How Does an Animal's Size Affect Metabolism?

The internal combustion engine provides a helpful, if imperfect, metaphor for the metabolism of an animal. Rather than gasoline, an animal uses food, but both engines and animals combine fuel with oxygen to generate energy. In this metaphor, an engine's speed -- usually measured in rotations per minute or RPM -- is roughly equivalent to an animal’s metabolism, which is usually measured in the amount of oxygen used in a unit of time.

An individual animal’s metabolic rate is not a single, consistent value. The speed of the chemical reactions taking place inside an animal’s body vary with a number of factors including the time of day, activity level and the presence or absence of food in the digestive system. In an attempt to standardize measurements of metabolic rate, scientists tend to study an animal’s standard, resting and maximum metabolic rates. An animal’s standard metabolism represents the minimum energy required to keep an animal alive. Scientists measure it at night for diurnal animals or during the day for nocturnal ones. Scientists measure an animal’s resting metabolic rate during the animal’s normal activity period but when the animal is not moving around. Scientists measure the maximum metabolic rate of an animal by employing a treadmill or similar device.

The metabolic rate of endothermic animals -- specifically birds and mammals -- increases with decreasing size. Animals like hummingbirds, shrews and bats have very fast metabolisms, while elephants, whales and buffalo have very slow metabolism. Reptiles exhibit a similar relationship between size and metabolism but, because they derive warmth externally, these ectothermic animals have much slower metabolic rates than similarly sized endothermic animals. The metabolic rate of reptiles is largely reflected in their ecology -- ambush hunters have slower metabolisms than widely foraging species do.

The ratio between an organism’s surface area and volume increases as the mass of the organism decreases. This means that relative to their mass, hummingbirds have more surface area than whales do. This is important, because the more surface area a species has the quicker it dissipates heat. Heat production is energetically expensive -- the quicker the animal cools off, the more food the creature has to eat. Therefore, tiny animals like hummingbirds, bats and shrews must eat during most of their waking hours to fuel their incredibly quick metabolisms. Because they do not have to produce body heat internally, coldblooded animals are able to exist at much smaller sizes than warmblooded animals are -- many lizard, frog and fish species exhibit smaller adult sizes than the smallest birds, shrews and bats do.