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How an elephant's trunk performs like a Swiss Army knife

An elephant’s trunk is capable of making a wide range of different movements to fulfill the requirements of the array of tasks it carries out. Some have even referred to this proboscis as a “Swiss Army knife” because of its functional versatility.

In addition to being used for smelling, eating and drinking, the trunk can curl, twist, lift, grasp, pluck, push, suck, blow and encircle an object. A team of scientists from the University of Geneva (UNIGE),  Switzerland, set out to investigate how elephants control the motion of this most extraordinary appendage. 

For all vertebrate animals, complex motion is possible because their bones are articulated at the joints and provide sites for muscle attachment. By flexing or relaxing their muscles, vertebrates can move their bones in well-defined and predictable ways. But an elephant’s trunk contains no bones and is flexible along its entire length. It is therefore something of a mystery how an elephant manages to use its trunk for tasks as diverse as plucking a single blade of grass and lifting a load of 270 kg (595 lb).

A multidisciplinary team led by Professor Michel Milinkovitch made use of motion-capture technology that is commonly employed in the movie industry, as well as state-of-the art medical imaging, to understand how an elephant deals with the huge variety of possible movements it can make with its trunk.

The results show that elephants use a suite of about 20 simple movements that they then combine in different ways to achieve more complex behavioral functions. In the same way that words are the building blocks of complex sentences, the 20 simple movements are selected and joined together by an elephant depending on the task it needs to perform with its trunk.

“When grasping and securing an object for transport, the trunk exhibits a localized flexion that travels from its tip to its most basal parts, while when the elephant reaches a target in front of it, it extends and retracts specific parts of its trunk in a modular fashion,” explained study co-author Paule Dagenais. 

For example, when an elephant reaches with its trunk for an object to the side, it extends its trunk in rigid segments, bending as though there are joints present in predictable places. These virtual joints give the impression of an elbow or a wrist but have no skeletal origin. 

Changing the characteristics of the object to be retrieved also alters the specific basic movements that are used. When picking up a light, wooden disk, an elephant will use suction to stick the disk to the end of its trunk while lifting it. However, when a heavier, metal disk is retrieved suction is only used initially, to secure the object’s position, while the elephant curls its trunk around the disk and lifts it with that movement.

“In addition, we discovered that how much the trunk slows down when following a curve can be predicted precisely on the basis of the local curvature of that path; remarkably, such a mathematical relation between speed and path curvature also exists for the human hand when drawing,” said Professor Milinkovitch. 

The use of computer tomographic (CT) scans, magnetic resonance imaging (MRI), and sectioning along the length of the trunk enabled the researchers to determine that the muscular anatomy of the trunk is what gives rise to its kinematic versatility. 

The researchers hope to use this new understanding of the biomechanics and physics behind trunk muscle movements to help design soft robotic manipulators that allow gentle and smooth interactions, such as gripping and grasping delicate objects. 

The research is published in the journal Current Biology.

By Alison Bosman, Staff Writer

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