Motor control: Elephant trunks ignore the many and choose a few

In "Motor control: Elephant trunks ignore the many and choose a few”, Scott Hooper (at Ohio University, USA) discusses the importance of the recent study published by the Milinkovitch's Lab on the elephant trunk biomechanics for understanding the complexity of motor control in muscular hydrostats, continuously flexible organs that lack the support of rigid elements such as bones or cartilage.

First, he addresses the redundancy problem, i.e. the fact that any given motor task can be achieved in a very large number of ways in vertebrates, and even more so in muscular hydrostats which display a virtually infinite number of degrees of freedom. Hooper points out how the results of the study offer an answer to this problem: the elephant solves the challenge of motor control by combining a finite set of relatively simple patterns to generate complex movements with its trunk. Such stereotyped movement primitives include the propagation of a local bend originating near the tip of the trunk and moving proximally towards the mouth, and the co-contraction of appropriate muscles generating the functional equivalent of an articulated arm.

Scott Hooper mentions the interesting behavioural similarity of these motor strategies with those found in another muscular hydrostat, the octopus arm, despite the very different neural control (locally-generated in the octopus arm, centralised in the cortex and spinal cord for vertebrates) between both species. In addition, he discusses the large development of brain regions responsible for controlling the face and mouth in some vertebrates, by remarking that many facial muscles resemble muscular hydrostats: they attach to other muscles and dermis rather than to two hard components (bone or cartilage), leading to an increase in the number of degrees of freedom. As a result, some parts of the motor cortex responsible for moving the nose and upper lip are organised, not anatomically (as for muscles directly responsible for rotating joints) but behaviourally, that is, according to motor tasks such as lip-licking or smiling. The author hypothesises that behaviourally-organised regions could be similarly present in the elephant motor cortex, representing the motor primitives that Milinkovitch's team identified in their study. Hence, Scott remarks that the hypertrophy of the elephant cerebellum, a region of the brain responsible for fine motor control, could be linked to the challenges of motor control in muscular hydrostats.

Hooper concludes that the new insights on the elephant trunk motor strategies and anatomy are great steps toward the development of biomimetic soft robots. As a final note, he warns against the temptation of searching for ‘best’ solutions when facing the redundancy problem in biological or robotic movements, reminding us that it is most likely that evolution has achieved not ‘best’, but ‘good enough’ strategies in motor control systems.

Download the article of Scott Hooper (Current Biology Dispatches) here:
Motor control: Elephant trunks ignore the many and choose a few
Scott L. Hooper
Current Biology 31, R1424–R1447 (2021)

Download the original Milinkovitch et al. article here:
Elephants evolved strategies reducing the biomechanical complexity of their trunk
Dagenais P, Hensman S, Haechler V and MC Milinkovitch
Current Biology 31, 4727–4737 (2021)