A fundamental but unanswered biological question asks how much energy, on average, Earth's different life forms spend per unit mass per unit time to remain alive. Here, using the largest database to date, for 3,006 species that includes most of the range of biological diversity on the planet-from bacteria to elephants, and algae to sapling trees-we show that metabolism displays a striking degree of homeostasis across all of life. We demonstrate that, despite the enormous biochemical, physiological, and ecological differences between the surveyed species that vary over 10 20 -fold in body mass, mean metabolic rates of major taxonomic groups displayed at physiological rest converge on a narrow range from 0.3 to 9 W kg ؊1 . This 30-fold variation among life's disparate forms represents a remarkably small range compared with the 4,000-to 65,000-fold difference between the mean metabolic rates of the smallest and largest organisms that would be observed if life as a whole conformed to universal quarterpower or third-power allometric scaling laws. The observed broad convergence on a narrow range of basal metabolic rates suggests that organismal designs that fit in this physiological window have been favored by natural selection across all of life's major kingdoms, and that this range might therefore be considered as optimal for living matter as a whole.allometry ͉ body size ͉ breathing ͉ scaling ͉ energy consumption T he process of life is critically dependent on consumption of energy from the environment. The amount of energy-per unit time per unit mass-required to sustain life can rightfully be considered one of the fundamental questions in biology. Yet a general quantitative answer to this question is lacking, despite the long history and the considerable number of studies devoted to various aspects of organismal energetics in all fields of bioscience. One reason for this persistent knowledge gap is that this fundamental question is typically approached in markedly different ways depending on the organisms being investigated. We show herein how differences in types, protocols, and units of measurements of metabolism have presented a challenge to the development of quantitative generalizations regarding the metabolic rates of organisms. We then use a comprehensive dataset to reconcile such differences and to characterize the remarkable similarity that emerges from comparisons of mass-specific metabolic rates across all of life. Problem SettingStudies of animal energetics have frequently focused on the allometric relationship between the whole-body metabolic rate Q and body mass M, Q ϭ Q 0 (M/M 0 ) b , where Q 0 is metabolic rate of an organism with body mass M 0 . Either M 0 or Q 0 can be chosen arbitrarily, whereas the second of these parameters is unambiguously defined by the choice of the first one. Usually, M 0 is chosen to be one mass unit-e.g., M 0 ϭ 1 g. For the mass-specific metabolic rate q ' Q/M, we have q ϭ q 0 (M/M 0 ) ␤ , ␤ ϭ b Ϫ 1, q 0 ϭ Q 0 /M 0 . Much of the current debate concerns the value of b, an...

A unified system of bioenergetic parameters that describe thermal regulation and energy metabolism in many passerine and non-passerine species has been developed. These parameters have been analyzed as functions of ambient temperature, and bioenergetic models for various species have been developed. The level of maximum food energy or maximal existence metabolism (MPE) is 1.3 times higher in passerines than in non-passerines, which is consistent with the ratio of their basal metabolic rates (BMR). The optimal ambient temperature for maximizing productive processes (e.g., reproduction, molting) is lower for passerines than for non passerines, which allows passerines to have higher production rates at moderate ambient temperatures. This difference in the optimal ambient temperature may explain the variation in bioenergetic parameters along latitudinal gradients, such as the well-known ecological rule of clutch size (or mass) increase in the more northerly passerine birds. The increased potential for productive energy output in the north may also allow birds to molt faster there. This phenomenon allows passerine birds to occupy a habitat that fluctuates widely in ambient temperature compared with non-passerine birds of similar size. Passerines have a more effective system for maintaining heat balance at both high and low temperatures. The high metabolism and small body sizes of passerines are consistent with omnivore development and with ecological plasticity. Among large passerines, the unfavorable ratio of MPE to BMR should decrease the energy that is available for productive processes. This consequence limits both the reproductive output and the development of long migration (particularly in Corvus corax). The hypothesis regarding BMR increase in passerines was suggested based on an aerodynamic analysis of the flight speed and the wing characteristics. This allometric analysis shows that the flight velocity is approximately 20% lower in Passeriformes than in non-Passeriformes, which is consistent with the inverted ratio of their BMR level. The regressions for the aerodynamic characteristics of wings show that passerines do not change the morphological characteristics of their wings to decrease velocity. Passerine birds prefer forest habitats. The size range of 5-150 g for birds in forest habitats is almost exclusively occupied by passerines because of their large energetic capability.

The molt cycle in Chaffinches (Fringilla coelebs) has a circannual periodicity under constant photoperiodic conditions (12:12LD, 20:4LD). The molt cycle and/or rate, however, can be modified by artificial alteration of daylength. The time of onset and completion of the post-nuptial molt in Chaffinches under natural conditions is a remote expression of vernal photostimulation. The timing of the molt is initiated at the end of the unifactorial phase of photoperiodic control. The onset of molt is induced after a latent period and its termination after another period, i.e. the molt begins and ends spontaneously. Under natural conditions, daylength immediately before and during the molt does not control either the time of onset or of termination of molt. The additional control system increases the rate of the molt in its early phase under short days, whereas under long days it decreases the rate of molt in its late phase. This system may play an adaptive role in synchronizing the end of the molt among birds that begin molt at different times. The postjuvenal molt begins and finishes spontaneously under control of a program of the individual development of juveniles. Short days increase the rate of molt and initiate earlier onset and completion, whereas long days decrease the rate of the molt and delay the times of onset and completion. Juvenile birds from late broods molt more rapidly than those from early broods. A combination of different programs and photoperiodic control of the molt synchronizes the termination of the molt in juveniles from early and late broods under natural conditions in summer and autumn.

The metabolic scaling in the animal has been discussed for over 90 years, but no consensus has been reached. Our analysis of 2126 species of vertebrates reveals a significant allometric exponent heterogeneity. We show that classes of terrestrial vertebrates exhibit the evolution of metabolic scaling. Both the allometric coefficient “a” and the allometric exponent “b” change naturally, but differently depending on the geological time of group formation. The allometric coefficient “a” shows the measure of the evolutionary development of systems that forms resting metabolism in animals. Endothermic classes, such as birds and mammals, have a metabolic rate that is in an order of magnitude higher than that in ectothermic classes, including amphibians and reptiles. In the terrestrial vertebrate phylogeny, we find that the metabolic scaling is characterized by 3 main allometric exponent values: b = 3/4 (mammals), b > 3/4 (ectotherms, such as amphibians and reptiles), and b < 3/4 (birds). The heterogeneity of the allometric exponent is a natural phenomenon associated with the general evolution of vertebrates. The scaling factor decreases depending on both the external design and the size (birds vs mammals) of the animal. The metabolic rate and uniformity of species within a class increase as the geological start date of formation of the class approaches the present time. The higher the mass‐specific standard metabolic rate in the class, the slower metabolic rate grows with increasing body size in this class. Our results lay the groundwork for further exploration of the evolutionary and ecological aspects of the development of metabolic scaling in animals.

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