Muscular hydrostats such as octopi’s arms and the elephant trunk have received much attention because of their dexterity in grabbing a wide range of objects. These organs have inspired soft-robotic manipulators and actuators with various applications, from working with humans in confined spaces and disaster relief to picking up fruit. For soft robots to apply large forces, they will need protective skin. This work shows that wrinkles and folds in skin can provide protection and flexibility to soft appendages. We hope this work will inspire future work in the mechanics of skin in both animals and robots.
In 1985 Bill Kier coined the term muscular hydrostats. Describing appendages that are purely muscular that are capable of elongation, shortening, bending, and torsion all while maintaining conservation of volume. However, the elephant trunk is unique as it has the two nasal passageways which allows the trunk to breathe and use fluid to manipulate objects.
In 2020 we performed elongation experiments at Zoo Atlanta with two elephants, Kelly and Msholo to understand how far elephants can strengthen their trunks while still maintaining strength in their skin.
Movie 1. Video abstract! Preliminary tests with Msholo, a 30-year-old male African bush elephant stretching to grab bran cubes on a plastic bin 3 meters away. From this video we see elephants can stretch up to 15-20% of their hanging length!
Elephants accomplish these large feats of elongation by extending their trunk towards food in high trees or low on the ground and grabbing using their prehensile fingers at the tip of the trunk. African elephants have two fingers on the tip of the trunk while Asian elephants only have one (Photo above by A. Thompson, Zoo Atlanta).
Movie 2. Using a technique known as Digital Image Correlation (DIC) we can track individual points of the trunk as it moves stretching to observe which sections are stretching the most.
Figure 1. Using the DIC automated tracking technique we see the elephant is starting to stretch with the tip of their trunk, followed by each segment until the base moves, behaving similarly to the chameleon tongue in telescoping to reach far objects.
Figure 2. Separating the trunk further by looking at the top folded portion of the trunk (Dorsal) and the bottom portion (Ventral) we see the dorsal portion is experiencing far greater stress, but why is that the case?
Figure 3. If we look to the muscle of the trunk there are several different muscles including oblique, radial, and longitudinal muscles that control the motion of the trunk.
Figure 4. Comparing this muscle to other muscles we see the elephant has varying muscle groups throughout their trunk with the base of their trunk primarily being for support and the tip for flexibility of motions.
Figure 5. Amazingly, skin is the answer. Elephants utilize the folders on the top of their trunk to help them stretch, while the wrinkles on the bottom of the trunk are likely only for added grip when manipulating objects, and not to help them stretch to further distances.
Video 3. The trials were all performed at the Zoo with a black tarp in the background for help tracking, and video at several hundred frames per second. Sped up elephants look to be stretching quickly, but its not all fun in games doing elephant biomechanics research.
Video 4. Video of preliminary trials with Msholo showing he doesn't like when the tarp was resisting him during elongation trails in his habitat at Zoo Atlanta.