A charging elephant might not look like it's merely walking, but that's exactly what it may be doing. Surprisingly, scientists don't agree on whether elephants run in the traditional sense. Now a new study splits the difference, suggesting that a rushing elephant may be walking and running at the same time.
Elephants break all the rules of animal movement. Most four-legged vertebrates change their stride when they move at high speeds so that all four feet leave the ground at once. Elephants take faster and longer steps, but they never take all four feet off the ground. That helps them spread out their massive weight as much as possible, says Norman Heglund, a biomechanics expert at the Catholic University of Louvain in Belgium. But then, are they really running?
That's been a tough question to answer. It turns out there's a lot more involved in running than just taking one's feet off the ground. The legs also flex in particular ways that change an animal's center of gravity and the force it exerts on the ground. Scientists can measure these changes in humans and other animals by making them walk across force sensing plates. But just try doing that with a 4000-kilogram elephant.
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The closest researchers have come is high-speed video of rushing elephants analyzed by a team of researchers in 2003. John Hutchinson, a biologist of the University of London's Royal Veterinary College, and colleagues concluded from the footage that, when elephants move at high speeds, their back legs bend slightly, like a runner springing from step to step. But scientists wanted to see more quantitative data.
So in the new study, Heglund and colleagues engineered a heavy-duty force-sensing plate that could withstand being trampled by a charging elephant. They shipped sixteen of the plates, along with computers and video cameras to the Thai Elephant Conservation Center in Lampang Province. There, they constructed a 2 meter by 8 meter track from the plates and filmed 34 elephants, each guided by a mounted trainer, as they traversed the track at a range of speeds.
Despite their size, elephants step lightly, the team reports today in The Journal of Experimental Biology. A human runner exerts peak forces of 3 times his or her body weight when running, while elephants exert at most 1.4 times their body weight. They also don't move their center of gravity much: Even at an 18 kilometer-per-hour charge, an elephant's center of mass moves up and down by about a centimeter, a smaller vertical movement than human runners make.
As to whether elephants actually run, the answer seems to be, well, sort of. While Hutchinson's team saw that an elephants' front legs walked while the back legs trotted, Heglund's team's measurements indicated the opposite: When stepping with a forelimb, the elephants' center of mass lowered slightly as the force on the ground increased, indicating a spring-like mechanism typical of a dog or a human's run. Meanwhile, even at high speeds, the elephants' back legs seemed to stay rigid, which is typical of a walk. "They don't really run in the classical sense," Heglund says. "They can't quite kick it into second gear, so they're stuck halfway in between" a walk and a run.
But that may not be the final answer, warns Hutchinson, who studied the same group of elephants alongside Heglund's team. He says that the elephants' feet likely touched more than one plate at once, making it hard to separate what individual limbs were doing. Still, he says, the research is important because it quantifies the forces with which elephants hit the ground. "It's nice to have the numbers."
If nothing else, the study illustrates that, when it comes to running in animals, "it's not one size fits all," says Daniel Schmitt, an expert in primate locomotion at Duke University in Durham, North Carolina. "That speaks to how evolution works."