Semicircular Canal Size Constrains Vestibular Function in Frogs

The vestibular system of vertebrates is a network of fluid-filled chambers and canals within the bone labyrinth of the inner ear that controls balance and spatial direction. It consists of three orthogonal Semicircular canals that sense angular acceleration and otolith organs that detect gravitational and linear acceleration. Endolymph, a fluid with a density similar to water, moves through sensory hair cells in the ampulla, physically deflecting them and causing a nerve impulse to be initiated. This sensory data is sent to the brain, where it is utilised to manage posture and movement by sending motor commands to skeletal muscle.

Large blood arteries, such as the aorta, have high flow rates due to low resistance, whereas tiny capillaries have low flow rates due to high resistance. This allows oxygenated blood to be supplied swiftly to where it is required while still allowing for gas exchange in the capillaries, where resistance causes blood flow to stall significantly. The movement of fluid within the tiniest animals might be hampered by Poiseuille’s law. The size—and perhaps function—of their anatomical systems, such as the size of the Semicircular canals and their sensitivity to changes in angular acceleration during movement, are limited by their tiny bodies.

The size of Semicircular canals in vertebrates is quite limited. Even animals with vastly different body masses have canals of similar size. Baleen whales, for example, have canals that are barely average in size among mammals. Except in the lowest species, where it is restricted by head size, Semicircular canal size is weakly reliant on body mass throughout vertebrates.