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Skin folds give elephant trunks more flexibility

The trunk of an elephant contains tens of thousands of different muscles that work together to enable the animal to carry out a range of complex movements from delicately retrieving a single small nut from the floor to stripping a giant tree of its branches and bark. New research adds to an understanding of this miraculous appendage by analyzing the way in which the folded skin helps telescope the trunk to its furthest extent.

The research from scientists at Georgia Institute of Technology, and in collaboration with Zoo Atlanta, finds that an elephant’s trunk skin doesn’t stretch uniformly. The skin on the dorsal surface is significantly folded and more stretchy than the ventral skin, which is only wrinkled. The two sections begin to diverge when an elephant extends its trunk more than 10 percent of its length. When stretching for food or objects, the dorsal section of the trunk extends further forward than the ventral section does.  

These findings are published in the Proceedings of the National Academy of Sciences by the same Georgia Tech team that authored a study last summer about how elephants use their trunk muscles to inhale food and water. 

“When people extend their tongue – a muscle-filled, boneless tissue similar in composition to an elephant’s trunk – it stretches uniformly. We expected the same when we challenged an elephant to reach for food,” said Andrew Schulz, the study’s lead author and a Ph.D. student in Georgia Tech’s George W. Woodruff School of Mechanical Engineering. He and the team filmed two African savanna elephants reaching for bran cubes and apples at Zoo Atlanta.  

“But when we looked at our high-speed camera footage and plotted the trunk’s movements, we were surprised. The top and bottom weren’t the same at all,” Schulz said.  

In order to understand how this was possible, Schulz made use of a dissected elephant and stretched it trunk to assess the skin’s elasticity. He found that the underside of the trunk has skin that is wrinkled while the dorsal skin has considerable folds, all the way from the head end to the far tip. This enables the dorsal skin to stretch 15 percent more than the ventral skin, and helps the elephant reach downwards to the ground, where most of its food will be found. 

The researchers realized they weren’t just seeing muscle movement as the zoo elephant stretched its trunk on the video – they were also tracking the movement of a thick enclosing sheet of skin. 

“Flexible skin folds are the elephant’s innovation,” said David Hu, Schulz’s advisor and a professor in the Woodruff School and the School of Biological Sciences. “They protect the dorsal section and make it easier for the elephant to reach downward, the most common gripping style when picking up items.”

The researchers also identified another way in which an elephant’s trunk differs from other boneless, muscle-filled appendages found in nature, such as squid and octopus tentacles. Instead of extending evenly, an elephant first extends the tip section of its trunk, followed by the adjacent section, gradually working back towards its body. 

This is reminiscent of how we extend the handle of a collapsible umbrella, telescopically pulling sections out after starting with the section closest to the umbrella itself. Schulz says the progressive movement of an elephant’s trunk from tip towards the base is intentional.

“Elephants are like people: they’re lazy,” he said. “The section at the [tip] end of the trunk is 1 liter of muscle. The section closest to its mouth is 11–15 liters of muscle. An elephant will first stretch the [tip] end of its trunk, then the adjacent section, because [it’s] easier to move. If an elephant doesn’t have to work very hard to reach something, it won’t.”

Schulz said he had to rely on a drawing from 1908 when learning about trunk anatomy because scientists and engineers haven’t done much research on the biomechanics of elephants during the last century. In addition to gaining a better understanding of elephants, Schulz (who is a mechanical engineer) also sees applications for the knowledge in the field of soft robotics. Today’s soft robots are typically built for either strength or flexibility, but not for both. An elephant can manage both these functions with its trunk. 

“Soft robotics created with biologically inspired design are always based on muscle movement. If they were wrapped with a protective skin, like an elephant’s muscle-filled trunk, the machines could apply larger forces,” he said. “Last year we learned that a trunk is a multi-purpose, muscular hydrostat. Now we know that skin is another tool at its disposal.”

By Alison Bosman, Staff Writer

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