Cold blood

Colloquially, the term “cold-blooded” is used to describe organisms that maintain their body temperatures differently from mammals and birds. The term is now archaic in the scientific context. It was initially assumed that cold-blooded animals (called poikilotherms) were absolutely incapable of maintaining their body temperatures, that they were “slaves” to their environment, and that whatever the ambient temperature, it was the same temperature of their bodies.

Since then, advances in the study of how creatures maintain their internal temperatures (''thermophysiology''), have shown that many of the preconceived notions about the meaning of “cold-blooded” and “warm-blooded” were far from true. reality. Nowadays it is known that the types of body temperature are not so simple as to use only two categories. Most creatures can best be classified with a graduation between “cold-blooded” at one extreme and “warm-blooded” at the other. Because of this, it is preferred to avoid using both terms and they have generally been replaced with one or more of their variants (see: Breaking down “cold-blooded”)

Decomposing “cold-blooded”

“Cold-blooded” generally refers to three separate areas of thermoregulation:

1. Ectotermia
2. Poiquilotermia
3. Bradimetabolism

• Ectotermia– refers to creatures that control body temperature by external means (from the Greek ecto – exterior and thermo – heat)
• Poiquilotermia – refers to creatures whose inner temperature varies, frequently being equal to the temperature of the immediate environment (from the Greek poikilosvaried, and thermo – heat)
• Bradimetabolism – It refers to a creature's rest metabolism. If such a creature has a low-rest metabolism, it is considered bradimetabolic (from the Greek bradys, slowness, and metabolism, change). Brazilian animals can often dramatically change the rate of metabolism, according to the availability of food and temperature. Many bradymetabolic creatures in deserts and areas of harsh winters can “stop” their metabolism and remain almost dead until they return favourable conditions.
  • Note: It is important to remember that a bradimetabolic animal has a low metabolism rest only. His metabolism in activity is often many times higher, so that a bradimetabolic creature should not be considered slow.

In reality, few creatures fall into the above three categories. Most animals use a combination of these three aspects of thermophysiology, along with their counterparts (endothermy, homothermy, and tachymetabolism), creating a wide range of body temperature types. Most of the time, creatures that use one of the previously defined aspects fall under the term: "cold-blooded." Physiologists have also invented the term heterothermy for creatures that exhibit a single case of poikilothermy.

Types of temperature control

Among examples of temperature control we have:

• Viper and lizards taking the sun on the rocks.
• Fish that change deep inside the water to find a proper temperature.
• Desert animals that bury themselves under the sand during the day.
• Insects that heat their flight muscles making them vibrate instead.
• Dilate or constrain peripheral blood vessels to adapt more or less quickly to ambient temperature.

Many warm-blooded, homothermic animals also use these techniques on occasion. For example, all animals are in danger of freezing to death in a drastic temperature decline; in the case of homothermic animals, most can shiver. Automatic reaction of the organism to survive in its environment.

Poikilotherms often have more complex organs than homotherms. For a major chemical reaction, poikilotherms may have four to ten enzyme systems that operate at different temperatures. As a result, poikilotherms often have larger and more complex genomes than homotherms in the same ecological niche. Toads are a notable example of this effect.

Because their metabolism is so variable, it is difficult for poikilotherms to have complex and energy-intensive organs, such as brains or wings. Some of the most complex adaptations known include poikilotherms with these types of organs. An example is the swimming muscle of tuna, which is heated by a heat exchanger. In general, poikilothermic animals do not use their metabolism to heat or cool themselves. For the same body weight, poikilotherms need 1/3 to 1/10 of the energy of a homothermic animal, so they eat 1/3 to 1/10 of the food that a homothermic animal needs.

Some large poikilotherms, due to a large volume/surface ratio, can maintain high temperatures and high metabolism rates. This phenomenon, known as gigantothermia, has been observed in sea turtles and the great white shark and was most certainly present in many dinosaurs and ichthyosaurs.

Ecological niches

It is comparatively easy for a poikilotherm to accumulate enough energy to reproduce. Poikilotherms in the same ecological niche often have much shorter generations than homotherms: weeks instead of years.

This difference in energy also means that a given niche in an ecology can support three to ten times as many poikilotherms as homotherms. However, given an ecological niche, homeotherms often end up extinguishing their poikilothermic competitors because homeotherms can forage for a greater proportion of the day.

Poikilotherms succeed in some niches, such as islands or particular bioregions (such as small bioregions in the Amazon Basin). They often do not have enough food to support a viable population of hot-thermic animals. In these niches, poikilotherms such as large lizards, crabs, and toads supplant homotherms such as birds and mammals.