The secret of an elephant’s trunk is superficial

The elephant has a hidden secret on its nose.

Its famous trunk, full of muscles and devoid of bones, can move in a virtually infinite number of directions and is capable of performing a multitude of tasks, such as tearing up foliage and sucking up water and tortillas. These abilities have inspired nature lovers as well as engineers working to build robots capable of similar feats of flexibility and strength.

But the trunk is more than just a muscle, and its abilities may also depend on something obvious but often overlooked: the skin of the appendix.

In a study published Monday in The Proceedings of the National Academy of Sciences, researchers report that due to major differences in skin flexibility in different areas, an elephant’s trunk stretches more on the top facing on the outside than on the underside closer to the mouth.

The trunk is “a muscular multitool that can do all of these things, but one of the tools it has in its back pocket is all this different skin,” said Andrew Schulz, a doctoral student in mechanical engineering at the Georgia Institute of Technology. and author of the new study.

As part of an ongoing collaboration with Zoo Atlanta, Schulz and his colleagues challenged two African elephants – a male and a female – to stretch their trunks horizontally to grab food from the far.

The simplicity of this reaching motion belies the complexity of what the trunk has to do.

When viewed on a high-speed camera, the elephant’s trunk does not stretch uniformly like other muscular appendages, such as the octopus’ arms or your tongue. Instead, the trunk telescopes outward, with the tip extending first, followed by the front half of the trunk.

The researchers hypothesize that this telescoping behavior might be more energy efficient than moving the entire trunk. If the trunk were divided into quarters, there would be about a liter of muscle at the tip but no less than 22 liters of muscle at the base, which would be heavy and energy-consuming if the movement were more uniform.

And with even more detailed analysis, the researchers noticed “weird asymmetries appearing everywhere, as if things were different up and down,” Schulz said. As the elephant’s trunk lengthened, the outward-facing half extended 15% further than the downward-facing half.

“I can still remember literally running into my advisor’s office like an idiot with my laptop in my hand to show him some of these results because it’s so amazing,” Schulz said.

Initially, the researchers thought this top-to-bottom difference in trunk stretch was a mistake, but further mechanical testing dispelled those doubts.

When they stretched samples of skin taken from a frozen trunk that had been preserved from a dead elephant in a zoo, the researchers found that the skin at the top of the trunk, with its long folds, was 15% softer that the skin covered with wrinkles. lower trunk skin.

These different characteristics correspond to the distinct functions that the skin performs. The top surface of an elephant’s trunk needs protection from the sun as well as other animals, and it has this “flexible armor like Kevlar that has these deep folds that are really, really easily stretchable,” he said. Mr Schulz. In contrast, the underside of the trunk is covered in smaller wrinkles and used for grasping and moving objects but rarely sees daylight.

The new study is a good reminder of “the involvement of the skin itself in biomechanics”, said Michel Milinkovitch, a professor at the University of Geneva who has conducted research on the biomechanical complexity of elephant trunks.

For engineers who are inspired by elephants, it is essential to realize that they should not only focus on the motors and other internal materials of their robots, but also think about “playing with the geometry of the package”, said Dr Milinkovitch, who was not involved. in the study. “No one has really built this into real robots yet,” he added.

As the research uncovers new possibilities for future robots that more accurately mimic the powers of the elephant’s trunk, it also underscores the importance of conserving the endangered species who best wields these wondrous implements.

“Bioinspiration is great until none of the animals that we benefit from bioinspiration exist,” Schulz said.

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