The evolution of endothermy in terrestrial vertebrates: Who? When? Why?

Avian and mammalian endothermy results from elevated rates of resting, or routine, metabolism and enables these animals to maintain high and stable body temperatures in the face of variable ambient temperatures. Endothermy is also associated with enhanced stamina and elevated capacity for aerobic metabolism during periods of prolonged activity. These attributes of birds and mammals have greatly contributed to their widespread distribution and ecological success. Unfortunately, since few anatomical/physiological attributes linked to endothermy are preserved in fossils, the origin of endothermy among the ancestors of mammals and birds has long remained obscure. Two recent approaches provide new insight into the metabolic physiology of extinct forms. One addresses chronic (resting) metabolic rates and emphasizes the presence of nasal respiratory turbinates in virtually all extant endotherms. These structures are associated with recovery of respiratory heat and moisture in animals with high resting metabolic rates. The fossil record of nonmammalian synapsids suggests that at least two Late Permian lineages possessed incipient respiratory turbinates. In contrast, these structures appear to have been absent in dinosaurs and nonornithurine birds. Instead, nasal morphology suggests that in the avian lineage, respiratory turbinates first appeared in Cretaceous ornithurines. The other approach addresses the capacity for maximal aerobic activity and examines lung structure and ventilatory mechanisms. There is no positive evidence to support the reconstruction of a derived, avian-like parabronchial lung/air sac system in dinosaurs or nonornithurine birds. Dinosaur lungs were likely heterogenous, multicameral septate lungs with conventional, tidal ventilation, although evidence from some theropods suggests that at least this group may have had a hepatic piston mechanism of supplementary lung ventilation. This suggests that dinosaurs and nonornithurine birds generally lacked the capacity for high, avian-like levels of sustained activity, although the aerobic capacity of theropods may have exceeded that of extant ectotherms. The avian parabronchial lung/air sac system appears to be an attribute limited to ornithurine birds.     

The marine mammal dive response is exercise modulated to maximize aerobic dive duration

When aquatically adapted mammals and birds swim submerged, they exhibit a dive response in which breathing ceases, heart rate slows, and blood flow to peripheral tissues and organs is reduced. The most intense dive response occurs during forced submersion which conserves blood oxygen for the brain and heart, thereby preventing asphyxiation. In free-diving animals, the dive response is less profound, and energy metabolism remains aerobic. However, even this relatively moderate bradycardia seems diametrically opposed to the normal cardiovascular response (i.e., tachycardia and peripheral vasodilation) during physical exertion. As a result, there has been a long-standing paradox regarding how aquatic mammals and birds exercise while submerged. We hypothesized based on cardiovascular modeling that heart rate must increase to ensure adequate oxygen delivery to active muscles. Here, we show that heart rate (HR) does indeed increase with flipper or fluke stroke frequency (SF) during voluntary, aerobic dives in Weddell seals (HR?=?1.48SF?-?8.87) and bottlenose dolphins (HR?=?0.99SF?+?2.46), respectively, two marine mammal species with different evolutionary lineages. These results support our hypothesis that marine mammals maintain aerobic muscle metabolism while swimming submerged by combining elements of both dive and exercise responses, with one or the other predominating depending on the level of exertion.     

The origin of homoiothermy--unsolved problem

The analysis of allometric dependence of energy expenditure on body mass among reptiles, birds and mammals has shown that standard metabolic rate of reptiles when they are warmed up to the temperature of homoiothermic animals is an order of magnitude lower than that of birds and mammals. Basal metabolism is originated as special feature historically related to the metabolism during active behavior, rather than thermal regulation. Facultative endothermy was not advantageous for large animals because of long time needed to warm up the body. The ancestors of birds and animals escaped negative consequences of van't-Hoff equation by choosing constant body temperature. Heat conductivity of reptile's covers is so great, that it cannot keep endogenous warm of resting animal at any temperature of the body. Reptile "dressed" in covers of bird or mammal would be able to keep warm under conditions of maximal aerobic muscular activity and body temperature similar to that of homoiothermic animals. The base of chemical thermoregulation in birds and mammals is a thermoregulatory muscle tonus which remains unknown. One can suppose that during evolution of birds and mammals the saltation-liked origin of endothermy "fixed" the level of metabolism typical for running reptile and transformed in into the basal metabolism. This event took place at the cell and tissue level. The absence of palaeontological evidences and intermediate forms among recent species does not allow easy understanding of homoiothermy origin.     

Energetics, body size, and the limits to endothermy

The scaling rate of metabolism with respect to body mass is analysed. Scaling of heat production implies that scaling also exists between temperature regulation and body mass. Most vertebrates follow a Kleiber relation down to a "critical mass, below which the scaling of metabolism must be changed to ensure the maintenance of endothermy. Such an adjustment is found interspecifically in birds and mammals, and is found intraspecifically in mammals during post-natal growth. If the Kleiber scaling relation is maintained below the critical mass, mammals and birds shiR from endothermic temperature regulation (above critical mass) to endothermy with obligatory torpor (below critical mass). If the Kleiber relation is followed to masses far below the critical mass, ectothermy results. Critical mass varies inversely with the level of energy expenditure, which therefore accounts for the fact that most mammals and birds are endotherms and most reptiles and fish are ectotherms. The same relationship permits the facultative endothermy found in some insects and plants.
The scaling relations existing among rate of metabolism, endothermy, and body mass can be written as a modification of the Kleiber relation. This analysis suggests that any organism, irrespective of phylogenetic position, can be endothermic at any body size, if its rate of metabolism is high enough, or can be endothermic with any rate of metabolism, if it is large enough. Consequently, it is difficult to distinguish minimal endothermy from inertial homoiothermy in animals having a large mass. The boundary conditions for effective endothermy are similar to the relationship described between metabolism and mass in the evolution of endothermy through a decrease in mass in the phylogeny of mammals. Even though endothermy may evolve with an increase in mass, its perfection may always require an evolutionary decrease in mass.     

Use of helium-oxygen to examine the effect of cold acclimation on the summit metabolism of a marsupial, Dasyuroides byrnei

To determine whether marsupial mammals increase their metabolic capabilities during cold acclimation, the metabolism of both warm and cold acclimated Dasyuroides byrnei was examined by exposure to cold in a helium-oxygen atmosphere. Mean values of heat production and conductance were significantly higher in a helium-oxygen atmosphere than in air. Body temperature did not change until metabolic capacity was exhausted. Both cold and warm acclimated groups could maintain a metabolic scope of 10-11 times the basal or standard level for this species. Such a metabolic scope is much higher than levels recorded for placental mammals. At very low ambient temperatures cold acclimated D. byrnei could sustain a high level of heat production longer than could warm acclimated animals. While there are some similarities between marsupial mammals and placental mammals in their responses to cold acclimation, an increase in maximum metabolism, as reported for placentals, does not seem to occur in marsupials.     

Genetic correlations between basal and maximum metabolic rates in a wild rodent: consequences for evolution of endothermy

According to the aerobic capacity model, endothermy in birds and mammals evolved as a correlated response to selection for an ability of sustained locomotor activity, rather than in a response to direct selection for thermoregulatory capabilities. A key assumption of the model is that aerobic capacity is functionally linked to basal metabolic rate (BMR). The assumption has been tested in several studies at the level of phenotypic variation among individuals or species, but none has provided a clear answer whether the traits are genetically correlated. Here we present results of a genetic analysis based on measurements of the basal and the maximum swim- and cold-induced oxygen consumption in about 1000 bank voles from six generations of a laboratory colony, reared from animals captured in the field. Narrow sense heritability (h2) was about 0.5 for body mass, about 0.4 for mass-independent basal and maximum metabolic rates, and about 0.3 for factorial aerobic scopes. Dominance genetic and common environmental (= maternal) effects were not significant. Additive genetic correlation between BMR and the swim-induced aerobic capacity was high and positive, whereas correlation resulting from specific-environmental effects was negative. However, BMR was not genetically correlated with the cold-induced aerobic capacity. The results are consistent with the aerobic capacity model of the evolution of endothermy in birds and mammals.     

Temperature,metabolic power and the evolution of endothermy

Endothermy has evolved at least twice, in the precursors to modern mammals and birds. The most widely accepted explanation for the evolution of endothermy has been selection for enhanced aerobic capacity. We review this hypothesis in the light of advances in our understanding of ATP generation by mitochondria and muscle performance. Together with the development of isotope‐based techniques for the measurement of metabolic rate in free‐ranging vertebrates these have confirmed the importance of aerobic scope in the evolution of endothermy: absolute aerobic scope, ATP generation by mitochondria and muscle power output are all strongly temperature‐dependent, indicating that there would have been significant improvement in whole‐organism locomotor ability with a warmer body. New data on mitochondrial ATP generation and proton leak suggest that the thermal physiology of mitochondria may differ between organisms of contrasting ecology and thermal flexibility. Together with recent biophysical modelling, this strengthens the long‐held view that endothermy originated in smaller, active eurythermal ectotherms living in a cool but variable thermal environment. We propose that rather than being a secondary consequence of the evolution of an enhanced aerobic scope, a warmer body was the means by which that enhanced aerobic scope was achieved. This modified hypothesis requires that the rise in metabolic rate and the insulation necessary to retain metabolic heat arose early in the lineages leading to birds and mammals. Large dinosaurs were warm, but were not endotherms, and the metabolic status of pterosaurs remains unresolved.     

Brain size, development and metabolism in birds and mammals

Recent hypotheses that variation in brain size among birds and mammals result from differences in metabolic allocation during ontogeny are tested.
Indices of embryonic and post-embryonic brain growth are defined. Precocial birds and mammals have high embryonic brain growth indices which are compensated for by low post-embryonic indices (with the exception of Homo supiens ). In contrast, altricial birds and mammals have low embryonic and high post-embryonic indices. Altricial birds have relatively small brains at hatching and develop relatively large brains as adults, but among mammals there is no equivalent correlation between variation in adult relative brain sizes and state of neonatal development.
Compensatory brain development in both birds and mammals is associated with compensatory parental metabolic allocation. In comparison with altricial development, precocial development is characterized by higher levels of brain growth and parental metabolic allocation prior to hatching or birth and lower levels subsequently. Differences between degrees of postnatal investment by the parents in the young of precocial birds versus precocial mammals may result in the different patterns of adult brain size associated with precociality versus altriciality in the two groups.
The allometric exponent scaling brain on body size differs among taxonomic levels in birds. The exponent is higher for some parts of the brain than others, irrespective of taxonomic level. Unlike mammals, the exponents for birds do not show a general increase with taxonomic level. These pattcrns call into question recent interpretations of the allometric exponent in birds. and the reason for changes in exponent with taxonomic level.     

Metabolism and longevity: is there a role for membrane fatty acids?

More than 100 years ago, Max Rubner combined the fact that both metabolic rate and longevity of mammals varies with body size to calculate that "life energy potential" (lifetime energy turnover per kilogram) was relatively constant. This calculation linked longevity to aerobic metabolism which in turn led to the "rate-of-living" and ultimately the "oxidative stress" theories of aging. However, the link between metabolic rate and longevity is imperfect. Although unknown in Rubner's time, one aspect of body composition of mammals also varies with body size, namely the fatty acid composition of membranes. Fatty acids vary dramatically in their susceptibility to peroxidation and the products of lipid peroxidation are very powerful reactive molecules that damage other cellular molecules. The "membrane pacemaker" modification of the "oxidative stress" theory of aging proposes that fatty acid composition of membranes, via its influence on peroxidation of lipids, is an important determinant of lifespan (and a link between metabolism and longevity). The relationship between membrane fatty acid composition and longevity is discussed for (1) mammals of different body size, (2) birds of different body size, (3) mammals and birds that are exceptionally long-living for their size, (4) strains of mice that vary in longevity, (5) calorie-restriction extension of longevity in rodents, (6) differences in longevity between queen and worker honeybees, and (7) variation in longevity among humans. Most of these comparisons support an important role for membrane fatty acid composition in the determination of longevity. It is apparent that membrane composition is regulated for each species. Provided the diet is not deficient in polyunsaturated fat, it has minimal influence on a species' membrane fatty acid composition and likely also on it's maximum longevity. The exceptional longevity of Homo sapiens combined with the limited knowledge of the fatty acid composition of human tissues support the potential importance of mitochondrial membranes in determination of longevity.     

Mitochondrial Oxygen Radical Generation and Leak: Sites of Production in States 4 and 3, Organ Specificity, and Relation to Aging and Longevity

Studies in heart and nonsynaptic brain mitochondria from two mammals and three birds showthat complex I generates oxygen radicals in heart and nonsynaptic brain mitochondria in States4 and 3, whereas complex III does it only in heart mitochondria and only in State 4. Theincrease in oxygen consumption during the State 4 to 3 transition is not accompanied by aproportional increase in oxygen radical generation. This will protect mitochondria and tissuesduring bursts of activity. Comparisons between young and old rodents do not show a consistentpattern of variation in mitochondrial oxygen radical production during aging. However, allthe interspecies comparisons performed to date between different mammals, and betweenmammals and birds, agree that animals with high maximum longevities have low rates ofmitochondrial oxygen radical production, irrespective of the value of their basal specificmetabolic rate. The sites and mechanisms allowing this, the recently described low degree ofmembrane fatty acid unsaturation of longevous animals, and their relation to longevity andaging are discussed.     

Mutagenicity, carcinogenicity and teratogenicity of aluminium

Aluminium and its salts, which are extensively used in the household and in industry, do not constitute a carcinogenic, mutagenic or teratogenic hazard, except, perhaps, in cases of extremely high exposure. The large majority of the experiments performed to assess the carcinogenicity of aluminium in laboratory animals gave negative results or even suggested some antitumor activity. Moreover, epidemiological studies have not provided clear evidence of a carcinogenic hazard of aluminium to man, and short-term tests made in vitro and in vivo to demonstrate mutagenic activity of A1 were negative except for some experiments in plants. The embryotoxic properties suggested by the studies on birds and mammals could result from the influence of A1 on phosphate and calcium metabolism or from interference with the polymerization of microtubules.     

Relationship between body size, Na+-K+-ATPase activity, and membrane lipid composition in mammal and bird kidney

We investigated the relationship between body size, Na(+)-K(+)-ATPase molecular activity, and membrane lipid composition in the kidney of five mammalian and eight avian species ranging from 30-g mice to 280-kg cattle and 13-g zebra finches to 35-kg emus, respectively. Na(+)-K(+)-ATPase activity was found to be higher in the smaller species of both groups. In small mammals, the higher Na(+)-K(+)-ATPase activity was primarily the result of an increase in the molecular activity (turnover rate) of individual enzymes, whereas in small birds the higher Na(+)-K(+)-ATPase activity was the result of an increased enzyme concentration. Phospholipids from both mammals and birds contained a relatively constant percentage of unsaturated fatty acids; however, phospholipids from the smaller species were generally more polyunsaturated, and a complementary significant allometric increase in monounsaturate content was observed in the larger species. In particular, the relative content of the highly polyunsaturated docosahexaenoic acid [22:6(n-3)] displayed the greatest variation with body mass, scaling with allometric exponents of -0.21 and -0.26 in the mammals and birds, respectively. This allometric variation in fatty acid composition was correlated with Na(+)-K(+)-ATPase molecular activity in mammals, whereas in birds molecular activity only correlated with membrane cholesterol content. These relationships are discussed with respect to the metabolic intensity of different-sized animals.     

Mammalian hibernation as a model of disuse osteoporosis: the effects of physical inactivity on bone metabolism, structure, and strength

Reduced skeletal loading typically leads to bone loss because bone formation and bone resorption become unbalanced. Hibernation is a natural model of musculoskeletal disuse because hibernating animals greatly reduce weight-bearing activity, and therefore, they would be expected to lose bone. Some evidence suggests that small mammals like ground squirrels, bats, and hamsters do lose bone during hibernation, but the mechanism of bone loss is unclear. In contrast, hibernating bears maintain balanced bone remodeling and preserve bone structure and strength. Differences in the skeletal responses of bears and smaller mammals to hibernation may be due to differences in their hibernation patterns; smaller mammals may excrete calcium liberated from bone during periodic arousals throughout hibernation, leading to progressive bone loss over time, whereas bears may have evolved more sophisticated physiological processes to recycle calcium, prevent hypercalcemia, and maintain bone integrity. Investigating the roles of neural and hormonal control of bear bone metabolism could give valuable insight into translating the mechanisms that prevent disuse-induced bone loss in bears into novel therapies for treating osteoporosis.     

Allometry of post-flight cooling rates in moths: a comparison with vertebrate homeotherms.

1. The rates of post-flight cooling in 25 saturniid moths of 8 genera ranging in weight from 81 to 2650 mg were measured and compared with cooling rates in sphingids, birds and mammals. 2. The initial and terminal cooling rates of the saturniids did not differ significantly. 3. Large saturniids have relatively smaller thoraxes than small ones. 4. In saturniids the rate of post-flight cooling is inversely related both to thoracic volume and total weight. 5. Cooling rate is less dependent on thoracic volume in saturniids than in sphingids. 6. Weight-specific conductance calculated on the basis of total weight, shows that moths are not as well insulated as birds or mammals. However, when considered on the basis of thoracic weight, the weight-specific conductance of saturniids and sphingids closely approximates that predicted by the regression of weight-specific conductance on total body weight in birds and mammals. 7. Since the insulation of saturniids and sphingids is no more effective for animals of their size than is that of birds and mammals, their high body temperatures during activity appear to depend primarily on high levels of heat production.     

浙江德清野化放归朱鹮的繁殖行为

人类干扰影响着野生动物的栖息、生存、繁衍等各个环节。干扰强度不同,对野生动物的负面影响也不同。为了了解不同强度人类干扰对野生动物栖息的影响,2021年1月,在浙江百山祖国家级自然保护区,根据人为干扰综合强度预先设定点位,布设40台红外相机。40个点位被分为4种干扰区,干扰强度从低到高依次为基本无干扰区、干扰轻微区、干扰较轻区和干扰较重区,每区10个点位,监测365 d,共监测14 585相机日。研究表明：(1)拍摄到野生动物独立照片4 256张,其中兽类3 485张,鸟类771张。除啮齿类动物外,拍摄到31种动物,其中兽类14种、鸟类17种,另外有3张因照片不够清晰而未能识别鸟的种类。国家一级重点保护野生动物2种,国家二级重点保护野生动物9种。(2)基本无干扰区、干扰轻微区、干扰较轻区及干扰较重区所拍摄到的野生动物独立照片数量和种类分别为1 798张29种(兽类15种,鸟类14种)、1 308张23种(兽类15种,鸟类8种)、756张19种(兽类12种,鸟类7种)、394张18种(兽类13种,鸟类5种),各区之间的独立照片数量差异极显著(P < 0.01)。随着人类干扰强度的增加,拍摄到的独立照片数量逐渐减少,拍摄到的独立照片数量与人类干扰强度之间存在极显著的负线性关系(P < 0.01)。同时,随着人类干扰强度的增加,拍摄到的野生动物种类也逐渐减少,尤其是鸟类减少幅度更大。(3)国有林内23台相机拍摄到3 163张,计33种(兽类16种、鸟类17种);集体林内17台相机拍摄到1 093张,计22种(其中兽类13种、鸟类9种)。集体林内平均每台红外相机拍摄到的照片数量为国有林的46.75%,差异极显著(P < 0.01),这可能是由于早些年份的林木采伐使栖息地遭受不同程度的破坏,并导致生境片段化,进而影响野生动物栖息。因而,对于野生动物保护来说,栖息地保护是一项相当重要的工作。     

Muscle structure and function in the goose, quail, pheasant, guinea hen, and chicken.

1. Metabolic functions, fiber composition and, in some cases, mitochondrial morphology have been investigated in the pectoralis major and sartorius of the goose, quail, pheasant, guinea-hen, broiler chicken and laying hen. 2. In the pectoralis only two types of fibers, red and white twitch fibers, are found in all birds studied except in the quail, where a third twitch fiber also occurs. Red fibers form the main part in the quail and goose, white fibers in the rest. In the sartorius at least three types of fibers, two twitch and one tonus are found in all birds except in the guinea-hen where only two fibers can be found. 3. The metabolic pattern of the muscles, based on determination of specific activities of metabolic key enzymes, varies greatly among the birds. Three groups can be discerned from the ratio between aerobic and anaerobic activities or between fatty acid oxidation and carbohydrate metabolism. The metabolic patterns are reflected in the fiber combinations of the muscles. 4. The size and number of the mitochondria vary among different animals and different fiber types. The metabolism of red and white fibers is discussed.     

A review of the multi-level adaptations for maximizing aerobic dive duration in marine mammals: from biochemistry to behavior

Marine mammals exhibit multi-level adaptations, from cellular biochemistry to behavior, that maximize aerobic dive duration. A dive response during aerobic dives enables the efficient use of blood and muscle oxygen stores, but it is exercise modulated to maximize the aerobic dive limit at different levels of exertion. Blood volume and concentrations of blood hemoglobin and muscle myoglobin are elevated and serve as a significant oxygen store that increases aerobic dive duration. However, myoglobin is not homogeneously distributed in the locomotory muscles and is highest in areas that produce greater force and consume more oxygen during aerobic swimming. Muscle fibers are primarily fast and slow twitch oxidative with elevated mitochondrial volume densities and enhanced oxidative enzyme activities that are highest in areas that produce more force generation. Most of the muscle mitochondria are interfibriller and homogeneously distributed. This reduces the diffusion distance between mitochondria and helps maintain aerobic metabolism under hypoxic conditions. Mitochondrial volume densities and oxidative enzyme activities are also elevated in certain organs such as liver, kidneys, and stomach. Hepatic and renal function along with digestion and assimilation continue during aerobic dives to maintain physiological homeostasis. Most ATP production comes from aerobic fat metabolism in carnivorous marine mammals. Glucose is derived mostly from gluconeogenesis and is conserved for tissues such as red blood cells and the central nervous system. Marine mammals minimize the energetic cost of swimming and diving through body streamlining, efficient, lift-based propulsive appendages, and cost-efficient modes of locomotion that reduce drag and take advantage of changes in buoyancy with depth. Most dives are within the animal’s aerobic dive limit, which maximizes time underwater and minimizes recovery time at the surface. The result of these adaptations is increased breath-hold duration and enhanced foraging ability that maximizes energy intake and minimizes energy output while making aerobic dives to depth. These adaptations are the long, evolutionary legacy of an aquatic lifestyle that directly affects the fitness of marine mammal species for different diving abilities and environments.     

Triglyceride lipase activity and fatty acid transport in physical endurance in rats adapted to the muscular activity

Aerobic training led to enhancement of lipase activity in type IIA type muscles. Still more obvious changes were found in rats trained to aerobic swimming with maximal intensity. In latter activity, a rise of the fatty acid-binding protein (FABP) was revealed in types I and IIA skeletal muscles. These adaptive changes led to enhancement of lipid metabolism. It was also shown that the FABP content decreased after physical exercise more obviously in the trained animals due, probably, to their substance turnover enhancement.     

Reptilian Cognition: A More Complex Picture via Integration of Neurological Mechanisms,Behavioral Constraints,and Evolutionary Context

Unlike birds and mammals, reptiles are commonly thought to possess only the most rudimentary means of interacting with their environments, reflexively responding to sensory information to the near exclusion of higher cognitive function. However, reptilian brains, though structurally somewhat different from those of mammals and birds, use many of the same cellular and molecular processes to support complex behaviors in homologous brain regions. Here, the neurological mechanisms supporting reptilian cognition are reviewed, focusing specifically on spatial cognition and the hippocampus. These processes are compared to those seen in mammals and birds within an ecologically and evolutionarily relevant context. By viewing reptilian cognition through an integrative framework, a more robust understanding of reptile cognition is gleaned. Doing so yields a broader view of the evolutionarily conserved molecular and cellular mechanisms that underlie cognitive function and a better understanding of the factors that led to the evolution of complex cognition.     

Fundamental avian energetics: 2. The ability of birds to change heat loss and explanation of the mass exponent for basal metabolism in homeothermic animals

The maximum ability of birds to generate heat due to increasing metabolism, as a result of both activity and heat stress, was determined in relation to the evaporative and nonevaporative heat losses at various temperatures in passerines and nonpasserines at the beginning and at the end of thermoneutral zones. The minimum (h _min) and maximum (h _max) nonevaporative thermal conductances in both species change similarly depending on the body mass, and the slopes of regression lines in h _min and h _max are identical. At the same time, h _max is approximately four times higher than h _min. Experimental data obtained both in this study and by other authors show that the ratio h _max/h _min = 4 is constant for all homeothermic animals and appears to be a sensible compromise found by the evolution between an increase in activity and the minimum effectiveness profitable for life of the transfer of metabolic power into mechanical power (?? = 1/4) during its fulfillment. An increase in the ratio h _max/h _min, although it allows an animal to augment its daily activity, leads to a reduction in the effectiveness and is, therefore, not used by homeothermic animals. The abilities of birds and mammals to change their heat loss are determined by the ratio h _max/h _min = 4, which is an integrated indicator of the level of development of blood circulation and respiration systems and the degree of development of external covers, as well as the ability of both to change heat loss. In homeothermic animals, this ratio is associated with the body mass exponent in allometric dependences for basal metabolism and determines the efficiency of transfer of metabolic power into mechanical work.