The animal world would be divided into two categories: so-called “warm-blooded” animals on one side and so-called “cold-blooded” animals on the other. Are these scientific expressions that correspond to reality or is it more complex? Here, we take a close look at cold-blooded animals and reveal the essentials of their biological functioning.
What does the expression “in cold blood” mean?
The expressions “warm-blooded” and “cold-blooded” are found in the scientific literature. In general, warm-blooded animals are birds and mammals, so all others are cold-blooded animals. However, not all animals have blood: sponges, corals, jellyfish do not.
Moreover, if we take the example of an animal which actually has blood and which is part of cold-blooded animals such as the lizard, one can only be surprised at the poor adequacy between the animal and its cold blooded animal quality. Indeed, on summer days, it is not uncommon to come across one, motionless, resting on a stone in full sun. If you take her blood temperature at this moment, you will measure it at 45 ° C. This is not what you might call a cold temperature. In comparison, the blood flowing through our fingertips, we warm-blooded animals, can be measured at 25 ° C when temperatures are low. Hence our need to wear gloves. These expressions which refer to the temperature of the blood are therefore widely used but ultimately constitute real puzzles when we look a little more closely. The reference to blood is therefore only symbolic, the expressions “warm blood” or “cold blood” ultimately referring to the animal’s ability to regulate body temperature.
Ectothermia, poikilothermia and bradymetabolism
Warm-blooded animals are said to be “homeothermic”, a name made up of two Greek terms:
- ὅμοιος, hómoios which means “similar”,
- and θερμός, thermos, which means “hot”.
These animals know how to maintain a constant or relatively constant body temperature, whatever the environment.
In cold-blooded animals, we identify three mechanisms distinct body temperature regulation.
There is theectothermy. This name is composed from the Greek ἐκτός, ektos, which means “outside”. Ectothermal animals are animals whose body heat is regulated by capturing it in the immediate external environment.
There’s the poikilothermia, from the Greek ποικιλος, poikilos, which means “changing, varied”. This means that the animal’s body temperature experiences significant variations.
There are animals that are both “ectothermic” and “poikilothermic”, but there are exceptions that prevent these two terms from being made synonyms. The difference is easy to understand. If the temperature of the environment in which the animal evolves does not change, its temperature does not change. It is therefore an ectotherm because it regulates its body temperature from its environment, but not a poikilothermic since its temperature is globally constant due to the constancy of its environment.
There is finally the bradymetabolism. In this case, the metabolism goes to rest. The best known examples are animals that hibernate.
These three mechanisms are not mutually exclusive and most animals combine them.
The winning strategy of cold-blooded animals
Mussels and oysters, for example, have developed protective mechanisms against dehydration and their predators, by making the shell. Other cold-blooded animals change location depending on the ambient temperature, such as lizards which will sometimes be in the sun, sometimes in the shade. Still others regulate their temperature in extremely hot places by playing on perspiration.
If the game of natural selection has allowed the appearance of this type of biological functioning, it is because it has advantages. The most important of them is that these animals require much lower energy requirements than those of other modes of operation. Cold blooded animals are observed in conditions in which warm blooded animals could not have survived.
Many reptiles heat up faster than they cool down. To do this, they modify the blood flow in the skin. As they warm up in the sun, their arterioles dilate to better capture heat. Conversely, when the animal is in the shade, the diameter of the arterioles decreases and the blood circulation slows down to limit heat loss.
The European pond turtle or brenne tortoise is a turtle that lives in muddy environments, measuring on average 14 cm. From November to March, the pond turtle winters in water or on land, sheltered under a good bed of leaves, its purpose being to avoid freezing. During periods of extreme cold, his heart hardly beats any more. In the spring, it is about regaining strength. It eats fish, insects, molluscs and crustaceans. She sunbaths for several hours a day to warm up and store the energy needed for her activity.
The royal python is the smallest of the African pythons measuring around 1.30m. For him, summer is a difficult period because the drought of the desert eliminates most of the vegetation and, therefore, the available prey. He therefore begins a period of rest called “aestivation”.
The West African crocodile dozes at the water’s edge to warm itself in the sun and regularly opens its mouth to cool off. When this is not enough, it immerses in water and then comes out to warm up again, etc.
Talking about the Atlantic salmon, which is a poikilothermic animal, also allows us to introduce a new term, that of stenotherm. This means that this fish only supports temperature variations of small amplitude around average values. Salmon’s preferred temperature range is between 9 and 17 ° C, with death occurring around 25 ° C. Apart from the optimal values, the functions allowing food, locomotion or even quite simply the perception of the environment are considerably reduced, requiring increased supplies of oxygen. As long as temperatures are only slightly outside the optimal range, oxygen supplies can be met through aerobic respiration. But during extreme variations, anaerobic respiration is triggered and the salmon goes into heat stress.
The common frog is the most common brown frog in Europe. It is an active frog day and night but it remains hidden on hot days. It hibernates during the winter months, sinking under a stump or silting up in the bottom of a stream or river, remaining motionless to reduce its vital needs to a minimum. Its breathing is then done through the skin.
One can only marvel at the richness of life. When one takes a close interest in its various manifestations, ingenuity is clearly at work: nature has great adaptability to respond to situations that sometimes appear very extreme to us.