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 ecological niches on the planet. There is no place within the biosphere where there would be no life. In the vents of volcanoes and on the peak of Everest - everywhere we find life in its various manifestations. And, undoubtedly, nature owes such diversity and distribution to the appearance in the process of evolution of the phenomenon of warm-bloodedness (homeothermic organisms).
The boundary of life is temperature
The basis of life is the body's metabolism, which depends on the speed and nature of chemical processes. BUTthese chemical reactions are possible only in a certain temperature range, with their own indicators and duration of exposure. For a larger number of organisms, the boundary indicators of the temperature regime of the environment are considered to be from 0 to +50 degrees Celsius.
But this is a speculative conclusion. It would be more accurate to say that the temperature limits of life will be those at which there is no denaturation of proteins, as well as irreversible changes in the colloidal characteristics of the cytoplasm of cells, a violation of the activity of vital enzymes. And many organisms have evolved highly specialized enzymatic systems that have allowed them to live in conditions far beyond these limits.
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 life in such conditions. More than 80% of the planet's biosphere are cold regions 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 lowers the activation energy of biological molecules and maintains 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 are organisms that are optimal forwhose lives are areas of high temperatures. Their vital activity is also ensured 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 balance of heat in a living system is the totality of its inflow and outflow. The body temperature of organisms depends on the ambient temperature (exogenous heat). In addition, an obligatory attribute of life is endogenous heat - a product of internal metabolism (oxidative processes and the breakdown of adenosine triphosphoric acid). The vital activity of most species on our planet depends on exogenous heat, and their body temperature depends on the course of ambient temperatures. These are poikilothermic organisms (poikilos - various), in which body temperature is variable.
Poikilotherms are all microorganisms, fungi, plants, invertebrates and most chordates. And only two groups of vertebrates - birds and mammals - are homoiothermic organisms (homoios - similar). They maintain a constant body temperature, 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 consequence, in homoiothermic organisms, all physiological processes are carried out at optimal and constant temperatures.
True and False
Some poikilothermsorganisms such as fish and echinoderms also have a constant body temperature. They live in conditions of constant external temperatures (ocean depths or caves), where the ambient temperature does not change. They are called falsely homoiothermic organisms. Many animals that experience hibernation or temporary torpor have fluctuating body temperatures. These truly homoiothermic organisms (examples: marmots, bats, hedgehogs, swifts, and others) are called heterothermal.
Dear aromorphosis
The appearance of homoiothermia in living beings is a very energy-consuming evolutionary acquisition. Scholars still argue about the origin of this progressive change in structure, which led to an increase in the level of organization. Many theories have been proposed for the origin of warm-blooded organisms. Some researchers admit that even dinosaurs could have this feature. But with all the disagreements of scientists, one thing is for sure: the appearance of homoiothermic 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 in a wide range of changes in body temperature is provided by biochemical adaptations and is called temperature compensation. It is based on the ability of some enzymes to change their configuration with decreasing temperature and increase their affinity with the substrate, increasing the rate of reactions. For example, in bivalves musselsIn the Barents Sea, oxygen consumption does not depend on ambient temperatures, which range from 25 °C (+5 to +30 °C).
Intermediate forms
Evolutionary biologists have found representatives of transitional forms from poikilothermic to warm-blooded mammals. Canadian biologists from Brock University have discovered seasonal warm-bloodedness in the Argentine black-and-white tegu (Alvator merianae). This almost meter lizard lives in South America. Like most reptiles, the tegu basks in the sun during the day, and hides in burrows and caves at night, where it cools. But during the breeding season from September to October, the temperature of the tegu, the respiratory rate and the rhythm of heart contractions in the morning increase sharply. The body temperature of a lizard can exceed the temperature in a cave by ten degrees. This proves the transition of forms from cold-blooded animals to homoiothermic animals.
Mechanisms of thermoregulation
Homoiothermic organisms always work to ensure the operation of the main systems - circulatory, respiratory, excretory - by generating a minimum of heat production. This minimum produced at rest is called basal metabolism. The transition to the active state in warm-blooded animals increases heat production, and they need mechanisms to increase heat transfer to prevent protein denaturation.
The process of achieving a balance between these processes is provided by chemical and physical thermoregulation. These mechanisms provide protection of homoiothermic organisms from low temperatures andoverheating. The mechanisms for maintaining a constant body temperature (chemical and physical thermoregulation) have different sources and are very diverse.
Chemical thermoregulation
In response to a decrease in environmental temperature, warm-blooded animals reflexively increase the production of endogenous heat. This is achieved by increasing oxidative processes, especially in muscle tissues. Uncoordinated muscle contraction (trembling) and thermoregulatory tone are the first stages of increasing heat production. At the same time, lipid metabolism increases, and adipose tissue becomes the key to better thermoregulation. Mammals of a cold climate even have brown fat, all the heat from the oxidation of which goes to warm the body. This energy expenditure requires the animal either to consume a large amount of food or to have substantial fat reserves. With a lack of these resources, chemical thermoregulation has its limits.
Mechanisms of physical thermoregulation
This type of thermoregulation does not require additional costs for heat generation, but is carried out by preserving endogenous heat. It is carried out by evaporation (sweating), radiation (radiation), heat conduction (conduction) and convection of the skin. Methods of physical thermoregulation have developed in the course of evolution and are becoming more and more perfect when studying the phylogenetic series from insectivores and bats to mammals.
An example of such regulation is the narrowing or expansion of the blood capillaries of the skin, which changesthermal conductivity, heat-insulating properties of fur and feathers, countercurrent heat exchange of blood between superficial vessels and vessels of internal organs. Heat dissipation is regulated by the slope of the fur hair and feathers, between which an air gap is maintained.
In marine mammals, subcutaneous fat is distributed throughout the body, protecting the endo-heat. 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 lying on the ice for hours. For animals living in hot climates, an even distribution of body fat over the entire surface of the body would be fatal. Therefore, their fat accumulates only in certain parts of the body (the hump of a camel, the fat tail of a sheep), which does not prevent evaporation from the entire surface of the body. In addition, animals of the northern cold climate have a special adipose tissue (brown fat), which is completely used for body heating.
More south - bigger 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 and its volume is important, because the volume of internal heat depends on the mass of the body, and heat transfer occurs through the integument. The protruding parts of the body have a large surface, which is good for hot climates, where warm-blooded animals need a lot of heat transfer. For example, large ears with many blood vessels, long limbs and a tail are typical for residents of a hot climate (elephant, fennec fox, Africanlong-eared jerboa). In cold conditions, adaptation follows the path of saving area to volume (ears and tail of seals).
There is another law for warm-blooded animals - the further north representatives of one phylogenetic group live, the larger they are. And this is also connected with the ratio of the volume of the evaporation surface, 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 homeothermic 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 mid-latitudes, at a depth of 1.5 meters, seasonal temperature fluctuations are imperceptible.
Group behavior is also used for thermoregulation. So, the penguins huddle 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 about +39 °C, and the fur of the outermost animals can be heated up to +70 °C.
Hibernation is a special strategy
Torpid state (stupor) or hibernation are special strategies of warm-blooded animals that allow using changes in body temperature for adaptive purposes. In this state, the animals stop maintaining body temperature and reduce it to almost zero. Hibernation is characterized by a decrease in metabolic rate andconsumption of accumulated resources. This is a well-regulated physiological state, when thermoregulatory mechanisms switch to a lower level - the heart rate decreases (for example, in a dormouse from 450 to 35 beats per minute), oxygen consumption decreases by 20-100 times.
Awakening requires energy and occurs by self-warming, which should not be confused with the stupor of cold-blooded animals, where it is caused by a decrease in ambient temperature and is a state unregulated by the body itself (awakening occurs under the influence of external factors).
Stupor is also a regulated state, but 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 torpor and hibernation. So, although we call the sleep of bears 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 she-bear gives birth to cubs.
Why are there few homoiothermic 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 associated primarily with an increase in metabolic processes, the basis of which is oxidation reactions. And oxygen plays a major role here. And, as you know, inin the aquatic environment, the oxygen content is not higher than 1% by volume. The diffusion of oxygen in water is thousands of times less than in air, which makes it even less available. In addition, with an increase in 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 unfavorable.
Pros and cons
The main advantage of warm-blooded animals over cold-blooded ones is their willingness to act regardless of the ambient temperature. This is an opportunity to withstand night temperatures close to freezing, and the development of the northern territories of the land.
The main disadvantage of warm-bloodedness is the high energy consumption to maintain 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.
