Ectothermic animals are generally thought to possess a species-specific preferred body temperature, which they protect behaviourally. For example,after feeding, many ectothermic animals appear to choose a higher temperature,termed postprandial thermophily, which may enable the animal to digest its meal at a higher rate. However, postprandial thermophily is generally investigated in laboratory thermal gradients. The open question is whether an animal's behaviour in this simplified setting is the same as observed in the wild? Does postprandial thermophily really exist? Some evidence suggests that it may not. For example, garter snakes maintain high constant temperatures in the field, but select relatively low temperatures in a thermal gradient,whereas other reptiles select higher temperatures in laboratory gradients than they do in the wild. Michael Wall and Rick Shine from University of Sydney,Australia, set out to investigate postprandial thermophily in an Australian pygopodid lizard (Lialis burtonis Gray) to test whether there was a discrepancy between field and laboratory measurements of postprandial thermophily.
To test the lizards' preferences when fed and unfed, the team designed three settings with different thermal patterns. First they constructed a field enclosure with a natural leaf litter layer that the lizards could burrow into,mimicking their natural habitat. The leaf litter provided good insulation,resulting in a temperature gradient ranging from 41.6°C at the surface to 24.8°C at the bottom of the leaf litter so that the team could measure the lizards' body temperatures after the reptiles had burrowed to a comfortable temperature. The authors also constructed a traditional temperature gradient in the lab with six visually separated sandy lanes. A combination of heating strips and cooling water created a linear temperature gradient at the sand surface with one end at 15°C and the other at 43°C. Placing fed and unfed animals at the warm end of the gradient, each in individual lanes, the team allowed the animals to run freely within the temperature gradient and measured their body temperatures for 3 days. But what if the fed lizards simply stayed put after feeding? The team had no way of knowing whether the reptile's immobility was due to postprandial thermophily or the animals simply settling down in any old spot to digest dinner. This time the authors adjusted the gradient so that the temperature ranged between 26.5°C and 50°C before placing fed and unfed animals in the cool end and waiting to see whether the reptiles moved. If the fed lizards moved to a warmer place than unfed lizards, then they were demonstrating postprandial thermophily. But if they simply settled down to digest their meal at the cold end of the gradient,then they would be cooler than the unfed animals; `reverse' postprandial thermophily.
The outcome was in fact all three possibilities! In the field experiment there were no differences between the fed and unfed lizards' mean body temperatures, i.e. no postprandial thermophily. However, in the first gradient experiment the fed lizards had higher body temperatures than the unfed lizards; classic postprandial thermophily. Finally, the second gradient experiment showed that the fed lizards were cooler than the unfed lizards– there was reversed postprandial thermophily.
Michael Wall and Rick Shine have elegantly shown that thermal gradient data may be misleading, or even plain wrong, compared with free-ranging animals'thermal ecology. As most postprandial thermophily research has been carried out in temperature gradients in the lab, the team suspects that postprandial thermophily could be less widespread than previously thought. Researchers should, therefore, take care in future experiments to provide animals with microhabitats that better approximate their natural environment to get more physiologically relevant readings.