What Is Warm-bloodedness, Anyway?

Jeff Poling

The question of dinosaurian warm-bloodedness has a long and tortuous history. Many paleontologists of the 19th century believed dinosaurs to be the closest relatives of birds, and therefore must have been warm-blooded. In the 1920s this view changed, possibly due to a wave of Christian fundamentalism that washed over the U.S. at that time. For the next 40 years, paleontologists believed dinosaurs to be cold-blooded failures. The tide turned in the 1960s when many paleontologists began to question the modern orthodoxy. Today, most paleontologists have returned to the view of dinosaurs of warm-blooded creatures. But what do the phrases "warm-blooded" and "cold-blooded" mean?

Warm-blooded and cold-blooded are catchall phrases that refer to three fundamental aspects of animal physiology:

1. body temperature regulation,
2. variation in body temperature, and
3. level of resting metabolism.

Body Temperature Regulation. Temperature regulation can be accomplished utilizing internal or external sources. Animals that regulate their body temperature internally are called endotherms. Endotherms regulate body temperature by utilizing internal processes such as altering bloodflow, muscle contraction, or burning calories. Animals that regulate their body temperature externally are called ectotherms. The body temperature of ectotherms respond directly to there temperature of the environment they are in. An ectotherm regulates body temperature by moving itself to an environment that will allow the animal to heat up or cool down to whatever temperature the animal desires. This is a key concept: ectotherms, like endotherms, regulate their own body temperature, but do so by utilizing its outside environment rather than internal biological processes.

Body Temperature Variation. Body temperature can be either widely variant or constant (or slightly variant). Maintaining a constant body temperature, usually well above the ambient temperature, is called homeothermy. Maintaining a widely variant body temperature, usually close to the variation in ambient temperature, is called poikilothermy.

Resting Metabolism. Metabolism refers to the level of internal activity that takes place in an animal, such as cellular respiration or burning food calories. Resting metabolism refers to the level of internal activity when an animal is at rest. An animal whose metabolism remains at a high level, burning calories and maintaining other metabolic functions at a high pace, is called tachymetabolic. An animal whose metabolism slows to a low level of activity when resting is called bradymetabolic.

Generally, animals referred to as warm-blooded are endothermic, homeothermic and tachymetabolic, while animals referred to as cold-blooded are generally ectothermic, poikilothermic and bradymetabolic. However, this is not always true.

The body temperature of the Naked Mole Rat, a mammal, varies with that of its burrow. It is ectothermic, homeothermic (the temperature of the burrow, and therefore the animal, does not vary widely), and, as a mammal, tachymetabolic. Is it warm-blooded or cold-blooded? The body temperature of a hibernating bear, a mammal, reflects the outside temperature, which remains fairly constant, and its metabolism is in a very low state. Therefore, when hibernating, it is ectothermic, homeothermic and bradymetabolic. Is it warm-blooded or cold-blooded? The body temperature of the Hawk Moth, an insect, reflects the temperature of its surroundings when at rest. However, its huge wing muscles can raise its body temperature much higher than that of its surroundings. It is endothermic, poikilothermic and bradymetabolic. Is it warm-blooded or cold-blooded?

Mammals are considered to be warm-blooded, yet some mammals exhibit characteristics, such as ectothermy and bradymetabolism, that are characteristic of cold-blooded animals. Insects are considered to be cold-blooded, yet some insects exhibit characteristics, such as endothermy, that are characteristic of warm-blooded animals. Therefore, care should be taken when using the terms "warm-blooded" and "cold-blooded" as many animals can exhibit characteristics that are a mix rather than being wholly one or the other. More than likely, this includes dinosaurs.