Postural control in an upright snake.

Abstract

Posture and its control are fundamental aspects of animal behaviour that capture the complex interplay between sensorimotor activity that is driven by muscular forces, and environmental feedback that is mediated by proprioception and active control. An extreme example of this is seen in brown tree snakes and juvenile pythons: they can stand almost upright, with 70% of their body length in the air. We quantify experimental observations of this behaviour and present a minimal theoretical framework for postural stability by modelling the snake as an active elastic filament whose shape is controlled by muscular forces. We explore two approaches to characterize the musculature needed to achieve a specific posture: proprioceptive feedback (whereby the snake senses and reacts to its own shape) and a control-theoretic optimization approach (whereby the snake minimizes the expended energy to stand up). Then we also analyse the dynamic stability of the snake in its upright pose. Our results lead to a three-dimensional postural stability diagram in terms of muscle actuation and strength, and gravity, consistent with experimental observations. In addition to general predictions about posture control in animals, our study suggests design principles for robotic mimics.