuring its preferred temperature in a thermal gradient, and selected body temperatures using radio telemetry, in relation to available operative environmental temperatures obtained using thermal models. Preferred body temperatures of geckos ranged from 31.4 ± 0.59-34.5 ± 0.55 °C in a laboratory thermal gradient. In the field, during winter, geckos were more effective thermoregulators than in the summer. In low thermal quality habitats, geckos sought rare, warm microclimates at night to maintain body temperatures warmer than most available environmental temperatures, and were highly effective thermoregulators. For ectotherms, appropriate environmental temperatures are a vital resource for survival, similar to food or shelter. The ability to exploit rare microclimates is especially important for nocturnal species, as heterogeneity of environmental temperatures is reduced at night compared to the day. In a warming world, it is vital to understand the thermal ecology of nocturnal ectotherms, as other species may shift to become more nocturnal to avoid lethal diurnal temperatures.