Abstract:
Most metabolic functions are optimized within a narrow range of body
temperatures, which is why thermoregulation is of great importance for the survival and
overall fitness of an animal. It has been proposed that lizards will thermoregulate less
precisely in low thermal quality environments, where the costs associated with
thermoregulation are high; in the case of lizards, whose thermoregulation is mainly
behavioural, the primary costs ofthermoregulation are those derived from locomotion.
Decreasing thermoregulatory precision in costly situations is a strategy that enhances
fitness by allowing lizards to be more flexible to changing environmental conditions. It
allows animals to maximize the benefits of maintaining a relatively high body temperature
while minimizing energy expenditure. In situations where oxygen concentration is low, the
costs of thermoregulation are relatively high (i.e. in relation to the amount of oxygen
available for metabolic functions). As a result, it is likely that exposures to hypoxic
conditions induce a decrease in the precision of thermoregulation. This study evaluated the
effects of hypoxia and low environmental thermal quality, two energetically costly
conditions, on the precision and level of thermoregulation in the bearded dragon, Pogona
vitticeps, in an electronic temperature-choice shuttle box.
Four levels of hypoxia (1O, 7, 5 and 4% 02) were tested. Environmental thermal
quality was manipulated by varying the rate of temperature change (oTa) in an electronic
temperature-choice shuttle box. Higher oT a's translate into more thermally challenging
environments, since under these conditions the animals are forced to move a greater
number of times (and hence invest more energy in locomotion) to maintain similar
temperatures than at lower oTa's. In addition, lizards were tested in an "extreme temperatures" treatment during which air temperatures of the hot and cold compartments of
the shuttle box were maintained at a constant 50 and 15°C respectively. This was
considered the most thermally challenging environment. The selected ambient (T a) and
internal body temperatures (Tb) of bearded dragons, as well as the thermoregulatory
precision (measured by the central 68% ofthe Ta and T b distribution) were evaluated. The
thermoregulatory response was similar to both conditions. A significant increase in the size
of the Tb range, reflecting a decrease in thermoregulatory precision, and a drop in preferred
body temperature of ~2 °C, were observed at both 4% oxygen and at the environment of
lowest thermal quality. The present study suggests that in energetically costly situations,
such as the ones tested in this study, the bearded dragon reduces energy expenditure by
decreasing preferred body temperature and minimizing locomotion, at the expense of
precise behavioural thermoregulation. The close similarity of the behavioural
thermoregulatory response to two very different stimuli suggests a possible common
mechanism and neuronal pathway to the thermoregulatory response.
