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Elephants, researchers, and mahouts preparing for our field experiments at the Thai Elephant Conservation Center. Photo: John R. Hutchinson
Why did we do it? The basic principles governing locomotion in animals larger than a horse remain poorly understood. Elephants are the best choice to begin studying as they are the largest land animals. Besides, elephants are strange and fascinating animals that are a joy to work with.
Why should the public care? Asian elephants are endangered; we should learn from them while healthy, active populations are still available to study. The general principles that we can learn from large animals like elephants apply even to humans, as these principles are about factors such as size, speed, strength, and stability. Also, the public should know about the way biomechanists define running today: as a bouncing spring-mass system rather than as necessarily having an aerial phase.
How did we do it? The elephants were motivated to move fast with food rewards, comfort, competition, cheering, playful chasing, other positive reinforcement, etc. One elephant, Pop, was so excited by the experiments that she had to be held back from running down the course, and broke free at full speed twice (once making one of our speed records). The scope of the project was broad: we selected 42 elephants that were known to be quick and healthy from over 300 potential subjects in Thai elephant camps. Thus we feel that we have found a reasonably close estimate of the maximum speed of Asian elephants, although it is likely some few can move slightly faster.
What next? Force platforms are being designed to definitively solve the running question. We also hope to work with African elephants as some have said they are faster than Asian elephants. Computer models can be used to test what features limit elephant speed and gait. Later, research on other large animals such as hippos, rhinos, and giraffes is needed.
SPEEDS: We provided the first well-documented study of the locomotion of elephants, the largest land animals. This subject has been surrounded by confusion and misinformation. The confusion about elephant locomotion extends back to the late 1800s, involving famous researchers such as Eadweard Muybridge (who was among the first to use cinematography, and so won a bet for Leland Stanford that all four legs of a horse leave the ground during a trot) and continuing to the present, when studies of animal locomotion cite widely different maximum speeds (and gaits) for elephants.
The myth that all elephants can move at 25 mph (and still walk!) mainly dates back to one anecdote from Roy Chapman Andrews (famed American Museum of Natural History paleontologist/archaeologist and a model for Indiana Jones). We still hear assurances from people familiar with fast African elephants that they can move 25 mph, but speeds are difficult to measure accurately. (we have a longer version of this speed story in essay form posted on the website)
This 15 mph speed is not that fast, but is 50% faster than previous reliable estimates at ~10 mph. For comparison, a typical high-school sprinter at 20 mph, or a champion Olympian 100m dash at 25 mph average speed, could outrun an Asian elephant without difficulty. An average human would be fairly evenly matched with a fast, healthy elephant in a footrace on level ground.
RUNNING: Elephants might be running at their faster speeds. It depends on the definition of "running" applied. The old, classical definition familiar to most people is that running is when all feet are off the ground at once (called an aerial phase) during a stride. Biomechanical engineers and biologists favor a more mechanistic definition today, which has come into favor over the last 40 years. This kinetic definition models the center of mass of the body as a rigid pendulum-like mechanism (in walking) vs. a bouncy spring-like mechanism (in running), explainable as follows:
In walking, kinetic energy and potential energy are out-of-phase: when the foot is on the ground, the center of mass swings up to its highest point (trading kinetic energy for potential energy), then back down again (regaining kinetic energy) when the foot leaves the ground.
In running, kinetic energy and potential energy are in-phase, so they cannot be exchanged. Instead, as the center of mass descends once the foot hits the ground, the leg(s) becomes compressed, storing elastic strain energy in its tissues (tendons, muscles, ligaments, bones, etc.). Rather than the center of mass being highest at the middle of ground contact, it is lowest, and the "leg-spring" is maximally compressed. Then, later in the step, the leg-spring rebounds, providing energy to raise the center of mass back up. Visualizing a pogo stick's gait might help you understand how running works. If the kinetic energy provided during bouncing on the ground is high enough, the body may leave the ground for an aerial phase. But that aerial phase is not necessary, and in many animals it does not occur.
Crabs and other many-legged animals bounce without leaving the ground, and lately scientists have found that other animals do this, including birds running at medium speeds, and humans who "Groucho run" by running with very flexed knees. Elephants might be doing this. We found three lines of evidence that suggest this:
In elephants, the dimensionless speed or Froude number, a ratio of the kinetic energy (or inertial forces) and potential energy (or gravitational forces) during locomotion, reaches values as high as 3.4. Theoretically, at Froude numbers greater than 1.0 animals should have to switch from walking to running, as the forces drawing them off their circular (pendular) path exceed the gravitational forces pulling them down. Indeed, most animals switch from walking to running at a Froude number of ~0.5, because it is either mechanically less stressful or energetically more efficient to switch gaits at a lower speed. But elephants surpass Froude numbers of 0.5, 1.0, and higher without their motions showing an obvious change of gait.
For the Froude number, think about it this way: if you are late for the bus and start walking faster as you realize that you might miss it, at some speed you cannot walk any faster. If you go faster, you must leave the ground because you fly right off the ground (inertial / centripetal forces overcome gravitational forces holding you down). "Groucho running" can be done without an aerial phase, unlike normal running, because the flexed legs are springier and can stay on the ground longer than more rigid legs. Humans can "Groucho run" to a Froude number of about the same as a really fast, small elephant: ~3. This is the same Froude number that a typical Boston marathon runner might use, in that case jogging rather than "Groucho running". In the upcoming Kentucky Derby, at 40 mph those horses will be using Froude numbers of about 20; much more relative exertion than any human runner or elephant can do. A cheetah at 70 mph might have a Froude number of about 30 or more.
Another explanation is that if a small child walks alongside a tall adult, because their legs are shorter, they must switch from a walk to a run at a lower speed than the adult. The Froude number is equal to the speed-squared, divided by hip height and gravity, so this makes sense. Shorter legs (lower hips) must switch gaits at slower speeds, for the same reasons as explained above. Elephants have long legs for their size, so perhaps this helps them move fast, maybe even run, without leaving the ground.
Clues that elephants may run at faster speeds include our observation that each foot spends less than 50% of the time on the ground (the duty factor). In most animals, this duty factor would require elephants to use an aerial phase -- e.g., a human whose feet are each only on the ground for 40% of the stride, a duty factor of 0.40, are both off the ground for 20% of the time, requiring an aerial phase. But the evenly spaced footfall pattern of elephants (see below) keeps at least one foot evenly on the ground, even at higher speeds with duty factors as low as 0.37. At one point in their stride, a fast elephant supports itself on only one leg. If it is a forelimb, that leg is held vertical and rigid, while the hindlimbs are airborne. If it is a hindlimb, it is more bent and compliant (looking like a spring), while the front of the elephant is airborne.
This ties into our Fig. 1b. While its foot is on the ground, the shoulder joint goes up then down regardless of speed, and the hip joint does the same thing until higher speeds, but then goes down and often back up. This is a strange mechanism that has suggested to some that the forelimbs are walking while the hindlimbs are running. We don't know if that's actually possible. And this is one reason why we can't say that elephants are definitely running, as the two sets of legs work oppositely and could change the mechanics in a major, unanticipated way. But I think it's rather likely, as the Froude number is so high, the duty factor is so low, and the hindlegs are so bent. We might be wrong, though; locomotion works in mysterious, complex ways.
For elephants, if they do indeed run, then this means they bounce. "Bounce" is not a word one often thinks about when considering elephants. "Rigid" or cumbersome more often come to mind. But if we are correct and the hints we see from the motions are hints of running, then indeed elephants do bounce.
On the running vs. walking definition, we see this as an opportunity to show the public that the old definition of running (aerial phase) is not satisfactory, and has fallen out of favor in the last 40 years. The modern definition shows that the center of mass is lowest at the midpoint of a step in running, vs. highest in walking. This requires that the legs act like a spring in running, vs. like a rigid pole in walking. Imagine a bouncing ball compared to a walking toy soldier and you can see the difference. When one realizes that animals of 2, 4, 6, or even 42 legs use this bouncing mechanism at higher speeds, and even switch from walking to that gait at the same relative speed (Froude number), one can see why the biomechanical definitions work better to unite how these animals work. Many of these animals (especially those with many legs) do not leave the ground at faster speeds, but still do use bouncing mechanisms.
One piece of evidence that might seem contradictory to our "running" speculation is the way the feet hit the ground. The same sequence (Fig. 1a in our paper) applies at all speeds for all elephants: 1 (brief pause), then 2, longer pause, 3 (brief pause), then 4 (imagine a 4-legged dance step, 1-2, 3-4). In elephants this 1-2, 3-4 pattern involves the legs on the same side of the body (1-2 = left side, 3-4 = right side for example). This footfall pattern is called a "lateral sequence singlefoot with lateral couplets" (Muybridge helped define this sequence; at faster speeds he called it the "amble", which is now an outdated term). It was previously recorded almost exclusively in animals that lacked an aerial phase and were thought to be walking. Yet as we now know from more studies of animals, that walking definition does not work well, because a walking footfall pattern does not necessarily coincide with a rigid, toy soldier-like gait.
Gait = a discrete pattern of locomotor movements. Running, walking, and hopping are the three main terrestrial gaits.
Stride = a complete cycle of footfalls, e.g., from the time the left hind foot hits the ground to the next time it hits the ground. In a walking biped, a stride consists of 2 steps, whereas in a quadruped it consists of 4 steps.
Kinetic energy = related to the mass and velocity of a body (e.g., inertia or momentum), it is the total energy propelling the animal in the direction of its velocity heading.
Potential energy = the energy an animal or other object has because of its height from the ground, its mass, and gravity.
Center of mass = a point at which the entire mass of an animal or other object can be abstracted as lying in 3D space.
Duty factor = the fraction of a stride that a given foot is on the ground.
Classical running definition = a gait in which there is an aerial phase, or the duty factor is less than 0.5. Walking therefore involves no aerial phase, and a duty factor 0.5 or more.
Kinetic running definition ("running" as defined here and in science in general today) = a gait in which the center of mass is lowest at the middle of ground contact during a stride. This mechanistic definition models the animal's limb and body as a spring-mass (bouncing) system, storing and releasing elastic strain energy to save energy while kinetic and potential energies oscillate in-phase and the body moves forward.
Kinetic walking definition = a gait in which the center of mass is highest at the middle of ground contact during a stride. This mechanistic definition models the animal's limb and body as a rigid pendular system (technically, an inverted pendulum). Kinetic and potential energies oscillate out-of-phase to save energy while the body moves forward.
"Groucho running" = running with more bent limbs. This unusual form of running gait often lacks an aerial phase. It is known among many animals with more than four legs, as well as some four-legged animals (lizards, sheep, opossums, and others) and bipeds (birds at medium speeds, comedic humans such as the namesake Groucho Marx).
John Hutchinson Staff Home Page Structure & Motion Lab
