Earth's energy flows: sun, wind, water, heat and life: How animals use energy, and the biosphere as one system

5.1 Metabolism scales with size: Kleiber's three-quarter rule

Every animal has a basal metabolic rate, the minimum power it needs at rest to keep its cells working, its heart beating and, for mammals and birds, its body warm. In 1932 the Swiss-American physiologist Max Kleiber showed that across mammals this rate rises not in proportion to body mass but roughly with mass to the power of three quarters. A handy version of the rule says that basal metabolism is about 70 times body mass in kilograms raised to the 0.75 power, in kilocalories per day.

The consequence is that bigger animals are more economical per kilogram. An elephant weighs about 100,000 times as much as a mouse, but needs only about 5,600 times as much energy a day. Per kilogram, the mouse burns energy almost twenty times faster. The smallest mammals and birds, shrews and hummingbirds, must feed almost constantly, while large animals can fast for days or weeks.

For a 70 kg adult human the formula gives about 1,700 kcal, or about 7 MJ, per day, which is a continuous power of roughly 80 W, like an old incandescent light bulb. The next module builds on this number.

Being warm-blooded is expensive. Mammals and birds keep their bodies at a steady high temperature, so they need several times more food than reptiles of the same size, which let their temperature follow their surroundings. The pay-off is the ability to stay active at night, in winter and in cold climates. Growth and reproduction add further costs on top of maintenance: young animals and pregnant or nursing females need much more energy relative to their size.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M2 'How animals use energy'
• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'How animals use energy'
• Smil, Size: How It Explains the World (2023) · via Smil Mastery Course, M2 'How animals use energy'

5.2 Moving, working and eating up the food chain

Animals also spend energy moving. Per kilogram of body mass and per kilometre travelled, swimming is the cheapest mode, because water supports the body; flying is intermediate; running and walking are the most expensive. Large animals move more cheaply per kilogram than small ones, another expression of scaling.

Muscle is the converter that turns food energy into motion, and it is only moderately efficient. Human and animal muscles turn roughly 20 to 25% of the chemical energy they use into mechanical work; the rest becomes heat. This is why exercise makes us hot, and why sweating, a use of the latent heat of evaporation described earlier, is so important for any animal that works hard in a warm climate. It is also why, in the Energy table, Muscle power appears as a natural source with modest output.

The largest loss, though, comes from eating. Each time energy passes from one feeding level to the next, from plants to herbivores, herbivores to carnivores, most of it is lost: some plant matter is not eaten, some of what is eaten is not digested, and most of what is digested is spent on metabolism rather than turned into new body mass. As a rule of thumb, only about 10% of the energy at one level becomes tissue at the next.

Two consequences follow. First, large predators are rare, because each needs a very large area of plant production beneath it. Second, meat is expensive in plant terms. Feeding grain to animals and eating the animals delivers only a fraction of the food energy that eating the grain directly would. Domestic animals do far better than wild ones on this score, chickens and fish better than cattle, but the losses remain large.

• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'How animals use energy'
• Smil, Energies: An Illustrated Guide to the Biosphere and Civilization (1999) · via Smil Mastery Course, M2 'How animals use energy'
• Smil, Harvesting the Biosphere: What We Have Taken from Nature (2013) · via Smil Mastery Course, M2 'Productivity and the mass of life'
• Bar-On, Phillips and Milo, 'The biomass distribution on Earth', PNAS 115 (2018) · total biomass about 550 Gt C, plants about 450 Gt C, humans about 0.06 Gt C; summary by the Weizmann Institute, https://www.weizmann.ac.il/WeizmannCompass/node/1200

5.3 The biosphere works as one linked system, and it is thin, old and now shaped by people

The Russian scientist Vladimir Vernadsky argued in 1926 that life is not just a passenger on Earth but a geological force. Plants and microbes have changed the chemistry of the air, the oceans and the rocks. Cyanobacteria began releasing oxygen more than 2.4 billion years ago, and the oxygen we breathe and the fossil fuels we burn are both products of that long history of photosynthesis.

The biosphere is remarkably thin. Microbes live kilometres down in rock and spores drift high in the air, but most life is crowded into a layer a few tens of metres thick on land and into the top 100 to 200 metres of the ocean, where there is enough light for photosynthesis.

Microbes run much of the system. Bacteria and archaea carry out chemistry that plants and animals cannot: they fix nitrogen from the air, decompose dead matter and return its nutrients, and drive the cycles of sulphur and carbon. In Cycles of Life Smil argues that the deepest environmental challenge is not any single pollutant but human interference in these grand cycles of carbon, nitrogen, sulphur and water: burning fossil carbon, making synthetic nitrogen fertiliser, releasing sulphur from coal.

The biosphere is resilient on geological time scales. It has recovered from at least five mass extinctions, including the asteroid impact 66 million years ago that ended the age of dinosaurs, but each recovery took millions of years. On the time scale of human societies, losses are effectively permanent.

Finally, civilisation rests on services that nature provides without charge: pollination, soil formation, water purification, the regulation of climate and the recycling of wastes. Smil stresses that no economy could replace these at the scale they operate, which is why the health of natural energy flows, not only our own, matters for any energy plan.

• Smil, The Earth's Biosphere: Evolution, Dynamics, and Change (2002) · via Smil Mastery Course, M2 'The biosphere as a system'
• Smil, Cycles of Life: Civilization and the Biosphere (1997) · via Smil Mastery Course, M2 'The biosphere as a system'
• Smil, Harvesting the Biosphere: What We Have Taken from Nature (2013) · via Smil Mastery Course, M2 'The biosphere as a system'
• Bar-On, Phillips and Milo, 'The biomass distribution on Earth', PNAS 115 (2018) · total biomass about 550 Gt C, plants about 450 Gt C, humans about 0.06 Gt C; summary by the Weizmann Institute, https://www.weizmann.ac.il/WeizmannCompass/node/1200

See it in the table

Check your understanding

An elephant weighs 100,000 times as much as a mouse. Roughly how many times more basal energy does it need per day?

Why is meat expensive in terms of plant energy?

Which organisms fix nitrogen from the air and run most of the biosphere's element cycles?