A terrain treadmill to study animal locomotion through large obstacles.

A challenge to understanding locomotion in complex three-dimensional terrain with large obstacles is to create tools for controlled, systematic experiments. Recent terrain arenas allow observations at small spatiotemporal scales (∼10 body lengths or cycles). Here, we created a terrain treadmill to enable high-resolution observation of animal locomotion through large obstacles over large spatiotemporal scales. An animal moves through modular obstacles on an inner sphere, while a rigidly attached, concentric, transparent outer sphere rotates with the opposite velocity via closed-loop feedback to keep the animal on top. During sustained locomotion, a discoid cockroach moved through pillar obstacles for up to 25 min (2500 cycles) over 67 m (1500 body lengths). Over 12 trials totaling ∼1 h, the animal was maintained within a radius of 1 body length (4.5 cm) on top of the sphere 90% of the time. The high-resolution observation enables the study of diverse locomotor behaviors and quantification of animal-obstacle interaction.