The common term “cold-blooded” used for iguanas is misleading because their blood is not necessarily cold. The accurate scientific classification is ectotherm. An ectotherm relies on external sources, such as sunlight or warm surfaces, to regulate its body temperature. This reliance means an iguana’s internal temperature fluctuates considerably depending on its surroundings, a survival strategy that shapes nearly every aspect of its life.
The Science of Ectothermy
The physiological difference between an iguana and a mammal lies in how each regulates its internal warmth. Mammals and birds are endotherms; they generate the majority of their body heat internally through a high metabolic rate. This internal heating allows endotherms to maintain a relatively constant body temperature, regardless of the outside environment. Iguanas are ectotherms, possessing a much lower internal heat production and relying on heat transfer from the environment to warm up.
This dependence on external sources means the iguana is also a poikilotherm, an organism whose internal temperature varies widely. While an endotherm’s core temperature is relatively stable, an iguana’s body temperature directly follows the thermal conditions of its habitat. It must actively seek out different thermal zones throughout the day to keep its body within a functional range.
Behavioral Thermoregulation
Iguanas employ conscious actions known as behavioral thermoregulation to manage their body temperature. To elevate their core temperature, iguanas bask, positioning themselves perpendicular to the sun’s rays and flattening their bodies to maximize surface area exposure. Studies on wild iguanas show they actively maintain daytime body temperatures between 96.8 and 98.6°F for optimal functioning.
When temperatures become too high, iguanas seek shade, retreat into water, or burrow underground to cool down. The goal of these behaviors is to reach and stay within the Optimal Temperature Zone (OTZ), which is necessary for fundamental biological processes. A sufficiently warm body temperature is required for efficient digestion and the mobility needed for escaping predators or foraging. They may even engage in thermoregulatory behavior within a burrow before emerging to start the day at a preferred, active temperature.
Energy Efficiency and Metabolic Rate
The ectothermic lifestyle results in a vastly reduced energy requirement compared to endotherms. The metabolic rate of iguanas is approximately one-sixth that of a mammal of a comparable size and temperature. This low rate is possible because iguanas do not expend large amounts of energy constantly generating their own body heat.
This energy efficiency translates directly into lower food needs for survival. Endotherms require frequent, substantial feeding to maintain a stable internal temperature. Iguanas, by contrast, can survive on far less food because their metabolism slows down when they are cool, allowing them to conserve energy during periods of low environmental heat or food scarcity.