The elephant has a secret hiding right on its nose.
Its famous trunk, full of muscle and devoid of bone, can move in a virtually infinite number of directions and is capable of performing an array of tasks, such as tearing up foliage and suctioning up water and tortilla chips. These abilities have inspired nature lovers as well as engineers working to build robots capable of similar feats of flexibility and force.
But the trunk is more than mere muscle, and its abilities may also depend on something obvious but often ignored: the appendage’s skin.
In a study published Monday in The Proceedings of the National Academy of Sciences, researchers report that because of key differences in the pliability of the skin in different zones, an elephant’s trunk stretches more at the topside that faces outward than at the underside closer to the mouth.
The trunk is “a muscular multitool that can do all these things, but one of the tools that it has in its back pocket is all of this different skin,” said Andrew Schulz, a doctoral student in mechanical engineering at the Georgia Institute of Technology and an author of the new study.
As part of a continuing collaboration with Zoo Atlanta, Schulz and his colleagues challenged two African elephants — one male and one female — to horizontally elongate their trunks to grab food placed far away.
The simplicity of this reaching movement belies the complexity of what the trunk needs to do.
When viewed on a high-speed camera, the elephant trunk does not stretch uniformly as do other muscular appendages, such as 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 this telescoping behavior may be more energy efficient than moving the entire trunk. If the trunk were divided into quarters, there is about 1 liter of muscle at the tip but a whopping 22 liters of muscle at the base, which would be heavy and energy intensive if the movement were more uniform.
And with even more detailed analysis, the researchers noticed “weird asymmetries popping up everywhere, like things were different on the top and the bottom,” Schulz said. As the elephant trunk elongated, the outward-facing half was stretching 15% farther than the half facing the ground was.
“I still remember literally running to my adviser’s office being an idiot with my laptop in my hand to show him some of these results because it’s so surprising,” Schulz said.
Initially, the researchers thought this top-bottom difference in trunk stretching was an error, but further mechanical testing put those doubts to rest.