Australasian Science: Australia's authority on science since 1938

Animal vs Human

By Tim Olds

How does the fitness of humans compare with other animals?

If we could go back a million years and pick an evolutionary winner from the zoosphere, humans would have been pretty low on the list. Weak, small, slow of foot, poorly armed in tooth and claw, prone to internecine wars – they really didn’t have much going for them. Clearly other things counted – social groups, opposable thumbs, thermoregulation in chase hunting, upright posture on the savannah, big brains …

How do humans actually compare with animals in the fitness department? During the 1980s, C. Richard Taylor and Ewald Weibel undertook a series of experiments measuring the maximal aerobic capacity of a wide range of animals. They used techniques that could only be described as innovative – and daring.

Maximal aerobic capacity (VO2max) is the highest rate at which an organism can use oxygen to produce energy, and is the key measure of stamina. In humans, VO2max is usually expressed in millilitres of oxygen per kilogram of body weight per minute. The average VO2max for a human is about 40 mL/kg/min.

VO2max depends on what is known as “the pathway for oxygen” – the flow of oxygen from the sea of air around us down to the muscles. Based on their animal work, Taylor and Weibel developed a much-debated hypothesis called “symmorphosis” – the idea that each stage of the pathway for oxygen (lungs, heart, tissues) is functionally adapted to the other stages. It’s a kind of Occam’s razor of physiology – there is no unused capacity. Working at our peak, our lungs can’t deliver more oxygen than the muscles can use.

VO2max depends on the ability of the heart to pump blood around the body (itself the product of heart rate and the amount of blood the heart squirts out with each beat), the oxygen-carrying capacity of the blood (mainly a function of the haemoglobin concentration), and the ability of the muscles to extract that oxygen (which depends on the density of the capillaries and the presence of aerobic enzymes). Greg Lemond, who won the Tour de France three times, had a VO2max of 92.5, while Lance Armstrong, who never won the Tour de France, had a VO2max of 84 mL/kg/min. But these aren’t the highest. The present record is 97.5, achieved by an 18-year old Swedish cyclist. Remember the name Oskar Svendsen – you heard it here first.

To measure VO2max, humans run on a treadmill that goes faster and faster, and at an ever steeper slope, until they literally can’t go on. They wear a face mask and are hooked up to a gas analysis machine, measuring how much oxygen they breathe in and how much comes out. I’ve done it, and I can tell you it’s not a pleasant experience.

It’s a bit more challenging measuring the VO2max of animals. Sometimes you can use a treadmill. Horses like treadmills, very big ones, and Taylor and Weibel used a horse treadmill for lions, giraffes and cows. For smaller animals, running wheels will often work – Syrian golden hamsters, for example, will run all day and all night, apparently for the sheer joy of it, until they collapse.

Science can be a cruel business. Rats are notoriously recalcitrant, so a treadmill with an electrified bar at the back is used. Whenever they slacken off, they get an electric shock. Or they can tie weights to their tails and make them swim for their lives.

Fish are put into circular tanks with an artificial current and made to swim like crazy after bait – the rate of oxygen depletion in the water is measured. Birds like turkeys can be put onto special treadmills or into flight chambers if they don’t run.

Crustaceans posed special problems. Eventually, Taylor and Weibel hit upon the idea of gradually freezing them so that eventually they reached VO2max by ramping up their shivering.

So how do humans compare? In two words, pretty poorly. Generally speaking, bigger animals have a lower mass-specific VO2max. Taylor and Weibel even developed a formula for land mammals: VO2max (mL O2/s) = 1.92 Mass (kg)0.81

Cows can barely hit 30, sheep, wildebeest and goats about 50, and lions about 60. These are all in the human range. Racehorses score about 180. Wolves, coyotes, foxes and dogs are very fit, with huskies topping the list at 240. As you would expect, birds require very high rates of energy production: 190 for a parrot in full flight, and 210 for a bowerbird.

The pronghorn is the champion large mammal with 300. These North American ruminants can do a sub-one minute mile, and are the second-fastest land animal. The champion of all land animals is the tiny 4 cm Etruscan shrew. With a maximal heart rate of 1300 beats per minute, and taking up to 800 breaths each minute, this little fellow scores 400.

And how did I score? About halfway between Lance Armstrong and a wildebeest. To be quite honest, maybe more a wildebeest than a Lance Armstrong.

Professor Tim Olds leads the Health and Use of Time Group at the Sansom Institute for Health Research, University of South Australia.