Summary
Metabolic rate scales with body mass to the three-quarter power, described by Kleiber's law.
In depth
Basal metabolic rate scales with mass^0.75 across animals, so small animals have higher mass-specific rates and shorter lifespans; explanations involve fractal-like distribution networks. A mouse's heart beats about 600 times a minute, an elephant's 30.
Allometry describes size-related changes. Scaling arises from supply network geometry. Exceptions occur.
Remember: mass-specific metabolic rate falls with body size.
Kleiber's law (1932) shows that metabolic rate scales with mass to the power of about 0.75, across 20 orders of magnitude from microbes to whales. A 1000-fold bigger animal has about 180-fold higher metabolism, so each gram of tissue uses less energy: a shrew's heart beats 800 times per minute, an elephant's 30. Explanations include fractal-like distribution networks (West, Brown and Enquist) and surface area constraints, though the exponent and theory are debated (2/3 or 3/4).
Temperature also affects rate (Q10), and the metabolic theory of ecology uses both to predict growth, lifespan and population density. Lifetime heartbeats are roughly similar across mammals (around a billion), though humans exceed it.
Example: small animals such as hummingbirds must eat constantly, and enter torpor at night, because of their high mass-specific metabolism.
Frequently asked questions
What is Metabolic Scaling?
Metabolic rate scales with body mass to the three-quarter power, described by Kleiber's law.
Which area of biology covers Metabolic Scaling, and how advanced is it?
Metabolic Scaling is one of the 53 topics in Zoology & Animal Diversity. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Hibernation and Torpor, Island Gigantism and Dwarfism, Animal Locomotion.
Can you give an example of Metabolic Scaling?
Shrews must feed almost constantly.
What should I learn before Metabolic Scaling?
Work through the Beginner topics in Zoology & Animal Diversity first, such as Animal Kingdom, Vertebrates and Invertebrates, Sponges, then return to Metabolic Scaling. Advanced topics assume the Intermediate vocabulary.
What topics are related to Metabolic Scaling?
The closest topics to Metabolic Scaling are Hibernation and Torpor, Island Gigantism and Dwarfism, Animal Locomotion, Biomechanics of Locomotion. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Metabolic Scaling?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Metabolic Scaling. To practise the whole of Zoology & Animal Diversity, use the Zoology & Animal Diversity quiz.
How is Metabolic Scaling different from Hibernation and Torpor?
Metabolic Scaling: Metabolic rate scales with body mass to the three-quarter power, described by Kleiber's law. Hibernation and Torpor: Reduced metabolism and body temperature allow animals to survive scarce winter food. Read both side by side and note what each one explains.
How should I study Metabolic Scaling step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Hibernation and Torpor and explain the difference aloud. Finally, take the Zoology & Animal Diversity quiz and come back to this page for any question you miss.