Secret to an elephant’s ‘supercharged’ sense revealed

By Stephen Beech

The whiskers on an elephant's trunk are key to its "amazing" sense of touch, reveals new research.

The 1,000 hairs that cover the trunk have unusual properties that highlight where contact happens along each whisker - allowing it to grab a peanut, say scientists.

The whiskers provide elephants with a supercharged sense of touch that compensates for their thick skin and poor eyesight, according to their findings.

The study, published in the journal Science, showed that the whiskers of elephants and domestic cats have stiff bases that transition to soft rubber-like tips, different from the uniformly stiff whiskers of rats and mice.

Known as a functional gradient, researchers found that the stiff-to-soft transition allows elephants and cats to brush past objects with ease, helps prevent whisker breakage, and provides unique contact encoding along the whisker’s length.

The German research team believe that the unusual stiffness gradient helps elephants know precisely where contact occurs along each of their 1,000 trunk whiskers so they can perform feats such as picking up a tortilla chip without breaking it or precisely grabbing a peanut.

Now the researchers are looking to invent new robotic sensing technologies inspired by the functional gradients they discovered in elephant and cat whiskers.

Study lead author Dr. Andrew Schulz and his colleagues used biological, materials science, and engineering techniques to image and characterise five centimetre-long whiskers from elephants and cats down to the length scale of one nanometre, which is one billionth of a metre.

The team examined elephant trunk whiskers to understand how they are shaped, how porous they are, and how soft they are.

The researchers initially expected elephant whiskers to be similar to the tapered whiskers of mice and rats, which have a circular cross-section, are solid throughout, and have approximately uniform stiffness.

Micro-CT scanning allowed the researchers to measure the 3D shape of several whiskers and showed that elephant whiskers are thick and blade-like, with a flattened cross-section, a hollow base, and several long internal channels that resemble the structure of sheep horns and horse hooves.

The researchers explained that the porous architecture reduces the whisker’s mass and provides impact resistance, allowing elephants to eat hundreds of kilos of food every day without worrying about damage to their whiskers, which never grow back.

Nanoindentation of both elephant and cat whiskers was performed with a diamond cube indenter the size of a single cell that cyclically pushed into the whisker walls.

Indentation performed at the base and the tip of the elephant and cat whiskers showed a transition from a stiff, plastic-like base to a soft, rubber-like tip that could not be permanently indented, a property known as resilience.

The researchers also compared these whiskers to elephant body hair.

Dr. Schulz said: “The hairs on the head, body, and tail of Asian elephants are stiff from base to tip, which is what we were expecting when we found the surprising stiffness gradient of elephant trunk whiskers.”

The discovery initially stumped the team as they were not sure how changing stiffness along a whisker would affect touch sensing.

To try to figure out why, Dr Schulz worked with colleagues at the Max Planck Institute for Intelligent Systems (MPI-IS) in Stuttgart to 3D print a scaled-up whisker with a stiff, dark base and a soft, transparent tip.

Having this physical “whisker wand” prototype helped the research team develop their intuition for what an elephant trunk feels through its whiskers.

Dr. Schulz left the wand with his mentor Professor Katherine Kuchenbecker after a meeting.

Prof Kuchenbecker carried the wand in her hand as she walked through the halls of the Institute, gently hitting the columns and railings.

She said: “I noticed that tapping the railing with different parts of the whisker wand felt distinct – soft and gentle at the tip, and sharp and strong at the base.

"I didn’t need to look to know where the contact was happening; I could just feel it.”

To test their hypothesis from the 3D-printed whisker wand, the research team developed a computational modelling toolkit to assess how the unique geometry, porosity, and stiffness gradients they had measured affect how a whisker responds to contact.

The simulations showed that the transition from a stiff base to a soft tip does indeed make it easier to feel where something is touching along the whisker, allowing the elephant to react appropriately and carefully manipulate even delicate objects, such as tortilla chips.

Dr. Schulz said: “It's pretty amazing.

"The stiffness gradient provides a map to allow elephants to detect where contact occurs along each whisker.

"This property helps them know how close or how far their trunk is from an object…all baked into the geometry, porosity, and stiffness of the whisker.

"Engineers call this natural phenomenon embodied intelligence.”

He says that domestic cat whiskers show the same kind of stiffness gradient.

The discovery has excited the research team who are working to apply the insights from nature to applications in robotics and intelligent systems.

Dr. Schulz said: “Bio-inspired sensors that have an artificial elephant-like stiffness gradient could give precise information with little computational cost purely by intelligent material design,."

Study co-author Dr. Lena Kaufmann, a neuroscience expert at the Humboldt University of Berlin, said: “Our findings contribute to our understanding of the tactile perception of these fascinating animals and open up exciting opportunities to further study the relation of whisker material properties and neuronal computation.”

Prof Kuchenbecker added: “I’m so proud of what we were able to figure out by working together across disciplines.

"Andrew pulled together an amazing team of engineers, materials scientists, and neuroscientists from five different research groups and led us on an exhilarating three-year-long journey to discover the secrets behind the powerful elephant’s gentle sense of touch.”