Elephants can carry enormous loads with their trunks, but can also perform incredibly sensitive movements. One of the secrets of this unique all-purpose organ lies in the more than a thousand fine tactile hairs that cover the elephant’s trunk. An interdisciplinary research team has now discovered what makes them special. Accordingly, the tactile hairs of the trunk have three special characteristics: They do not have a round cross-section, but rather a flattened one, which makes them easier to bend. They are traversed by porous channels, which makes them lighter and more stable. And elephant trunk hairs have a decreasing stiffness towards the tip – similar to cats’ whiskers. This makes them particularly sensitive. Together, these properties give the elephant trunk its amazing sensitivity.
The most striking feature of elephants is their movable and versatile trunk – it takes on many of the tasks that our hands and arms do. This muscular extension of the nose and upper lip allows the animals to bend large tree branches and pick up trunks. At the same time, the trunk is sensitive enough to grasp even a peanut without crushing it. No wonder that the brain region that controls the trunk in elephants contains more nerve cells than in any other land mammal. The trunk itself is controlled by more muscles than we humans have in our entire body.
Proboscis whiskers are flattened and porous
Another special feature of the elephant trunk is its tactile hairs: “Around 1,000 of these immobile hairs are located on the thick-skinned upper and lower surfaces of the trunk,” explain Andrew Schulz from the Max Planck Institute for Intelligent Systems in Stuttgart and his colleagues. “They expand the sensory capabilities of the elephant’s trunk and help it to precisely manipulate a wide variety of objects.” The researchers have now examined in more detail how these tactile hairs on the trunk are structured and what makes them so sensitive. To do this, they first analyzed the shape of the trunk hairs using microcomputed tomography. This showed that unlike cat whiskers or rat whiskers, elephant whiskers do not have a circular cross-section but are flattened. This is especially true for the hair near the tip of the trunk. “This makes the whiskers bend more easily in the longitudinal direction of the trunk,” report Schulz and his team. This makes sense because the trunk can stretch and contract significantly lengthwise and the hair can follow this better due to its flat shape.
The second peculiarity was also revealed by microtomography: the elephants’ trunk hairs have a hollow base and are traversed by several hollow channels. “Such channels promote the distribution of energy in keratin structures and reduce weight,” explains the team. Similar tubules can also be found in antlers, the horns of bighorn sheep or in horse hooves. The hollow structures give the tactile hairs on the elephant trunk reduced weight, increased shock resistance and more protection against breakage, as the researchers explain. In the next test, they checked the stiffness and compressive strength of the whiskers. To do this, they pressed the hair in various places near the base in the middle and near the tip of the hair on the side and observed how much the material gave way and how quickly the dent disappeared again. They carried out the same test with the body hair of elephants and the whiskers of cats.
(Video: Max Planck Institute for Intelligent Systems)
Stiff at the bottom, rubbery at the tip
These tests showed that, unlike rats, mice or the body hair of elephants, the trunk hairs are not equally stiff everywhere. Instead, they have a stiff, plastic-like base that transitions into a soft, rubbery tip along the hair. This stiffness prevents the tactile hairs from breaking and ensures unique contact coding along the entire length of the hair. “This is pretty amazing! The stiffness gradient provides a map that allows elephants to see where along each whisker contact occurs,” says Schulz. “This characteristic helps them detect how close or how far their trunk is from an object.” This is the only way elephants can pick up a thin tortilla chip with their trunk without breaking it, or grab a tiny peanut. The whiskers of domestic cats also show such a stiffness gradient.
Taken together, these results reveal what makes elephant trunks so sensitive. “All of this is contained in the geometry, porosity and stiffness of hair. Engineers refer to this natural phenomenon as embodied intelligence,” says Schulz. This form of material intelligence is more sophisticated than that of the well-researched whiskers of rats and mice. “Our results contribute to our understanding of the tactile perception of these fascinating animals and open up exciting opportunities to further investigate the relationship between the material properties of the tactile hairs and the neuronal information processing in the animals’ brains,” adds co-author Lena Kaufmann from the Humboldt University of Berlin. However, the new findings could also be used to optimize technical sensors. “Bio-inspired sensors with artificial stiffness gradients similar to proboscis hairs could provide precise information with little computational effort through intelligent material design alone,” says Schulz.
Source: Andrew Schulz (Max Planck Institute for Intelligent Systems, Stuttgart) et al., Science, doi: 10.1126/science.adx8981