Warm-blooded

This article is about the biological term. For the song, see Hot Blooded.

Thermographic image: a coldblooded snake is eating a warmblooded mouse

The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes. These are characteristics of mammals and birds.

Both the terms "warm-blooded" and "cold-blooded" have fallen out of favour with scientists because of the vagueness of the terms and an increased understanding of the field. Body temperature types are not discrete categories. Each term may be replaced with one or more variants (see the next section for examples). Body temperature maintenance (thermoregulation) incorporates a wide range of different techniques that result in a body temperature continuum.

Characteristics of warm-bloodedness

Warm-bloodedness generally refers to three separate aspects of thermoregulation.

• Endothermy is the ability of some creatures to control their body temperatures through internal means such as muscle shivering or increasing their metabolism (Greek: endon = "within", thermē = "heat"). Some writers restrict the meaning of endothermy to mechanisms that directly raise the animal's metabolic rate in order to produce heat. The opposite of endothermy is ectothermy.
• Homeothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This temperature is often, though not necessarily, higher than the immediate environment (Greek: homoios = "similar", thermē = "heat"). The opposite is poikilothermy.
• Tachymetabolism is the kind of thermoregulation used by creatures that maintain a high "resting" metabolism (Greek: tachys/tachus = "fast, swift", metabolēn = "throw beyond"). Tachymetabolic creatures are, essentially, "on" all the time. Though their resting metabolism is still many times slower than their active metabolism, the difference is often not as large as that seen in bradymetabolic creatures. Tachymetabolic creatures have greater difficulty dealing with a scarcity of food.

Reasons for term falling into disuse

A large proportion of the creatures traditionally called "warm-blooded", such as mammals and birds, fit all three of these categories. However, over the past 30 years, studies in the field of animal thermophysiology have revealed many species belonging to these two groups that do not fit all these criteria. For example, many bats and small birds are poikilothermic and bradymetabolic when they sleep for the night, or day, as the case may be. For these creatures, the term heterothermy was coined.

Further studies on animals that were traditionally assumed to be cold-blooded have shown that most creatures incorporate different variations of the three terms defined above, along with their counterparts (ectothermy, poikilothermy and bradymetabolism), thus creating a broad spectrum of body temperature types. Even some fish have warm-blooded characteristics. Swordfish and some sharks have circulatory mechanisms that keep their brains and eyes above ambient temperatures, and thus increase their ability to detect and react to prey. Tunas and some sharks have similar mechanisms in their muscles, improving their stamina when swimming at high speed.

External links

• www.earthlife.net
• Dinosauria.com: What is Warm-bloodedness anyway?
• The Reptipage: What is cold-blooded?