Speedy Elephants Use A Biomechanical Trick To 'Run' Like Groucho

Even at fast speeds, it might seem to the casual observer that elephants don't run. Their footfall pattern remains the same as that in walking, and never do all four feet leave the ground at the same time - a hallmark of running. But biomechanists are finding that an elephant's center of mass appears to bounce at high speeds. If that turns out to be true, an elephant's gait meets the biomechanical definition of running.

Biomechanists have dubbed this gait "Groucho running" after the silly, crouched walk of Groucho Marx. They say the elephants seem to bend their limbs slightly in order to move their bodies more smoothly. This research may provide insight into the biomechanical tricks that help large animals, from extinct dinosaurs to obese people, overcome the physical forces that restrict their motion.

"We do find evidence that elephants run in a sense," said first author John Hutchinson, a Stanford postdoctoral research fellow in the Department of Mechanical Engineering. "It's an intermediate sort of gait, but it looks like what we biomechanically would call running. They don't leave the ground, which is the classical definition, but they do seem to bounce, which is the biomechanical definition."

Last year Hutchinson co-authored another Nature paper that used a computer model of physical forces to show that Tyrannosaurus rex probably was too big to run quickly. For his recent paper, he teamed up with Dan Famini, a veterinary student at the University of California-Davis; Richard Lair, an adviser and international relations director at the Thai Elephant Conservation Center; and Rodger Kram, an associate professor of kinesiology and applied physiology at the University of Colorado-Boulder. They focused on an extant biggie rather than an extinct one: the Asian elephant (Elephas maximus), which can tip the scales at more than 4 tons.

From Africa USA to Thailand

In 1997, Hutchinson, Kram and Famini were all at UC-Berkeley. Kram, the first with colleagues at Harvard to measure the rate of oxygen consumption in walking elephants, was advising Hutchinson and Famini about "normal" elephant biomechanics during the duo's kinematic experiments with African elephants at Six Flags Marine World in Vallejo, Calif. Earlier, Kram had noted that elephants preferred to walk at a slow but efficient speed that gave them what he called the "best gas mileage."

Hutchinson began to correspond with Lair, the author of Gone Astray: The Care and Management of the Asian Elephant in Domesticity, published by the United Nations Food and Agriculture Organization in 1997. Once an elephant trainer at Marine World/Africa USA in Redwood City, Calif., before the park relocated to Vallejo, Lair moved to Thailand in 1980 to help save Asian elephants from extinction. He has trained Asian elephants for films, notably Disney's Operation Dumbo Drop, and now works at the Thai Elephant Conservation Center, which provided crucial support for the Nature study.

"[Hutchinson asked] if I thought Thai elephants could run faster than the speeds he and Dan had got from U.S. zoo and circus elephants [about 10 mph]," recalled Lair in an e-mail interview. "I said that I knew they could because I had timed them much faster at the Surin Elephant Round-up in northeast Thailand in 1984."

Thanks to a traveling fellowship from the Journal of Experimental Biology, Hutchinson and Famini went to Thailand in 2000 and 2001 to put some elephants to the test.

For their experiments, Hutchinson palpated the animals' limbs to find their joints, and then the duo marked the joints with large dots of water-soluble, nontoxic paint. They videotaped 188 trials of 42 Asian elephants walking and running through a 100-foot course and measured their speed with photosensors and video analysis.

The average walking speed was 4.5 mph. But 32 of the elephants moved faster than previously documented - up to 15 mph. Three were especially fleet of foot, exceeding 15 mph - 50 percent faster than anyone had ever reliably recorded, Hutchinson said.

Past references gave anecdotes, not data. The result was a lot of confusion about elephant speeds.

"The vast majority of statements regarding the maximum speed of African elephants descend from one of two apocryphal hunches dating back over 60 years," Famini wrote in an e-mail.

Said Hutchinson: "Here we actually have the videotape and data to back it up, whereas with an anecdote, like some big game hunter clocking an elephant with a speedometer on a car, it's just not reliable."

Seeing was believing - these elephants were fast. "When I saw the speed trap times and videos I was convinced," Kram wrote in an e-mail. "I ran the mile in 4:30 when I was in high school and I am still a competitive Master's runner. I can only just barely sprint as fast as the fastest elephants we measured."

To run or not to run - that is still the question

So what turns a walk into a run? It isn't just speed, although that plays a part.

Kinematically, one thing that distinguishes walking from running is the footfall pattern. Typical quadrupeds use a walk at slow speeds, a trot at medium speeds and a gallop at fast speeds.

In the footfall pattern of a trot, diagonal limbs contact the ground at the same time. "So a quadruped goes left hind, right front together and then right hind, left front together," Hutchinson explained. "It's acting like a biped."

In contrast, in the footfall pattern of a gallop, the two hindlimbs touch the ground one after the other, followed by a pause, after which the two forelimbs touch the ground one at a time. If an animal's feet are on the ground less than half of the time, Hutchinson said, it meets the kinematic definition of running.

But elephants are weird because no matter how fast they go, their footfall pattern doesn't change. They use a walking footfall pattern even at 15 mph, the researchers found. That pattern has the left hind foot moving first, followed by a brief pause, after which the left front foot moves. Then there's a long pause, after which the same thing happens on the right side.

An all-aerial phase - where no feet are touching the ground - also kinematically differentiates running from walking. But elephants never have all their feet off the ground.

"Elephants probably don't run with an aerial phase because it would be too mechanically stressful on their bodies," said Hutchinson. It turns out that a lot of other animals - including running birds like chickens, emus and rheas - have limbs that release elastic strain energy like the rebound of a stretched rubber band without ever getting propelled so forcefully that all feet are off the ground at the same time.

That led biomechanists to redefine running more than 30 years ago to better describe the physical forces at work, Hutchinson said. "We're just beginning to understand which animals can break the rules and bounce without leaving the ground - and how they do it."

A deeper biomechanical mechanism may explain running better than the aerial phase frequently observed. All legged land animals, Hutchinson said," whether they have two legs, four legs, six legs or even in the case of a centipede, 42 legs," use the same mechanism to switch from a walk to a run. That switch often occurs at the same relative point, or Froude number, which is a dimensionless measurement that gives an animal's speed relative to its hip height. So even though a cockroach has shorter legs than an elephant, in terms of how many body lengths it can move in a certain amount of time, it may still scurry with greater relative speed than a charging elephant.

"At the same Froude number, any animal, regardless of size, should be moving with the same mechanism," Hutchinson said. "It should be exerting itself in relatively the same way." A Froude number of 0.01 is slow for any animal; a Froude number of 20, fast. Most animals should switch from a walk to a run at about the same Froude number, at 0.5 or so, he said. Animals shift from a walk to a run because at faster speeds walking becomes less energetically efficient, or more mechanically stressful, than running, he said.

"The stunning thing about our elephants is they were going at a Froude number as fast as 3.4, which is over three times the dimensionless speed that an elephant should be switching from walking to running," Hutchinson said. "A horse would be well into a gallop by this point. But the elephants were still using a walking footfall pattern."

If you think of the body abstractly as just a stick swinging back and forth as it moves, he explained, its center of mass moves differently during walking compared to running. "Walking is a stiff, pendulum-like gait; the limb stays pretty straight and swings back and forth. Running is a bouncing gait in which the limb actually compresses and bounces back with a spring."

The researchers' kinematic measurements suggest that fast-moving elephants may switch from a pendulum-like gait to a bouncing gait. If they do, they fit the biomechanical definition for running.

But there's only one way to find out for sure. The animals would have to move across a force platform - a special device that registers the forces that elephants exert on the ground - to see if their center of mass swings like an inverted pendulum (as in walking) or bounces like a spring (running).

"That's a problem because the force platforms that are generally available would break if an elephant ran across them," Hutchinson said. "That's been the obstacle for years. That's one reason why no one has ever done it."

He and Kram are building a prototype force platform in Colorado to answer once and for all if elephants can run. So there's still time to place your bets.

Relevant Web URLs:

Stanford University Neuromuscular Biomechanics Lab: http://www.stanford.edu/group/nmbl/

Thai Elephant Conservation Center: http://www.changthai.com