While commonly called “cold-blooded,” this term is scientifically misleading. Lizards are accurately described as ectotherms, meaning their body temperature is primarily dictated by external sources rather than internal heat generation. Understanding this distinction is fundamental to grasping the biology and survival strategies of these reptiles.
Ectothermy Versus Endothermy
The key difference between lizards and mammals lies in the source of their body heat, distinguishing ectothermy from endothermy. Ectothermy describes animals that rely on the environment to regulate their temperature. Conversely, endotherms, such as birds and mammals, generate most of their body heat internally through metabolic processes.
The term “cold-blooded” is a misnomer because a lizard’s body temperature is not consistently low; it can often be quite high. Many lizard species actively seek out heat, sometimes maintaining temperatures that exceed those of humans. The biological distinction is the mechanism used to achieve the preferred operating temperature, which for a lizard, is achieved by absorbing heat from the outside world.
Behavioral Regulation of Body Temperature
Since lizards cannot rely on an internal furnace to maintain a stable temperature, they engage in complex, active behaviors to manage their heat levels. This behavioral thermoregulation is the primary way they optimize body functions, such as digestion and locomotion. A common strategy is heliothermy, which involves solar basking to gain heat directly from radiation.
Lizards often change their body posture to maximize or minimize sun exposure, such as flattening their bodies against a surface to absorb more warmth. They also utilize thigmothermy, which is the absorption of heat through direct physical contact with a warm substrate, like a sun-baked rock. When temperatures become too high, lizards move to cool microclimates, such as seeking shade or retreating into burrows, demonstrating constant, active management of their thermal state.
Metabolic Consequences of Ectothermy
The reliance on external heat sources results in significant differences in a lizard’s overall physiology and energy budget compared to an endotherm. Ectotherms have substantially lower metabolic rates because they do not have to continuously expend energy to maintain a stable, high internal temperature. A resting endotherm may spend more than ten times the energy of a similarly-sized ectotherm.
This efficiency means that lizards require significantly less food and can survive for long periods without eating. However, this energy saving comes at the cost of being heavily dependent on environmental temperature for all physiological activity. When temperatures drop below their preferred range, a lizard’s metabolism slows down, making them sluggish and unable to perform functions like effective digestion. Activity levels thus rise and fall directly with the available environmental heat.