The diversity of life on our planet is striking in its scale. Recent studies by Canadian scientists give a figure of 8.7 million species of animals, plants, fungi and microorganisms that inhabit our planet. Moreover, only about 20% of them are described, and this is 1.5 million species known to us. Living organisms have populated all the ecological niches on the planet. Within the biosphere there is no place where life would be absent. In the vents of volcanoes and at the peak of Everest - everywhere we find life in its various manifestations. And, of course, nature owes such diversity and distribution to the appearance of the phenomenon of warm-bloodedness (homeothermic organisms) in the process of evolution.
The boundary of life is temperature
The basis of life is the metabolism of the body, which depends on the speed and nature of the flow of chemical processes. And these chemical reactions are possible only in a certain temperature range, with their own indicators and the duration of exposure. For a larger number of organisms, the boundary indicators of the temperature regime of the environment are considered from 0 to +50 degrees Celsius.
But this is a speculative conclusion. More precisely, it will be said that the temperature boundaries of life will be those at which protein denaturation does not occur, as well as irreversible changes in the colloidal characteristics of the cytoplasm of cells, impaired activity of vital enzymes. And many organisms in the process of evolution acquired highly specialized enzymatic systems that allowed them to live in conditions far beyond these boundaries.
Environmental classification
The boundaries of optimal life temperatures determine the division of life forms on the planet into two groups - cryophiles and thermophiles. The first group prefers cold for life and is specialized for living in such conditions. More than 80% of the planet’s biosphere is cold areas with an average temperature of +5 ° C. These are the depths of the oceans, the deserts of the Arctic and Antarctic, the tundra and highlands. Increased cold resistance is provided by biochemical adaptations.
The enzymatic system of cryophiles effectively reduces the activation energy of biological molecules and supports the metabolism in the cell at a temperature close to 0 ° C. At the same time, adaptations go in two directions - in the acquisition of resistance (opposition) or tolerance (resistance) to cold. The ecological group of thermophiles is organisms that are optimal for life in areas of high temperature. Their livelihoods are also provided by the specialization of biochemical adaptations. It is worth mentioning that with the complication of the organization of the body, its ability to thermophilia decreases.
Body temperature
The heat balance in a living system is a combination of its arrival and consumption. The temperature of the body of organisms depends on the ambient temperature (exogenous heat). In addition, endogenous heat, a product of internal metabolism (oxidative processes and the breakdown of adenosine triphosphoric acid), is a mandatory attribute of life. The vital activity of most species on our planet depends on exogenous heat, and their body temperature on the course of ambient temperatures. These are poikilothermic organisms (poikilos - various), in which the body temperature is variable.
Poikilotherm - all microorganisms, fungi, plants, invertebrates and most chordates. And only two groups of vertebrates - birds and mammals - are homoothermal organisms (homoios - similar). They maintain a constant temperature of their body, regardless of the ambient temperature. They are also called warm-blooded animals. Their main difference is the presence of a powerful flow of internal heat and a system of thermoregulatory mechanisms. As a result, in homoothermal organisms, all physiological processes are carried out at optimal and constant temperatures.
True and false
Some poikilothermic organisms, such as fish and echinoderms, also have a constant body temperature. They live in conditions of constant external temperatures (depths of the ocean or caves), where the ambient temperature does not change. They are called falsely homeothermic organisms. Many animals, which are characterized by hibernation or temporary stupor, have a changing body temperature. These true homeothermic organisms (examples: marmots, bats, hedgehogs, swifts and others) are called heterothermal.
Dear Aromorphosis
The emergence of homeothermia in living things is a very energy-intensive evolutionary acquisition. Scientists today argue about the emergence of this progressive change in structure, which led to an increase in the level of organization. Many theories of the origin of warm-blooded organisms have been proposed. Some researchers assume that even dinosaurs could possess this feature. But with all the disagreements of scientists, one thing is for sure: the appearance of homeothermic organisms is a bioenergetic phenomenon. And the complication of life forms is associated with the functional improvement of heat transfer mechanisms.
Temperature compensation
The ability of some poikilothermic organisms to maintain a constant level of metabolic processes over a wide range of changes in body temperature is provided by biochemical devices and is called temperature compensation. It is based on the ability of some enzymes to change their configuration at lower temperatures and to increase affinity for the substrate, increasing the reaction rate. For example, in bivalve mussels of the Barents Sea, oxygen consumption is independent of ambient temperatures, which range from 25 ° C (+5 to +30 ° C).
Intermediate forms
Evolutionary biologists have found representatives of transitional forms from poikilotherm to warm-blooded mammals. Canadian biologists from Brock University have discovered seasonal warm-bloodedness in the Argentinean black and white tegu (Alvator merianae). This almost meter long lizard lives in South America. Like most reptiles, the tagu is basked in the sun during the day, and hides in holes and caves at night, where it cools down. But during the breeding season from September to October, the temperature of the tagu, the respiratory rate and the rhythm of the heart contractions in the morning increase sharply. The body temperature of a lizard can exceed the temperature in the cave by ten degrees. This proves the transition of forms from cold-bloodedness to homoothermic animals.
Thermoregulation Mechanisms
Homoothermal organisms always work to ensure the functioning of the main systems - circulatory, respiratory, excretory - by developing a minimum of heat production. This minimum, produced at rest, is called basal metabolism. The transition to an active state in warm-blooded animals increases heat production, and to prevent protein denaturation, they need mechanisms to increase heat transfer.
The process of achieving a balance between these processes is ensured by chemical and physical thermoregulation. These mechanisms protect homeothermic organisms from low temperatures and overheating. The mechanisms for maintaining a constant body temperature (chemical and physical thermoregulation) have various sources and are very diverse.
Chemical thermoregulation
In response to lowering the temperature of the warm-blooded animals, a reflex increase in the production of endogenous heat occurs. This is achieved by enhancing oxidative processes, especially in muscle tissue. Inconsistent muscle contraction (trembling) and thermoregulatory tone are the first stages of increasing heat production. In this case, lipid metabolism increases, and adipose tissue becomes the key to better thermoregulation. Cold climate mammals even have brown fat, all the heat from oxidation of which goes to heat the body. This energy expenditure requires the animal to either consume large amounts of food, or substantial fat reserves. With a lack of these resources, chemical thermoregulation has its limits.
The mechanisms of physical thermoregulation
This type of thermoregulation does not require additional costs for heat production, but is carried out by maintaining endogenous heat. It is carried out by evaporation (sweating), radiation (radiation), heat conduction (conduction) and convection of the skin. The methods of physical thermoregulation developed in the course of evolution and become more and more perfect when studying phylogenetic series from insectivores and bats to mammals.
An example of such regulation is the narrowing or expansion of blood capillaries of the skin, which changes the thermal conductivity, heat-insulating properties of fur and feathers, countercurrent heat transfer of blood between the surface vessels and vessels of the internal organs. Heat transfer is regulated by the slope of the hair of the fur and feathers, between which the air gap is maintained.
In marine mammals, subcutaneous fat is distributed throughout the body, protecting endotherm. For example, in seals, such a fat bag reaches up to 50% of the total weight. That is why the snow does not melt under the seals that lie on the ice crust for hours. For animals living in hot climates, a uniform distribution of body fat over the entire surface of the body would be disastrous. Therefore, they accumulate fat only in certain areas of the body (the hump of a camel, the tail of a sheep in sheep), which does not prevent evaporation from the entire surface of the body. In addition, animals of the northern cold climate have a special fatty tissue (brown fat), which is fully used to heat the body.
Closer to the south - more ears and longer legs
Different parts of the body are far from equivalent in terms of heat transfer. To maintain heat transfer, the ratio of the surface of the body to its volume is important, because the amount of internal heat depends on the mass of the body, and heat is exchanged through the integument. The protruding parts of the body have a large surface, which is good for a hot climate, where warm-blooded animals require large heat transfer. For example, large ears with many blood vessels, long limbs and a tail are typical for residents of a hot climate (elephant, fox fox, African long-eared jerboa). In cold conditions, adaptation goes along the path of saving area to volume (ears and tail of seals).
There is another law for warm-blooded animals - the north the representatives of one phylogenetic group live, the larger they are. And this is also associated with the ratio of the volume of the surface of evaporation, and, accordingly, heat loss, and the mass of the animal.
Ethology and heat transfer
Behavioral features also play an important role in heat transfer processes, both for poikilothermic and homoyothermal animals. This includes changes in posture, and the construction of shelters, and various migrations. The greater the depth of the hole, the smoother the course of temperatures. For middle latitudes at a depth of already 1.5 meters, seasonal temperature fluctuations are invisible.
For thermoregulation, group behavior is also used. So, penguins are knocked together, tightly clinging to each other. Inside the heap, the temperature is close to the body temperature of penguins (+37 ° C) even in the most severe frosts. Camels do the same - in the center of the group the temperature is around +39 ° C, and the fur of the extreme animals can heat up to +70 ° C.
Hibernation is a special strategy
The torpid state (numbness) or hibernation are special strategies of warm-blooded animals that allow the use of changes in body temperature for adaptive purposes. In this state, animals stop maintaining body temperature and lower it to almost zero. Hibernation is characterized by a decrease in the level of metabolism and the consumption of accumulated resources. This is a well-regulated physiological state when the thermoregulatory mechanisms switch to a lower level - the heart rate decreases (for example, in a Sonya regiment from 450 to 35 beats per minute), oxygen consumption decreases by 20-100 times.
Awakening requires energy and occurs through self-heating, which should not be confused with the numbness of cold-blooded animals, where it is caused by a decrease in ambient temperature and is an unregulated state of the body (awakening occurs under the influence of external factors).
Numbness is also an adjustable state, but at the same time the body temperature drops by only a few degrees and often accompanies circadian rhythms. For example, hummingbirds become numb at night when their body temperature drops from 40 ° C to 18 ° C. There are many transitions between numbness and hibernation. So, although we call the bear’s sleep in winter hibernation, in fact, their metabolism decreases slightly, and their body temperature drops by only 3 - 6 ° C. It is in this state that the bear gives birth to cubs.
Why are there few homoeothermic organisms in the aquatic environment?
Among hydrobionts (organisms living in the aquatic environment) there are few representatives of warm-blooded animals. Whales, dolphins, fur seals are secondary aquatic animals that have returned to the aquatic environment from land. Warm-bloodedness is primarily associated with an increase in metabolic processes, the basis of which is the oxidation reaction. And the main role here is played by oxygen. And, as you know, in an aqueous medium the oxygen content is not higher than 1% of the volume. Diffusion of oxygen in water is thousands of times less than in air, which makes it even less accessible. In addition, with increasing temperature and enrichment of water with organic compounds, the oxygen content decreases. All this makes the existence of a large number of warm-blooded organisms in the aquatic environment energetically disadvantageous.
Advantages and disadvantages
The main advantage of the warm-blooded over the cold-blooded is its readiness for action, regardless of the ambient temperature. This is an opportunity to withstand night temperatures close to frost, and the development of the northern land.
The main disadvantage of warm-bloodedness is the high energy consumption for maintaining a constant body temperature. And the main source for this is food. A warm-blooded lion needs ten times more food than a cold-blooded crocodile of the same weight.