Body size, energy metabolism and lifespan.
It is most common to refer to the resting metabolic rate, which operationally a relationship between the resting metabolic energy requirement per unit mass of magnitudes of body size and widely differing phylogenetic groups the rates are . It predicts that the power (log–log slope) of metabolic scaling relationships should vary between 2/3 and 1, in a systematic way with metabolic. body size effects. The relationship of metabolic rate with body size has been a subject of speculation and investigation since Max Rubner, in the. 's.
This example illustrates how scaling arguments work.
Metabolic rate (article) | Khan Academy
For many inanimate systems the energy produced has to be removed through the bounding surface area, and each unit of area allows a constant energy flux. At the same time the volume, V, scales as R3. Assuming constant density this will also be the scaling of the total mass, M. According to our assumption above, the energy is removed through the surface at a constant rate, and thus the total energy produced should be proportional to A, i. Does this simple scaling result based on simple considerations of energy transfer also hold for biological systems?
The resting energy demand of organisms has recently been compared among more than different organisms spanning over 20 orders of magnitude in mass! In contrast to the Kleiber law prediction, this recent work found a relatively small range of variation with the vast majority of organisms having power requirements lying between 0. Further evidence for breaking of Kleiber scaling was provided recently for protists and prokaryotes J.
The metabolic rate of an organism is condition dependent, and thus should be strictly defined if one wants to make an honest comparison across organisms. The most extreme example we are aware of is that bees in flight increase their oxygen consumption and thus their energy consumption by about fold in comparison to resting conditions BNID Similarly, humans taking part in the strenuous Tour de France consume close to 10, kcal a day, about five times the normal resting value.
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It is most common to refer to the resting metabolic rate, which operationally means the animal is not especially active but well fed. Using interspecific data to test the rate of living hypothesis is, however, confused by several major problems.
For example, appeals that the resultant lifetime expenditure of energy per gram of tissue is 'too variable' depend on the biological significance rather than the statistical significance of the variation observed. Moreover, maximum lifespan is not a good marker of ageing and RMR is not a good measure of total energy metabolism. Analysis of residual lifespan against residual RMR reveals no significant relationship.
However, this is still based on RMR. A novel comparison using daily energy expenditure DEErather than BMR, suggests that lifetime expenditure of energy per gram of tissue is NOT independent of body mass, and that tissue in smaller animals expends more energy before expiring than tissue in larger animals.
Some of the residual variation in this relationship in mammals is explained by ambient temperature.
In addition there is a significant negative relationship between residual lifespan and residual daily energy expenditure in mammals. A potentially much better model to explore the links of body size, metabolism and ageing is to examine the intraspecific links.
These studies have generated some data that support the original rate of living theory and other data that conflict. In particular several studies have shown that manipulating animals to expend more or less energy generate the expected effects on lifespan particularly when the subjects are ectotherms.
However, smaller individuals with higher rates of metabolism live longer than their slower, larger conspecifics.