Elephant trunks have long fascinated researchers and animal lovers alike. Strong and powerful, yet flexible and precise enough to grab a single blade of grass, the elephant trunk has also served as a natural model for engineers building robots.
Now, scientists say they have a much better idea of how elephant trunks work—and they were surprised to discover the important role that skin plays in the appendage's impressive dexterity.
The top side of an elephant’s trunk is more pliable and can stretch 15 percent farther than the underside of the trunk, according to a new paper published Monday in Proceedings of the National Academy of Sciences.
The findings not only contribute to the scientific understanding of trunk biomechanics, which hadn’t received much attention during the 20th century, but may also prove useful to engineers working in soft robotics, who may have previously underestimated the importance of skin-like coverings when developing robotic appendages.
“Most hydraulic limbs are like a fluid-filled balloon—no one’s ever thinking about what’s on the outside of the balloon material,” Andrew Schulz, a mechanical engineer at the Georgia Institute of Technology and one of the study’s authors, tells the New Scientist’s Clare Wilson. “In order to have these complex movements, you need these structures on the outside to help you internally.”
Muscular and boneless, human tongues are similar to elephant trunks. And, when humans stick out their tongues, the organ stretches uniformly. The same is true for octopus and squid tentacles. Because of these and other examples in nature, scientists expected elephant trunks to behave similarly.
But when they took a closer look, they realized elephant appendages were asymmetrical. To analyze elephant trunks, researchers challenged two African savanna elephants—one male, one female—to reach for bran cubes and apples outside of their enclosure at Zoo Atlanta.
Researchers filmed the elephants using a high-speed camera, then analyzed the footage. When they took a closer look, they realized the top and bottom of the trunks were moving differently. At first, they thought this was an error, so they stretched out frozen skin samples from an elephant who had died at the zoo. This test confirmed that the top of the trunk was, indeed, more flexible and could stretch farther than the bottom. They also discovered that the top skin is folded, while the bottom skin is wrinkled.
"Flexible skin folds are the elephant's innovation," David Hu, a mechanical engineer at Georgia Tech and one of the study’s authors, says in a statement. "They protect the dorsal section and make it easier for the elephant to reach downward, the most common gripping style when picking up items."
For engineers who are developing soft robots, these elephant trunk revelations are a good reminder of the importance of skin to biomechanics and a sign that they may want to consider “playing with the geometry of the wrapping” of their technologies, says Michel Milinkovitch, a biologist at the University of Geneva who was not involved in the study, to the New York Times’ Richard Sima.
Understanding elephants better may also help bolster efforts to protect and conserve them, per the researchers.
“Bioinspiration is great until none of the animals exist that we are gaining bioinspiration from,” Schulz tells the New York Times.