Abstract
The elephant proboscis (trunk), which functions as a muscular hydrostat with a virtually infinite number of degrees of freedom, is a spectacular organ for delicate to heavy object manipulation as well as social and sensory functions. Using high-resolution motion capture and functional morphology analyses, we show here that elephants evolved strategies that reduce the biomechanical complexity of their trunk. Indeed, our behavioral experiments with objects of various shapes, sizes, and weights indicate that (1) complex behaviors emerge from the combination of a finite set of basic movements; (2) curvature, torsion, and strain provide an appropriate kinematic representation, allowing us to extract motion primitives from the trunk trajectories; (3) transport of objects involves the proximal propagation of an inward curvature front initiated at the tip; (4) the trunk can also form pseudo-joints for point-to-point motion; and (5) the trunk tip velocity obeys a power law with its path curvature, similar to human hand drawing movements. We also reveal with unprecedented precision the functional anatomy of the African and Asian elephant trunks using medical imaging and macro-scale serial sectioning, thus drawing strong connections between motion primitives and muscular synergies. Our study is the first combined quantitative analysis of the mechanical performance, kinematic strategies, and functional morphology of the largest animal muscular hydrostat on Earth. It provides data for developing innovative "soft-robotic" manipulators devoid of articulations, replicating the high compliance, flexibility, and strength of the elephant trunk. VIDEO ABSTRACT.
Highlights
The elephant proboscis functions as a muscular hydrostat:[1] the coordinated contractions of antagonist muscles are translated into torsion, bending, elongation, shortening, and stiffening, not via the support of an articulated skeleton, but through shape changes that rely almost entirely on the near-incompressibility of the self-supporting trunk tissues
Basic trunk behaviors and adaptation The elephant trunk exhibits an impressive versatility in its prehension capabilities, which is built upon a finite set of motor strategies, combined to compose complex trajectories
We quantified the prevalence of 17 basic prehension strategies of African elephant trunks in point-to-point transport tasks, reflecting the compositionality of motion as a crucial simplification mechanism for controlling this muscular hydrostat
Summary
The elephant proboscis (trunk) functions as a muscular hydrostat:[1] the coordinated contractions of antagonist muscles are translated into torsion, bending, elongation, shortening, and stiffening, not via the support of an articulated skeleton, but through shape changes that rely almost entirely on the near-incompressibility (constant volume) of the self-supporting trunk tissues. The elephant trunk can perform very delicate tasks such as manipulating a single blade of grass, but it is capable of carrying heavy loads up to 270 kg.[2,3] The proboscis serves elephants in multiple additional functions: breathing, olfaction, mechanosensation, vocalization, posture-based communication, siphoning/spraying water, sprinkling dust, and tool handling.[2,3,4,5,6,7,8,9,10,11,12] In principle, a muscular hydrostat could exhibit a virtually infinite number of degrees of freedom. The composition of trajectories from building blocks (motion primitives) provides a solution to reduce the behavioral landscape complexity in muscular hydrostats.[14,15,16,17,18,19]
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