You can't use expressions like "warm-blooded" and "cold-blooded" and sound informed. In fact, most "cold-blooded" vertebrates have warm blood when they are active. So it's really about endotherms (endo - inside; therm - heat), organisms that regulate their temperature internally, and ectotherms (ecto - outside), organisms that use external sources of heat to regulate their temperatures.
In some organisms, called poikilotherms (poikilo - changing), temperature fluctuates, but in others, called homeotherms (homeo - same), the temperature remains constant. Humans are endothermic homeotherms: when we are unable to maintain our body temperature, we get sick. Ectotherms, such as lizards, can tolerate decreases in core temperature, while endotherms must internally regulate their core temperatures.
Temperature control is not the only issue in endothermic and ectothermic tetrapods. Indeed, more central is the very nature of metabolism itself, that is the sum of the chemical reactions in the cells of the organism, and the effect that the differences in endothermic and ectothermic metabolisms have on activity (Box 12.1).
Ectothermy and endothermy are two biochemically and biophysically different methods of obtaining heat. The terms poikilotherm and homeotherm, however, are endpoints in a spectrum that runs from maintaining a constant temperature to having a fluctuating temperature. While many animals do cluster at the familiar metabolic endpoints, many do not (Box 12.2). The metabolisms of dinosaurs, as we shall see, likely did not closely match those found in living vertebrates.
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