Decoding the Sensory Power of Elephant Whiskers

An interdisciplinary German research team, headed by the Max Planck Institute for Intelligent Systems' Haptic Intelligence Department, has discovered the key to the elephant trunk's delicate dexterity. Elephants have a remarkable sense of touch that makes up for their thick skin and poor vision. This is due to the unique material qualities of the 1000 hairs that cover the elephant’s trunk, which emphasize the points along each whisker where contact occurs.

Image credit: Independent birds/Shutterstock.com

Variable Stiffness of a Tactile Hair

In contrast to the uniformly stiff whiskers of rats and mice, the whiskers of domestic cats and elephants have stiff bases that transition to soft, rubber-like tips, according to a recent study titled “Functional gradients facilitate tactile sensing in elephant whiskers” that was published in Science. This stiff-to-soft transition, called a functional gradient, helps keep whiskers from breaking, gives distinctive contact encoding over the length of the whisker, and enables elephants and cats to easily brush past things.

The researchers believe that this peculiar stiffness gradient allows elephants to recognize exactly where touch happens along each of their 1000 trunk whiskers, allowing them to complete tasks such as picking up a tortilla chip without breaking it or accurately catching a peanut. The study team hopes to develop new robotic sensing technologies based on the functional gradients they identified in elephant and cat whiskers.

Schulz and colleagues employed a variety of biological, materials science, and engineering approaches to image and describe 5-cm-long whiskers from elephants and cats down to the length scale of one nanometer, or one billionth of a meter.

I came to Germany as an elephant biomechanics expert who wanted to learn about robotics and sensing. My mentor, Prof. Kuchenbecker, is an expert on haptics and tactile robotics, so a natural bridge was for us to work together on touch sensing through the lens of elephant whiskers.

Andrew K. Schulz, Study Lead Author and Postdoctoral Researcher, Haptic Intelligence Department, Max Planck Institute for Intelligent Systems

Analysis of Natural Properties

The interdisciplinary team studied elephant trunk whiskers to determine their form (geometry), porosity (porosity), and softness (material stiffness). They anticipated elephant whiskers to be similar to mice and rats' tapering whiskers, which have a circular cross-section, are solid throughout, and are about uniform in stiffness.

The researchers used micro-CT to examine the 3D shape of several whiskers, which revealed that elephant whiskers are thick and blade-like, with a flattened cross-section, a hollow base, and multiple lengthy internal channels resembling the structure of sheep horns and horse hooves. This porous construction minimizes the bulk of the whiskers while also providing impact resistance, allowing elephants to eat hundreds of kilos of food every day without fear of damaging their whiskers, which never grow back.

Both elephant and cat whiskers were nanoindented using a diamond cube indenter the size of a single cell, which was pressed into the whisker walls cyclically. Indentation at the base and tip of elephant and cat whiskers revealed a shift from a rigid, plastic-like base to a soft, rubber-like tip that could not be permanently indented, a feature known as resilience. The scientists compared the whiskers to elephant body hair.

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.

Andrew K. Schulz, Study Lead Author and Postdoctoral Researcher, Haptic Intelligence Department, Max Planck Institute for Intelligent Systems

While interesting, this discovery initially puzzled the scientists since they were unsure how changing the stiffness along a whisker would affect touch sensing.

An Imitation Elephant Hair from a 3D Printer

To figure out why, Schulz collaborated with MPI-IS colleagues to 3D print a larger whisker with a rigid, black base and a soft, transparent tip. Having a tangible "whisker wand" prototype helped the researchers build an idea for how an elephant's trunk feels through its whiskers.

After a meeting, Schulz left the wand with his mentor. A few days later, Kuchenbecker walked through the Institute's corridors, holding the wand and softly tapping the columns and railings.

Katherine Kuchenbecker (left) and Andrew Schulz (right) with a 3D-printed replica of an elephant's trunk hair, which helped the research team understand how a transition in material stiffness facilitates contact sensing in the tactile hairs of elephants and cats. Image Credit: MPI-IS/W. Scheible

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.

Katherine J. Kuchenbecker, Haptic Intelligence Department, Max Planck Institute for Intelligent Systems

To verify their hypothesis based on the 3D-printed whisker wand, the researchers used a computational modeling toolset to determine how the unique shape, porosity, and stiffness gradients they detected impact how a whisker responds to touch. The simulations revealed that switching from a rigid base to a soft tip improves the elephant's ability to feel where something is contacting along the whisker, allowing it to react correctly and delicately manage even fragile things like tortilla chips.

“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,” Schulz added.

Interestingly, the same type of stiffness gradient is seen in the whiskers of domestic cats.

Transfer from Nature to Robotics

Schulz and Kuchenbecker, who are attempting to apply these insights from nature to robotics and intelligent systems applications, are excited by this result.

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

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.

Dr. Lena V. Kaufmann, Study Co-Author and Academic Researcher, Humboldt University in Berlin

Kuchenbecker reflected back on the entire project, “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.”

Elephant whiskers exhibit material intelligence for touch sensing. A research team led by the Max Planck Institute for Intelligent Systems discovered that the secret to the elephant’s amazing sense of touch is in its unusual whiskers. Video Credit: MPI-IS/A. Posada