Asymmetric Stretch: Skin folds make elephant trunks extensible

Up to one and a half meters long and weighing 130 kilograms: An elephant's trunk is anything but a filigree tool.

Asymmetric Stretch: Skin folds make elephant trunks extensible

Up to one and a half meters long and weighing 130 kilograms: An elephant's trunk is anything but a filigree tool. Nevertheless, the animals can use it in a variety of ways and very precisely. Researchers are finding out exactly how the wrinkled skin of the snout helps.

It's not just the muscles that give an elephant's trunk its amazing mobility, the folded trunk skin also plays an important role. This is the result of a US study reported in the specialist magazine "PNAS". Only the combination of muscles and skin allows the pachyderms to grasp delicate grass and to clear heavy tree trunks out of the way. With their findings, the researchers hope to improve robot technology, which today is usually designed for either great strength or flexibility.

Not only is the trunk one of the most distinctive features of elephants, but it is also a sophisticated multifunctional tool that the animals use to smell, eat, drink and communicate. In addition, the organ, which is up to one and a half meters long and weighs up to 130 kilograms, is used in combat and in reproduction. Around 40,000 transverse and longitudinal muscles give the trunk strength and a wide range of mobility at the same time, although it lacks bones and therefore joints.

Just last year, scientists at the Georgia Institute of Technology also reported that elephants can precisely control the sucking function of their trunks. The same team has now taken a closer look at the skin of elephant trunks: For their new study, the researchers filmed two African elephants (Loxodonta africana) reaching for cubes of bran and apples at the Atlanta Zoo.

"When humans stick out their tongue -- a muscular, boneless tissue similar in structure to an elephant's trunk -- it stretches evenly," lead author Andrew Schulz said in a statement. The team expected elephant trunks to behave in a similar way: "But when we looked at the footage from our high-speed camera and recorded the movements of the trunks, we were surprised." In fact, the skin on the upper side, especially near the tip of the trunk, stretched differently than on the underside.

To better understand this difference in elasticity, Schulz stretched the tissue of a dissected elephant and found that the wrinkled skin on the top of the trunk is 15 percent more flexible than that on the wrinkled underside. The asymmetric elongation is likely due in part to differences in the skin's wrinkle patterns, with the asymmetry showing up when the trunk is stretched more than 10 percent. The results indicated that the stiffness of the elephant skin also affects the trunk extensor mechanism.

"Flexible skin folds are the elephant's innovation," comments co-author David Hu. They would make it easier for the animals to reach down, as is most often done when picking up food or objects. The study also found that elephants extend their trunks telescopically - similar to how the handle of an umbrella is extended. In female and male animals, the trunk can lengthen by 20 and 13 percent, respectively.

The scientists describe the sequence of movements in pachyderms as follows: First, an elephant stretches out the area that encompasses the tip of the trunk, then the adjacent section, and so on, gradually working its way back to its body. According to Schulz, this progressive movement towards the base of the trunk is intentional: "Elephants are like people: they are lazy." The section at the end of the trunk consists of one liter of muscle mass, while the area closest to the mouth contains eleven to 15 liters of muscle. Accordingly, the movement of the tip of the trunk costs less energy.

"An elephant first stretches the end of its trunk, then the adjacent section, because it's easier to move," explains Schulz. "If an elephant doesn't have to work very hard to get something, it won't do it either." The lead author believes that a better understanding of the animals could lead to better protection measures, but as a mechanical engineer, he also sees possible applications of the study results in robotics.

Specifically, Schulz refers to soft robotics, i.e. robots made of flexible materials whose design is biologically inspired. Encasing these in a skin-like structure could give the machines protection and strength while still remaining flexible. Schulz summarizes, "Last year we learned that a trunk is a versatile, muscular hydrostat. Now we know that the skin is another tool at its disposal."

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