Metabolic rate is affected by body size.

In addition to activity level, an animal’s size influences its metabolic rate. At rest, larger animals consume more energy and have higher metabolic rates than smaller ones. However, resting metabolic rate does not increase linearly with an animal’s mass. Instead, measurements of resting metabolic rate in a wide range of organisms show that metabolic rate increases with animal mass raised to the ¾ power (Fig. 40.3). This means that the average rate at which each gram of body tissue consumes energy is less in larger animals compared to smaller ones. Or, put another way, the larger the organism, the lower the metabolic rate per gram of body tissue. This scaling pattern of cellular energy metabolism is remarkable because it holds across a diverse size range of unicellular and multicellular organisms, as can be seen in Fig. 40.3.

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FIG. 40.3 Resting metabolic rate and body size in diverse organisms. The relationship plotted here shows that larger animals have lower metabolic rates per gram of tissue than smaller animals. Source: Adapted from A. M. Hemingsen, 1960, “Energy metabolism as related to body size and respiratory surfaces, and its evolution.” Re. Steno Memorial Hospital Nordisk Insulinlaboratorium 9:1–110.

In addition to comparing resting metabolic rate in different-sized animals, we can also examine how metabolic rate varies with respect to level of activity in different-sized animals. In this case, for example, we are looking at how energy is used to allow animals to move at different speeds. The American comparative physiologist C. Richard Taylor has extensively studied how metabolic rate varies with running speed in different-sized terrestrial animals (Fig. 40.4). He found that metabolic rate increases linearly with speed, and that larger animals expend less energy per unit mass than smaller ones.

HOW DO WE KNOW?

FIG. 40.4

How is metabolic rate affected by running speed and body size?

BACKGROUND In the 1970s and 1980s, the American physiologist C. Richard Taylor and his colleagues performed studies on the relationship between metabolic rate and running speed in mammals of different sizes. They were interested in understanding the energetic costs of running in different animals.

EXPERIMENT Taylor and colleagues measured oxygen consumption during running for a wide range of organisms. They trained each animal to run on a treadmill at different speeds and measured oxygen consumption using either a face mask or enclosure. Oxygen consumption was used as a measure of metabolic rate.

RESULTS The researchers found that there is a linear increase in metabolic rate with speed in different-sized animals and that larger animals expend less energy per unit body mass to move a given distance compared to smaller ones, as shown on the graph.

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FIG. 40.4
Photo: Cary Wolinsky/Getty Images.

FOLLOW-UP WORK Similar studies were performed with kangaroos, which move by hopping rather than running. Interestingly, it was found that it is “cheaper” (that is, it requires less energy) to hop than to run. This finding perhaps explains why kangaroos and other hopping animals survived, while many animals that run on four legs became extinct in Australia 30,000 to 20,000 years ago with the arrival of early humans. Studies have also compared the energetic costs of running on two legs and running on four legs, with the finding that the cost is the same.

SOURCES Dawson, T. J., and C. R. Taylor. 1973. “Energetic Cost of Locomotion in Kangaroos.” Nature 246:313–314; Taylor, C. R., N. C. Heglund, and G. M. O. Maloiy. 1982. “Energetics and Mechanics of Terrestrial Locomotion.” Journal of Experimental Biology 97:1–21.

Quick Check 2 Does a dog that is twice as heavy as a cat have twice the resting metabolic rate?

Quick Check 2 Answer

No. Metabolic rate increases with increasing mass, but the relationship between the two is not linear. Instead, metabolic rate increases with mass raised to the ¾ power. This means that a dog that is twice as heavy as a cat has a metabolic rate that is less than twice as much as the cat.