Decreased thermoregulatory precision contributes to the hypoxic decrease in body temperature in lizards

Abstract

Low oxygen induces a decrease in body temperature (Tb) in most vertebrates. This response has been attributed to a decrease in the in the Tb set‐point, protecting organs from oxygen depletion. In addition, hypoxia decreases activity levels and, therefore, the propensity to move. In ectotherms, where thermoregulation is mainly behavioural, hypoxia is, thus, expected to impact the precision of thermoregulatory control. To determine if thermoregulatory precision is indeed decreased in hypoxia, we evaluated the variability and level of thermoregulation of bearded dragons at five oxygen levels in three different experimental settings: a dynamic temperature‐choice shuttle box, a constant temperature dual‐choice shuttle box, and a thermal gradient. A significant increase in the size of the Tb range was observed at the lowest oxygen level (4% O2) in the dynamic shuttle box, but not in the thermal gradient. This was accompanied by a Tb drop of 2‐ 4°C, the drop being greatest when Tb must be actively defended. Situations that force lizards to continually choose temperatures lead to an increase in Tb variability, which is further exaggerated in hypoxia. This study reveals that a decrease in thermoregulatory precision caused by a diminished propensity to move or effect appropriate thermoregulatory responses may partially explain the lowering of Tb observed in hypoxic ectotherms.