Posture and support geometry, rather than body size, dictate lateral dynamic stability in walking mammalian quadrupeds.

Body size and limb posture vary widely across mammals and are expected to shape locomotor stability, yet direct comparative evidence remains limited. Here, we tested whether smaller, crouched mammals exhibit greater lateral dynamic stability than larger, more upright species by comparing treadmill walking in mice and cats at dynamically similar speeds. Using kinematic analyses and size normalized measures of stability, we show that mice are substantially more laterally stable than cats. This increased stability is associated with relatively wider step widths and more crouched limb posture, indicating that support geometry and posture play dominant roles in stabilizing locomotion. Despite these differences, both species regulate lateral balance on a step-by-step basis, as revealed by relationships between center of mass motion and subsequent adjustments of the border of support. Our findings demonstrate that locomotor stability does not scale simply with body size but depends critically on posture dependent strategies that differ across species. These results identify lateral stability as a key factor of locomotor adaptation and suggest that crouched postures in small mammals may reduce reliance on active neural control while enhancing robustness in complex environments.