Analyze This: Why the fastest creatures are neither tiny or huge

Cheetahs sprint across the savanna. Impalas, antelopes and other prey gallop away. All these creatures were built for speed. In fact, mid-sized land animals are generally faster than huge or tiny creatures. New research now unveils why.

Christofer Clemente led the work. He studies biomechanics — how animals move — at the University of the Sunshine Coast. That’s in Sippy Downs, Australia.

To probe the role of size in swiftness, he and his colleagues could have compared how different animals — such as an elephant, cheetah and mouse — move. But those creatures’ leg muscles and movement differ.

“Too many things are changing,” says Clemente. It would be hard to identify which factors limit how fast an animal runs.

Instead, Clemente and his colleagues used a computer model of one type of animal — a human — at many different sizes. The team modeled humans ranging from mouse-sized at 100 grams (0.2 pound) to elephant-sized at 2,000 kilograms (4,400 pounds).

The model showed that being bigger lets a creature take longer strides. But there are drawbacks. If a body keeps the same proportions as it grows taller, its mass grows faster than its muscle strength does. The result is an animal that becomes worse at carrying its own weight. That cuts into its speed. The most massive model humans were so heavy their muscles couldn’t move them at all.

Small animals are comparatively strong. But that doesn’t make them the fastest. The mini model humans showed why. Small critters’ muscles push them off the ground too quickly when they take a step. That decreases the amount of force they can generate to move forward.

“Once they produce enough force, they just become airborne,” Clemente says. This is similar what happens when astronauts walk on the moon.

To lessen their air time, small animals crouch more than large animals do. Bending their legs more keeps them on the ground a little longer during each step. But they can’t take steps as quickly as more upright creatures.

For speed, there’s a Goldilocks zone, Clemente says. “That sweet zone where your muscles are still strong, but your body mass is big enough that you can actually use that strength.” The researchers shared their results last October in Nature Communications. The findings might help engineers figure out how to design robots for motion.

When muscles can’t keep up with mass

As an animal gets taller, its mass grows faster than its strength does. That’s because mass is directly related to volume. But strength depends on the area of a slice of muscle. To see how this works, picture a cube 1 centimeter tall. Its volume is 1 cubic centimeter, and the area of one slice of it is 1 square centimeter. Now, say you have a cube 2 centimeters tall. Its volume is now 8 cubic centimeters. But the area of one slice of it is only 4 square centimeters.

Data Dive:

1. Look at Figure A. What size of animals have the fastest speeds? What speeds can those animals reach?
2. How much faster is this than the speeds achieved by 1-kilogram animals? How much faster is it than the speeds achieved by animals that weigh around 1,000 kilograms?
3. Look at Figure B. How fast do the smallest model humans move? How fast do the biggest model humans move?
4. What’s the peak speed for a human? How does that compare with the peak speed for four-limbed animals?

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