The top side of an elephant’s trunk stretches more than the bottom

On a sunny day at Zoo Atlanta in 2020, Kelly the African bush elephant reached for a snack and revealed something strange.

High-speed cameras tracking her movements suggested that the skin on top of Kelly’s trunk stretched more than the skin underneath. “But that didn’t make any sense,” says Andrew Schulz, a mechanical engineer at Georgia Tech in Atlanta.

Scientists had assumed that elephant trunk skin largely stretches the same way all over. When Schulz sent data from Kelly and a male elephant, Msholo, to colleagues, they said, “Oh yeah, your data is wrong,” he remembers.

Follow-up experiments stretching pieces of elephant skin in the lab showed the same peculiar phenomenon: The skin on the top and bottom of the trunk are two entirely different beasts. “Talk about a great day as a scientist!” Schulz says. “That’s when we really started to believe that what we were saying was true.”

The tough upper skin of an elephant’s trunk, which crumples into creases and crags like the folded furrows of a shar-pei puppy, is about 15 percent stretchier than the gently wrinkled skin on the underside, Schulz and his colleagues report July 18 in the Proceedings of the National Academy of Sciences.

When an elephant elongates its trunk, the upper skin stretches more than the skin on the underside. Green crosses represent initial positions on the skin. As the trunk extends, the crosses turn red and move to the right, indicating how much the skin stretched.

That extra stretch probably helps elephants reach down and wrap their trunks around a log or a tree branch, while the wrinkled skin underneath gives the animals a good grip, Schulz says.

The team also observed that the trunk extends like a telescope, the tip reaching out first, followed by sections farther up. And at the trunk tip, it’s the skin that does most of the straining, not the muscle, mathematical modeling experiments suggest.

Scientists have long studied the muscles in elephant trunks (SN: 3/26/88). But skin has largely been overlooked, says Lucia Beccai, a soft roboticist at the Italian Institute of Technology in Genoa who wasn’t involved in the research. The new study “tells us that the structure of elephant skin is not all the same.”

Artificial skin is often modeled after human skin, but researchers could learn a lot from other animals, Beccai says. Understanding how Kelly’s and Msholo’s folds and wrinkles stretch is “surely information that will improve the design of soft robots,” she says.