Amazing stuff!
"... The 1,000 whiskers that cover an elephant's trunk have unusual material properties that highlight where contact happens along each whisker, giving elephants an amazing sense of touch that compensates for their thick skin and poor eyesight. ..."
"To the point
- Sense of touch despite thick elephant skin: Researchers have discovered that the hairs on elephants' trunks are responsible for their extraordinary sense of touch.
- Special material properties: Elephant sensory hairs have a stiff base and a soft tip, which enables them to precisely feel objects and recognize where contact is made. These properties are similar to the whiskers of cats and differ from the completely stiff sensory hairs of rats and mice. ...
- Applications in robotics: The findings will be used in the development of robot-assisted sensor technologies that mimic the stiffness gradient of elephant tactile hairs.
..."
From the editor's summary and abstract:
"Editor’s summary
Mammals such as cats and rats use whiskers to help sense their environment. In rats, the short whiskers and long whiskers resonate at different frequencies, helping rats map out their surroundings as the keratin-based fibers contact the edges and surfaces of nearby objects.
Elephants also have whiskers, which line the length of their trunks. Schulz et al. used micro–computed tomography imaging, electron microscopy, mechanical testing, and finite element analysis to map out the structure and properties of these whiskers.
At the base of the trunk, the whiskers are thick, circular, porous, and stiff, but they progress toward being thin, ovular, dense, and soft toward the tip, which contrasts with whiskers found in most other mammals. This combination of structure and form helps magnify the signals transmitted to the trunk. ...
Structured Abstract
INTRODUCTION
Animals have evolved a diverse array of sensing systems that help them traverse complex terrain, locate food, and detect predators. Many terrestrial and aquatic mammal species use specialized sensory hairs, known as whiskers, as active tactile sensory organs to monitor their environment. A follicle surrounds each whisker’s base with mechanoreceptors that respond to physical whisker stimulation and thereby extend the animal’s sense of touch. Most research focuses on how the geometry and/or neuromechanics of the whisker-follicle structure affect tactile sensitivity. This study analyzes variations in intrinsic whisker properties, including how porosity and stiffness change along the whisker.
RATIONALE
The boneless elephant trunk is covered with about 1000 whiskers that expand the sensory volume of this highly dexterous appendage. These whiskers do not possess the innervated local muscles that allow the characteristic “whisking” behavior commonly seen in rats and mice, and they cannot regrow, so we hypothesized they may also differ in other fundamental ways. This study applies several precise measurement approaches to characterize the geometry, porosity, and stiffness of Asian elephant (Elephas maximus) whiskers and uses mechanical simulation to show how the captured characteristics may affect trunk touch.
RESULTS
We measured the geometry, porosity, and material stiffness from the base to the tip of elephant whiskers and entered these properties into our open-source, customizable finite element model that allows whisker properties to vary longitudinally. This simulation was then used to compare elephant whiskers with rat whiskers, which exhibit uniform material stiffness along their length. By contrast, elephant whiskers showcase three independent functional gradients. The geometry of elephant whiskers shows a tapered ovular cross section, facilitating bending as the trunk extends between obstacles. Elephant whisker porosity is characterized by a network of hollow tubules in the inner cortex; this horn-like microstructure at the base merges into a dense whisker tip.
A porous base provides functional benefits of mass reduction and impact resistance, similar to the horns of bighorn sheep.
Our stiffness analysis shows that elephant whiskers transition from a stiff base (modulus of elasticity = 2.99 GPa) to a soft, resilient tip (0.0706 GPa), a shift of two orders of magnitude, although elephant body hair has approximately constant stiffness from base (2.20 GPa) to tip (1.15 GPa). The stiffness gradient of elephant whiskers provides two key benefits over homogenous whiskers: reduction of base stress during large deflection and amplification of signal differences along the whisker length, strengthening the encoding of contact location.
CONCLUSION
The geometry, porosity, and stiffness gradients of Asian elephant whiskers seem tuned to augment tactile sensing. Their tapered ovular geometry increases interaction with textures and allows preferred bending directions; the shift from a porous base to a dense tip reduces mass, increasing the whisker’s resonant frequency and reducing breakage; and the transition from a stiff base to a soft tip increases tip deflection and facilitates contact encoding along the whisker. The physical intelligence of these three functional gradients found together in elephant whiskers expands our understanding of touch and could inspire new approaches in artificial tactile sensing."
Elephant trunk whiskers exhibit material intelligence (original news release)
Functional gradients facilitate tactile sensing in elephant whiskers (no public access)
Functionally graded keratin facilitates tactile sensing in elephant whiskers (preprint, open access)
... 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.
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