Most residents of Tamborine Mountain have at least a few lizards living in their garden. As the weather cools they tend to disappear, why does this happen?
To survive, animals need to maintain their body temperature within certain limits. Mammals and birds are endothermic, meaning that they provide their body heat internally through metabolic processes, they are generally homoeothermic, that is, they maintain a near constant internal temperature. Reptiles, amphibians, fish, insects and arachnids are ectothermic, meaning they derive body heat from their environment, they are generally poikilothermic, that is, their internal temperature may fluctuate.
Ectothermic animals such as lizards need to eat and breathe far less than endothermic animals, which use most of their energy in maintaining a constant internal temperature. The disadvantage of being ectothermic is that changes in environmental temperature cause changes in internal body temperature. Ectothermic animals thermoregulate through behaviour and metabolism. When lizards need to heat up they bask in the sun, expand their rib cage and darken their skin to absorb more heat. If they become too hot they lighten their skin and open their mouth or move to the shade. They can also change their heart rate and blood flow to increase when hot, and decrease when cool. When temperatures are high lizards are active, when the temperature drops they become sluggish and lethargic as the chemical activity in their muscle slows.
As days shorten and temperatures fall, lizards may enter a hibernation-like state called brumation. Lizards in true brumation are dormant and hide in burrows, rock crevices, caves and leaf litter until an increase in temperature and length of days triggers a return to normal activity. Not all lizards experience true brumation, in mild weather some may only exhibit a slight change in activity and feeding patterns.
It was thought that lizards were descendents of dinosaurs, in fact the term dinosaur, first coined by Sir Richard Owen in 1842, is derived from the Greek deinos (fearfully great) and sauros (lizard). However studies have shown that lizards and dinosaurs are not closely related. It is now generally accepted that birds are the descendents of dinosaurs. One of the largest relations of modern terrestrial lizards was not a dinosaur, but Megalania prisca, a giant goanna of 5 metres which roamed Eastern Australia in ancient times.
Mainly because dinosaurs were regarded as giant lizards, it was assumed that they were ectothermic, sluggish and unintelligent. There is a new theory that dinosaurs were active, intelligent endotherms, however this is not universally accepted with some scientists still believing that dinosaurs were too large to be endotherms.
New research into thermoregulation in living creatures is revealing a whole spectrum of adaptations and exceptions which show that many of our old labels such as “cold blooded” and “warm blooded” are too simplistic. It is possible that dinosaurs could have been different from or a combination of ectotherm and endotherm.