Humans Are Slow Sprinters, So Why Can We Run Animals Down Over Long Distances?
― We Have a Rare Body That Cools Itself With Sweat While Running
Over a short distance, a human can't beat a deer or a dog. But in a contest that lasts for hours, things change. Our bodies are thought to be built to compete with other animals not on speed, but on the ability to shed heat while running.
Race a dog across a park and it leaves you behind in no time. Even the world-record sprinters top out at around 44 km/h. A cheetah is said to reach nearly 100 km/h, so we don't even get halfway.
Yet tens of thousands of people enter city marathons and run the full 42 km. Take a dog along for the same distance, and many dogs would stop partway, tongues out, panting hard.
Why do the animals that should be faster give out first? The key is the heat that builds up inside a running body.
Only Two Reasons We're Good Over Long Distances
Our skin is packed with sweat outlets, and our body hair is thin. Sweat takes heat with it as it dries, so we can keep cooling down without ever stopping.
An animal galloping on four legs is thought to manage only one breath per stride. A two-legged human can freely change how many steps go with each breath.
When we run, our muscles turn most of the energy they use into heat. Over long distances, what decides the winner is not how fast you can run but how you get rid of that heat. Let's take the two reasons in turn.
Reason 1: Sweat Is a Cooling System You Can Use While Running
Of the energy a muscle gets from food, only about a quarter can go into the work of moving the body forward. All the rest becomes heat. Even at a slow jog, your body keeps giving off heat on the scale of several electric heaters.
If that heat isn't shed, body temperature climbs and climbs. So humans sweat. When the water in sweat turns to vapour on the skin, it pulls a large amount of heat from its surroundings. The tiny outlets that release sweat (sweat glands) number between 2 and 4 million across the whole body.
Many other animals can't do this. A dog has sweat outlets only on the soles of its feet, and cools itself by evaporating water from its tongue. In furry animals, even if sweat comes out, the fur is slow to dry and the heat has trouble escaping. Our thin body hair is thought to be a design for letting sweat dry.
So how far can sweat take us? Look at Figure 1. The faster you run, the more heat you produce (the rising solid line). Body temperature starts to climb once that line passes the heat that sweat can carry away (the horizontal dotted line).
If a 60 kg person sweats one litre an hour, the sums say they can shed all their heat up to about 12 km/h (the calculation is in the fold-out section at the end). Trained athletes are said to sweat close to 2 litres an hour, and can keep up a faster pace as a result.
Reason 2: We Can Choose Our Own Breathing Rhythm
When a four-legged animal gallops flat out, its body bends and stretches a great deal from front to back. Each time, the contents of the chest and belly are pushed like a piston. For that reason, it is thought to manage only one breath per stride.
Dogs and deer shed heat by fast breathing (panting) instead of sweat. But while galloping flat out, their breathing rate is tied to their stride. As heat builds up, they have no choice but to stop and catch their breath.
A human running on two legs has a chest that is not so tightly tied to the stride. We can pick the rhythm: one breath every two steps, or every three. And because sweat does the cooling, our breath can focus on taking in oxygen.
In the Kalahari Desert of southern Africa, hunts have been recorded in which people chase a large antelope for hours in the heat of the day. The prey builds up heat with every bout of fleeing, is caught up with before it can rest and cool down, and eventually is said to be unable to move. Horses, however, are one of the few animals that sweat as humans do. In a British race where people and horses compete over mountain trails, the horses usually win, and a human victory is said to be rare.
The Achilles tendon at the heel and the arch of the foot stretch a little at each landing and store energy. They give it back in the next push-off. Thanks to these "springs", some of the energy of running is thought to be reused.
This way of cooling has a weak point, though. Sweat only takes heat away once it dries. On a muggy day when sweat won't dry, or a day when you are short of water, the dotted line in Figure 1 drops a long way. When running far in summer, it matters to drink often and slow your pace. What happens when the body's heat passes its limit is explained in detail in our article on heatstroke.
Summary
Humans lose to other animals in raw running speed. But with plenty of sweat glands and thin body hair, we shed heat as we run, and by freeing our breathing rhythm from our stride we can keep going for hours. What decides a long-distance contest is how long you can keep getting rid of heat.
Humans are animals that compete not on speed,
but on being able to run while staying cool.
How animals that can't sweat cool themselves is covered in why dogs pant with their tongues out, and how the body makes heat when it is cold is in why the body shivers.
- Step on a bathroom scale before you run and again after 10–20 minutes of running (don't drink on the way, and keep it short). Most of the weight you lose is water that left as sweat and in your breath.
- While running, count how many steps you take for each breath in. The rhythm should change between a slow pace and a slightly faster one.
- Wrap a damp towel around your arm and fan it. The cool feeling as it dries is the same mechanism by which sweat takes heat away.
Do the weighing experiment on a day when you feel well, at a cool time of day. If you feel dizzy or unwell, stop at once and rest.
For Those Who Want More ― Terms, Equations, and Links to TextbooksWe mark which level each part is, from junior-high science to university specialist courses
- Junior highCovered in junior-high science
- High schoolCovered in high-school biology and physics
- High school+Advanced high-school material, or textbook sidebars
- UniversityUniversity specialist courses (exercise physiology, comparative physiology, anthropology), not taught in high school
- ResearchNot taught even at university as settled fact; researchers are still investigating it
Junior highTerms: This Phenomenon Has Names
- Sweat gland (eccrine gland): A small organ in the skin that releases sweat. In humans it is spread over the whole body and serves to lower body temperature.
- Heat of vaporisation (latent heat of evaporation): The heat a liquid takes from its surroundings when it turns into gas. One gram of sweat drying is said to take about 2.4 kilojoules of heat.
- Panting: Cooling the body by evaporating water from the tongue and mouth through shallow, fast breathing. Used by animals with little sweat, such as dogs and deer.
Junior highHigh schoolChecking With an Equation: What Is the Fastest Pace Sweat Can Cool?
We find the speed at which the heat produced by running balances the heat taken away by drying sweat. We assume a hot day and ignore heat lost in any way other than sweat.
| In symbols | v ＝ L × m ÷ （ f × C × M ） |
| In words | Fastest speed you can keep cool at ＝ heat sweat can remove per hour ÷ heat produced per km run |
| Where it comes from | A heat balance for keeping body temperature steady. It is the energy-conservation equation "heat produced by running ＝ heat lost through sweat", solved for speed |
| L | Heat of vaporisation of sweat (kilojoules per gram) |
| m | Amount of sweat that dries in one hour (grams) |
| f | Fraction of the energy used that becomes heat |
| C | Energy used to run 1 km per kg of body weight (kilojoules) |
| M | Body weight (kg); v is speed (km/h) |
| Body weight M | 60 kg |
| Energy to run 1 km per kg of body weight, C (said to be nearly constant regardless of speed) | About 1 kilocalorie ＝ about 4.2 kilojoules |
| Fraction that becomes heat, f | About 0.8 |
| Heat of vaporisation of sweat, L | About 2.4 kilojoules per gram |
| Sweat that dries in one hour, m | 1000 grams (1 litre) |
| Heat sweat can remove in one hour (kilojoules) | 1000 × 2.4 ＝ 2400 |
| Energy used to run 1 km (kilojoules) | 60 × 4.2 ＝ 252 |
| Of that, the part that becomes heat (kilojoules) | 252 × 0.8 ＝ 201.6 |
| Fastest speed you can keep cool at (km/h) | 2400 ÷ 201.6 ≒ 11.9 |
| Sweat cooling converted to watts (1 hour ＝ 3600 seconds) | 2400000 ÷ 3600 ≒ 667 |
About 12 km/h is a pace of roughly 5 minutes per km. That is close to the speed of a recreational runner finishing a full marathon in about three and a half hours, and with one litre of sweat, anything faster means heat builds up in the body. With 2 litres of sweat, the limit doubles to nearly 24 km/h. In reality, muggy weather and sweat that runs off without drying push the limit lower.
High schoolHigh school+Temperature Control and the Evolution of Running
High schoolBody temperature is kept steady by the hypothalamus in the brain, which adjusts blood flow to the skin and the amount of sweat. When exercise raises body temperature, the blood vessels in the skin widen, carry warmed blood to the surface, and sweat cools it.
High school+In four-legged animals, a one-to-one link between each galloping stride and each breath is known, called locomotor–respiratory coupling. A 1983 study showed that in humans this ratio can vary, as 4 to 1, 3 to 1, 2 to 1 and so on.
UniversityThe Endurance Running Hypothesis and Heat Balance
In 2004, the biologist Bramble and the anthropologist Lieberman proposed the "endurance running hypothesis": that many features of the human body were selected for running long distances. The evidence they cite includes the length of the Achilles tendon, the arch of the foot, the ligament at the back of the neck (nuchal ligament) that steadies the head, and the large buttock muscles. In exercise physiology, the flow of heat through the body is treated with a "heat balance equation", which takes heat produced by metabolism and subtracts terms for work, conduction, convection, radiation and evaporation. The calculation in this article is an approximation that keeps only the evaporation term.
📖 Derivations and further reading: Bramble and Lieberman's paper (Nature, 2004) ／ Heat of vaporisation (Wikipedia, in Japanese)
ResearchWhat We Still Don't Know for Sure
- How widespread was persistence hunting? Recorded examples are limited, and some researchers disagree that it was a main method of hunting in human history.
- When did body hair become thin? Hair does not survive in fossils, so the timing can only be estimated indirectly, from genes and similar evidence, and the range is still wide.
- How far can a heat-tolerant body be trained? It is known that getting used to heat makes sweat start earlier and flow more, but individual differences and the mechanisms behind the limits are still being studied.
In other words, what this article says is "an explanation within what is known today". In particular, the idea that "humans evolved to run" is one leading hypothesis, not a settled fact.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: changes of state, structure of animal bodies | Drying sweat takes heat; sweat glands |
| High school | Biology: maintaining the internal environment / Physics: quantity of heat | Temperature control by the hypothalamus; the heat-balance calculation |
| High school+ | Advanced biology: animal behaviour and evolution | Coupling of stride and breathing |
| University | Exercise physiology, physical anthropology | Endurance running hypothesis; heat balance equation |
| Research | Human evolution, heat acclimatisation | Spread of persistence hunting; when body hair thinned |
| ― | Links to daily life | Why you should drink and slow down when running in summer |
- Bramble, D. M. & Lieberman, D. E. (2004) Endurance running and the evolution of Homo. Nature 432, 345–352.
- Bramble, D. M. & Carrier, D. R. (1983) Running and breathing in mammals. Science 219, 251–256.
- Liebenberg, L. (2006) Persistence hunting by modern hunter-gatherers. Current Anthropology 47, 1017–1026.
- Sweat gland (汗腺) - Wikipedia (Japanese)
- Daniel E. Lieberman, The Story of the Human Body (Japanese edition: 『人体600万年史』, Hayakawa Publishing)
※This article is a popular-science explanation. The figures given are rough guides to help you understand how things work. Sweat volume and heat tolerance vary greatly from person to person. When exercising on hot days, don't push yourself; listen to your body and follow the heatstroke guidance of event organisers and local authorities.