Weird Science | The long and short of an elephant’s trunk

It’s one of the most versatile organs among all mammals exhibiting a rare combination of immense strength and dexterity and holds great inspiration for robotics

The elephant’s trunk is an all-in-one tool that allows it to perform a range of activities from eating and drinking to bathing and uprooting trees. The same trunk can lift hundreds of kilos and also remove a peanut from its shell, a rare combination of strength and dexterity that robotics has long been trying to achieve.

An elephant trunk, folded on the upper side and wrinkled on the underside, contains 120,000 muscles(Georgia Tech)

The trunk’s morphology has been studied for centuries. Research in the biomechanics of its operations, however, has picked up only in recent years. A series of studies has examined what makes the trunk such an efficient tool — and a role model for robots.

When the trunk is elongate to longer distances, the upper side of the trunk stretches more than the underside(Illustration by Ben Seleb, provided by Andrew Schulz, Georgia Tech)

The multitasker

The trunk is a complex structure of tens of thousands of muscles and nerves, covered in skin that has a series of folds on the upper side and is more wrinkled on the underside. At the tip are “fingers” — Asian elephants have one of these; African elephants have two — which enable the elephant to pick up tiny objects from the ground, and examine them before deciding what to do with them (like eat or discard).

Researcher Robert Harrison was clearly impressed with he described the elephant’s trunk in the Proceedings of the Royal Irish Academy in 1844: “From the anatomical examination of the complex structure of this very curious appendix, we can understand the powers it displays, and the purposes it effects in the economy of the animal to which it appertains. With it he can strike down an animal, or can raise it into the air and dash it to the earth; or he can bend it around its body or its neck, and crush it by powerful compression. It can tear down large branches of trees, and raise and propel great weights; hence the great value of the elephant in warfare, and in marshy countries, as a beast of burden, for the transport of heavy guns and cumbrous baggage.”

The trunk is also the elephant’s all-in-one organ for smell, touch, and respiration. It can go underwater, keeping only the tip of the trunk over the surface, so it can breathe.

An elephant uses its trunk to jams (a) 32 mm cubes and (b) 16 mm cubes, forming a joint for the smaller items. (David Hu et al, Journal of the Royal Society Interface)

Many thousand muscles

In his 1844 paper, Harrison described attempts to count the number of muscles in the trunk as “useless”. Indeed, the estimate has varied: the Smithsonian Institute gives a count of 150,000 muscles for the Asian elephant’s trunk, while the World Wildlife Fund puts it at 40,000 muscles, without specifying the kind of elephant.

Different studies may have used different criteria for grouping muscle fascicles (bundles of fibre), explained Lena Kaufmann, a PhD student at the Bernstein Center for Computational Neuroscience (BCCN) in Berlin. She is part of a lab led by Dr Michael Brecht at BCCN, which recently put the number of muscle fascicles in the African elephant’s trunk at 120,000, made of eight types.

“The number of muscle fascicles colleagues at my lab found is around 120,000. What might have contributed to differences in numbers reported before is that multiple fascicles might have been grouped together and counted as one muscle… and criteria for grouping or borders between groups seem to be unclear. So, the most reliable thing to do is counting the clearly distinguishable fascicles,” Kaufmann said in an email.

The same team published a paper in Current Biology last year, describing the network of nerves in the trunk and parts of the face; there are two sets of 400,000 nerves each. The sensory nerve (trigeminal nerve) in the trunk is thicker than the spinal cord of the elephant, Kaufmann said.

This month, the team published a paper in Nature describing a feature that is less widely studied: the whiskers on the elephant’s trunk. Other mammals such as rodents tap their whiskers against objects to get a sense of what it is; this is called whisking. Elephants do not whisk but, Kaufmann said, their whiskers transmit tactile stimulation, or signals via the nerves to the brain.

The trunk uses suction to gather tortilla chips(Provided by Andrew Schulz, Georgia Tech)

The physics behind the movement

At the Georgia Institute of Technology, another team has been working on the biomechanics of the elephant trunk, and has published a series of studies over the past five years.

One study, published in the journal PNAS last year, examined the role that skin plays when an elephant stretches its trunk. The tip extends first, then a section immediately next to it, and so on, quite like a telescope.

“For objects that are very far away, the elephant stretches to much higher amounts, extending their trunk up to 25%. When this occurs, the stiffness difference in the skin starts to change the motion. The top of the trunk stretches using its folds, but the bottom of the trunk does not stretch as much with its wrinkles,” postdoctoral researcher Andrew Schulz, who led the study, said over email.

In 2021, the same team observed how elephants consume food and liquid, and conducted an ultrasonic probe to analyse the physics at play. According to that study, which appeared in the Journal of the Royal Society Interface, the elephant dilates its nostrils to create more space in its trunk; this allows the trunk to store up to 5.5 litres of water. The team estimated that an elephant inhales at speeds comparable to Japan’s bullet trains.

To eat, the elephant uses its trunk finger(s) to grab and collect large items. But it also uses suction for smaller ones. “Elephants utilise suction to grab fragile items, such as a tortilla chip. When grabbing a large rutabaga cube, the African elephant utilises the two fingers at the tip of the trunk to pincer the large cube up to their trunk,” Schulz said. “As the items get smaller, the elephant vacuums the items into their trunk using its large lungs. One of the primary reasons for this is, as the cubes get smaller, suction is easier.”

In the same journal in 2018, Georgia Tech Professor David Hu had shown how an elephant uses its trunk to assemble food items and crush them using muscle strength — and the weight of the trunk. To do this, it forms a joint in the trunk; the portion below it serves as a soft pillar. The study found that the elephant can raise or lower the location of the joint according to the force required: the higher it is, the more force that can be applied.

What robots can learn

These studies from the Georgia Tech team were supported by the US Army Research Laboratory, and each can have implications for robotics.

For example, the team suggested that future robots could be built with several joints, which could apply various weight pressures, as required, to help compress objects together for lifting them efficiently.

It remains to be seen if and when roboticists work on these suggestions. What is clear, however, is that the findings relate directly to some of the challenges that robot developers have been trying to meet.

Given that grasping is a difficult task for a robotic hand to achieve, the Georgia Tech team has suggested that studying the combinations of suction and grasping in the elephant trunk could help find new ways to build robots.

Again, in their quest to build more efficient robots, researchers have often tried to build materials matching the properties of human skin. Based on the findings on the role that skin plays when an elephant stretches its trunk, the Georgia Tech team has suggested that wrapping soft robotics with a skin-like structure could achieve the rare combination of strength and flexibility.

Kabir Firaque is the puzzles editor of Hindustan Times. His column, Weird Science, tackles a range of subjects from the history of inventions and discoveries to science that sounds fictional, but isn't.