How snakes rise upright without limbs, according to researchers

A new study in the Royal Society journal Interface examines how snakes can stand upright and overcome gravity without legs. Led by biophysicist L. Mahadevan of Harvard University, the researchers argue that a snake’s main task is not simply rising from the ground, but maintaining stability while it does so.

The team tested two climbing species, the brown tree snake (Boiga irregularis) and a young bush python (Simalia amethistina), both averaging about one meter long. In experiments, the snakes were placed on a low shelf and encouraged to reach toward a higher shelf that was gradually raised from 30 to 80 centimeters. Both species succeeded, with much of their bodies suspended in the air by muscular force alone. The main muscles involved were the spinal and semispinal muscles along the back.

The researchers modeled the snake as an “active elastic filament” and compared two possible control strategies. One is local proprioceptive feedback, in which the snake adjusts muscle activity based on how curved each body segment is. The other is optimal control, a more integrated strategy in which the snake manages its body as a whole to minimize energy use and maximize stability. The authors say the second method is more energy efficient and may be the one evolution favors, though they note more biological work is needed to know whether snakes use one method, both, or additional mechanisms.

One surprising finding was that the higher a snake rises, the less muscular effort it needs. The animal appears to use gravity to help hold its posture, making small, slow motions similar to balancing an inverted pendulum or a broomstick on a fingertip. In both approaches, the snake forms a characteristic S-shape near the base of its body, where most of the effort is concentrated in what the researchers call a boundary layer. Above that point, once the body is already upright, less force is needed. The study may help engineers design soft robots and medical devices such as endoscopes that can move more steadily inside the human body.