In five minutes a human will burn around 350 joules (J) of energy per kilogram of body mass. A kilogram of mice will burn 3,000 J in the same time, and a 4,000- kg African elephant will burn just 200 J per kilogram. On a gram-for-gram basis, large animals burn less energy and require less food than small ones .The exact relationship between metabolic rate and body mass, and the mechanistic basis for it, remains an open question. Elephants are some 200,000 times greater in mass than mice, but require only 15,000 times more energy each day.
In Nature April 1 2010 p 753 Kolokotrones et at. show that the standard power equation that has been used for decades to describe the relationship may not be appropriate for mammals.
They use the deviation from this equation to test one of the most prominent explanations for the relationship.
In Paris in the 1830s, Sarrus and Rameaux suggested that, because the heat produced by an animal as a by-product of metabolism must be lost through the body surface, the rate at which it produces heat (metabolic rate) should be proportional not to its body mass, but to the surface area over which the heat is lost. Rubner then demonstrated empirically in 1883 that the basal metabolic rate of dogs is proportional to their body surface area.
In 1916, Krogh suggested that the relationship between body mass and metabolic rate is best described by a power function, so that metabolic rate is not proportional to mass, but to mass raised to some power p. When p is equal to one, the relationship is a straight line and metabolic rate is said to scale isometrically (in proportion) with mass. When p does not equal one, the relationship is curved, and metabolic rate is said to scale allometrically.
Krogh also suggested that for endotherms (birds and mammals) p is close to the value of 2/3 suggested by Rubner’s ‘surface law’, whereas for ectotherms, such as insects, fish, amphibians and reptiles, the p value is closer to 1.
However, in 1932 Kleiber, Brody and Proctor showed that metabolic rate was on a scale with a p value close to ¾. ( Kleiber’s law).
Kolokotrones et al. however demonstrate that a standard power equation may not be appropriate for describing the relationship between basal metabolic rate and body mass in mammals. They show that the value of p increases with body size: metabolic rate increases more rapidly with mass for large mammals than for small ones. They show that the relationship between mass and metabolic rate has convex curvature on a logarithmic scale, and is therefore not a pure power law, even after accounting for body temperature.
These results extend our understanding of a very complicated topic .
Size, surface are , heat production and loss, rate of movement, mass, types of muscles and more.
White 2010 There is no single p Nature vol 464 p 691
Kolokotrones et al 2010 Curvature in metabolic scaling . Nature vol 464 pp 753-756
- Martin Eastwood