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The Joy of Pronking

A deer can effortlessly jump high into the air; so can a flea. But are humans capable of the same tasks with our dumb legs?

Eadweard Muybridge’s study of a deer in motion. (Photo: Wellcome Trust V0048764/CC BY 4.0)

The other day, I was driving along a road in upstate New York when I saw a deer. This isn’t an unusual sight in itself, but this fawn, young enough to still have was still spots,  was bounding, flying, across the lawns beside the road, so fast and heedless that my husband worried it would jump right into a car and die.

What was even more striking than its speed, though, was how high it seemed to get, as it left the earth and almost hovered, for a moment, in the air, over and over again. It look something like this, but even more dramatically air-borne:

So effortless, so much air. Why couldn’t I do that?

In theory, say some experts, jumping should not be just the provence of a few lucky animals. One evening in 1949, after dinner at the Royal Institution in London, a scientist named Archibald Vivian Hill delivered a speech in which he argued that, based on the basic properties of muscle and on simple dimensional reasoning, “similar animals of different size should be able to jump the same height.”

Jumping relies on basic physics: any animal that jumps is using energy to leave the ground. This is true whether they are bounding, leaping, high jumping, jumping straight up into the air, or lifting all four feet off the ground while “stotting” or “pronking.”

Jump from a running start, and you have horizontal energy to put into it. Jump from a standstill, and you only have the energy your body can create from that state of rest. No matter what, though, the aim of a jump is to lift the center of gravity from the ground, as high as possible. 

You’d think that a smaller animal might get less air, since it has less muscle and shorter legs and can create less power. But it also has less mass to move. Hill noted that in the standing long jump, kangaroo rats and humans could jump about the same distance. As a rule, animals with equal proportion of the muscles used for jumping can cover about equal heights or distances.

The idea that all animals should jump the same distance had been around for centuries before Hill outlined it, but, of course, it’s not that simple. Think about it this way: small animals have shorter legs, which means that as they bend their legs and push off their ground their muscles, even if they were incredibly strong, don’t have much time to do the work of jumping. They basically can use only one of two tricks to improve their jumping prowess, explains Jim Usherwood, a senior research fellow at the Royal Veterinary College’s Structure & Motion Lab. They can extend the time in which they make the jump or increase its power.

Longer legs allow more time for muscles to gather energy before the animal leaves the ground. This is why good jumpers have disproportionately long or strong legs. And excellent jumpers, like fleas, locusts, and other insects, do not rely on the power of muscle alone. They have other tricks—their legs include catapult-like mechanisms that store energy and release it at the right moment.

Still, even accounting for these tricks, Usherwood points out that humans aren’t that bad. “I can jump higher than a locust, or frog, or flea is able to,” he says. In the end, it does matter how big you are to begin with. 

A frog with long legs. (Photo: Carey James Balboa/Public domain)

The tiny fawn didn’t have noticeably long legs, so I asked Usherwood if he could explain how it go so much air. We considered this image, by Eadweard Muybridge of a similarly airborne fawn:

The front legs of these sorts of animals would have good, long tendons, Usherwood explained, which help it slow, right before the jump, and ping up its legs into the air. Then, it relies on its back, muscly legs, to kick off.  

But part of this deer’s jump is just show. It’s pulled up its legs up towards its body, leaving an impressive gap of air beneath it. But the measure of a good jump isn’t actually how much air you can clear, so much as how high can you get your center of mass away from the ground. In other words, the deer wasn’t doing anything so special. 

Humans are decent at this, although not great. We don’t run particularly fast, and we don’t have springy legs. But, really, there’s not much separating me from that fawn. In fact, in one of Hill’s examples, mule deer and humans were measured as jumping the same distances if they started from a run.

People are often impressed that deer can jump 8-foot fences, but humans can, too, if properly trained. The current high jump record for humans is just over 8 feet, and that’s under rules requiring jumpers to launch from one foot, a restriction that limits the height they can reach. Using two feet, and a series of energy-gathering flips, people have cleared bars even higher, of 9 or 10 feet. These are highly trained athletes, whereas among deer clearing a tall fence seems more commonplace. But the deer often have a stronger motivation for jumping fences (like access to a tasty garden). If the reaching best food required jumping 8 foot fences, we’d all probably be pretty good at it, too.