Hot and covered: how dragons face the heat and thermoregulate

Regulating body temperature is a critical function for many animals. Ectotherms use multiple thermoregulatory behaviours, including habitat selection, sun-shade shuttling, posture, orientation, gaping, and panting. According to thermoregulatory control theory, gaping and postural behaviours should act in coordination with microhabitat selection, providing a fine-tuned counterbalance to more costly behaviours. However, gaping and panting have also been considered indicators of stress in lizards, which would counter a homeostatic thermoregulatory interpretation, especially during expression of voluntary behaviours. Careful adjustments in rostral orientation toward warmth have been observed in bearded dragons, analogous to well-described solar gain and solar avoidance postures in the wild. Little is known about the sensory drivers of these behaviours. Although skin temperature changes faster than core, it is not uniform across the body, and is subject to evaporative cooling, and thus could be crucial to directing behavioural thermoregulatory decision making. To examine the subtle coordination between thermoregulatory behaviours, and to test if inhibiting gaping would lead to thermoregulatory compensatory behaviours, bearded dragon lizards (Pogona vitticeps) were allowed to behaviourally thermoregulate, while their ability to show spontaneous gaping behaviour was disrupted non-pharmacologically. Gaping acted in concert with thermoregulatory behaviours, although at lower rates than predicted from earlier steady-state models, suggesting that respiratory cooling mechanisms are perceived as costly. Bearded dragons enhanced their rostral orientation to heat when gaping was inhibited, while reducing their selected temperatures in a thermal gradient. Combined, these observations indicate the presence of coordination between these various thermoregulatory behaviours.