Warm-blooded

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Warm-blooded is an archaic term used to describe an animal that keeps its core body temperature at a nearly constant level regardless of the temperature of the surrounding environment (that is, to maintain thermal homeostasis). This can involve not only the ability to generate heat, but also the ability to cool down. Warm-blooded animals control their body temperature by regulating their metabolic rates (e.g. increasing their metabolic rate as the surrounding temperature begins to decrease).

Thanks to more thorough research in the field of animal physiology, scientists have come to realize that body temperature types do not easily fit a simple either/or scenario. Body temperature maintenance incorporates a wide range of different techniques that result in a body temperature spectrum, with the traditional ideals of warm blooded and cold-blooded being at opposite extremes.

Because of the generalness of the terms, as well as an increased understanding in this field, both warm blooded and cold-blooded have mostly fallen out of favour. They have since been replaced with one, or more, of their variants (see: Breaking down warm-bloodedness).

1 Breaking down warm-bloodedness
2 Mechanisms
3 Warm-blooded versus cold-blooded
4 Between cold and warm blooded
5 References
6 External links

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Breaking down warm-bloodedness

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

Endothermy
Homeothermy
Tachymetabolism

Endothermy is the ability of some creatures to control their body temperatures through internal means such as muscle shivering, fat burning, and panting (Greek: endo = "within," therm = "heat").

Homeothermy is the kind of thermoregulation used by those creatures that maintain a stable internal body temperature regardless of external influence. This temperature is often higher than the immediate environment (Greek: homoios = "same, identical," therm = "heat").

Tachymetabolism is the kind of thermoregulation used by creatures that maintain a high resting metabolism (Greek: tachy = "fast, swift," metabol = "to change"). 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 a harder time dealing with a scarcity of food.

A large proportion of the creatures traditionally called "Warm Blooded" (namely mammals and birds) fit all three of these categories. Over the past 30 years, studies in the field of animal thermophysiology has shown that there are still quite a few members of these two groups that don't fit all this criteria (e.g. many bats and small birds are poikilothermic and bradymetabolic when they sleep for the night, or day). For creatures such as these, another term was coined: heterothermy.

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 (see In between cold and warm blooded).

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Mechanisms

Endotherms include birds and mammals. The advantages of endothermy are increased enzyme activity and a constant body temperature, allowing these animals to be active in cold temperatures. On the other hand, the disadvantage is the need to maintain thermoregulation, even during inactivity, otherwise the organism will die.
Other living creatures such as fish and reptiles are called ectothermic or cold blooded, meaning that they cannot control their internal temperature and so were assumed to have the same temperature as their surroundings.

In winter, there may not be enough food to enable an endotherm to keep its metabolic rate stable all day, so some organisms go into a controlled state of hypothermia called hibernation, or torpor. This deliberately lowers the body temperature to conserve energy. In hot weather, endotherms expend considerable energy to avoid overheating: they may pant, sweat, lick, or seek shelter or water.

Diverse mechanisms can come into play to regulate body temperature such as shivering (when muscles are forced to contract, their cells respire more - respiration releases heat and is incidentally the main heat source in the body), blanching (circulatory changes to direct less heat to the skin), flushing (circulatory changes to radiate more heat from the skin), panting or sweating (to increase heat loss through evaporation).

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Warm-blooded versus cold-blooded

Biochemical processes are heat dependent. The rule of thumb is that they go faster when they are warm and slower when they are cold. The advantage of being homeothermic is that you can always maintain yourself near one optimum temperature and all your internal chemical reactions will function at an optimum level. This means that you can think, move, digest, etc. with your best possible speed and efficiency.

Warm blooded animals warm themselves by digesting food. The disadvantage of being warm blooded is that you must always consume large amounts of food energy. When the core temperature of a warm blooded animal does change, even by a few degrees, the animal will rapidly lose its ability to function.

The advantage of being cold blooded is that an organism needs much less food. This means that it can survive famine, long ocean voyages, and shortage of prey when warm blooded organisms would surely die.

The disadvantage of being cold blooded is that an organism needs to have multiple chemical pathways available to it, some of them for cooler temperature functioning, others for warm. Such an organism may also find itself moving or thinking more slowly than normal, simply because the temperature is colder.

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Between cold and warm blooded

It has been a while since the original distinction was made between warm and cold blooded animals. Time has passed, science has advanced, the warm cold business has been studied in closer detail. It turns out that the cold blooded animals all use behavioral means to adjust their temperatures, sometimes quite effectively. There are also creatures that do not properly fall into either category.

Some examples of in between creatures include:

Tuna and Swordfish. Fish have long been thought to be cold blooded. Tuna and swordfish dive deep into the ocean to where the water is quite cold. Swordfish are able to raise the temperature of their brains and eyes in cold water, allowing for faster eye movements when hunting. Tuna are able to warm their entire bodies through a heat exchange mechanism called the rete mirabile, which helps keep heat inside the body, and prevents the loss of heat through the fish's gills into the cold water. They also have active muscles for swimming near the center of their body instead of closer to the cold surface.
Bees. An individual bee is perfectly cold blooded. Bees, however, do not live by themselves. In summer if the nest starts to overheat they will go to the entrances to the nest and fan air in and out of the nest to cool it. In winter if the nest becomes too cold, they will shiver their wing muscles until they grow warm from their efforts. Any solitary bee doing this by itself would not produce any notable effect. Done collectively, this will raise the temperature of the nest.
Skunk Cabbage. Plants are normally thought of as having the exact same temperature as their surroundings. The skunk cabbage uses chemical means to warm itself at the end of winter. The warming is modest by animal standards, but is enough to enable them to get an early start in the spring. This permits them to start growing while all their predators and competition are still asleep because of the cold.

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