Cold-blooded and Warm-blooded Explained and Corrected

Jul 22, 2010
Ken Chan

: Basking Lizards Can Have High Body Temperatures - Ken Chan

For a number of reasons, the once useful terms "warm-blooded" and "cold-blooded" are no longer helpful in describing physiological conditions of animals.

Body temperature is the amount of heat generated in an animal’s body. Heat is naturally produced as a by-product of metabolism, which is a process involving chemical reactions in body cells to convert consumed food into energy. A higher rate of chemical reactions in cells means a higher metabolic rate, resulting in greater heat production.

But the heat an animal produces is constantly lost to the environment. This is why we shiver when it is cold outside; shivering increases metabolism and therefore heat production to replace some of the lost heat.

Endothermy and Ectothermy

It is common to see the term “warm-blooded” used interchangeably with the biological term “endothermic” (“endo” = internal, “therm” = temperature), which describes those animals able to produce enough heat to offset any changes to the external temperature. Because of this, it is argued that their body (and blood) temperature is always “warm”.

Mammals and birds were thought to be the only true warm-blooded, or endothermic, animals. Humans are an example because their resting body temperature is maintained at a high 36–37°C (96.8–98.6°F) regardless of what the outside temperature is.

Most other animals (reptiles, amphibians, fish, insects and other invertebrates) produce so little metabolic heat that they rely on the external environment (e.g. the sun) to raise their body temperature. They are said to be cold-blooded because if the environmental temperature is low, their body temperature would also be low and their blood would be cold. Cold-blooded animals are often referred to as “ectotherms” (“ecto” = external).

However, because an ectotherm’s internal temperature varies directly with the environmental temperature, a hot surrounding will generate a high body temperature. Indeed, the body temperature of a lizard basking on a hot day can reach more than 40°C (104°F). Therefore if endotherms are warm-blooded, then ectotherms can be hot-blooded.

Furthermore, “endotherm” and “ectotherm” are really biological terms describing the main method by which an animal obtains heat. By contrast, “warm-blooded” and “cold-blooded” express a measure of heat in the blood (or body) of an animal. There are therefore problems concerning the interchangeability of these terms.

Homeothermy and Poikilothermy

Often “warm-blooded” and “cold-blooded” are used synonymously with the biological terms “homeotherm” and “poikilotherm”, respectively. A homeotherm is an animal whose body temperature is constant. A poikilotherm is an animal whose body temperature is variable.

In the past, it was thought that all endotherms are homeothermic mammals and birds and that all ectotherms are poikilotherms because they have a low rate of heat production. We know now that some deep-sea fishes living in very thermally stable environments have more stable body temperatures than do many mammals and birds.

Similarly, an ectotherm that lives in the constant temperature of a cave will have a constant body temperature. Even the ectothermic lizard can maintain a constant body temperature by continually moving to cooler and warmer places.

Clearly, just as they are misleading terms for endotherms and ectotherms, “warm-blooded” and “cold-blooded” are misleading terms for homeotherms and poikilotherms, respectively, because many poikilotherms can become quite warm and many so-called homeotherms can likewise become cold by undergoing torpor or hibernation at certain times.

Usage in Formal and Informal Situations

The terms “warm-blooded” and “cold-blooded” have become outdated colloquial words in our vocabulary, and their essentially incorrect expressions can cause confusion to truth-seeking debates on such topics as dinosaur thermoregulation.

Since the biological terms “endotherm”, “ectotherm”, “homeotherm”, and “poikilotherm” are available to cover all aspects of body-temperature descriptions of animals, “warm-blooded” and “cold-blooded” should be used in the most informal situations only, if they are used at all.