Aerial maneuvers of leaping lemurs: The physics of whole-body rotations while airborne

Several prosimian species begin a leap from a vertical support with their back toward the landing target. To reorient themselves from this dorsally facing, head-first lift-off to a ventrally facing, feet-first landing, the animals combine an initial twist with a partial backward somersault. Cinefilms of a captive colony of ringtailed lemurs (Lemur catta) revealed that during leaps from vertical poles to horizontal supports, the backward somersaulting rotations were often initiated while the animals were airborne. How could these prosimians initiate rotations in the absence of externally applied forces without violating angular momentum conservation? The problem was approached through vector analysis to demonstrate angular momentum (H) changes about the three principal (symmetrical) axes of rotation for a series of critical body positions that were extracted from the filmed sequences. One L. catta specimen was segmented to provide the dimensions and weights necessary for modeling the various body positions. These data were also used to calculate moments of inertia about the three principal axes in order to predict if rotations about these axes were stable or metastable. Lemurs, like any projectile, must conserve the total angular momentum (H_T ) established at lift-off. H_T , however, is a vector quantity that is the resultant of component vectors about the three principal axes. Thus, H about the individual axes may change as long as H_T remains constant. Strategically timed tail movements tilted the body, thereby changing the H value about the head-to-toe (twisting) axis. To conserve H_T , also aligned along the twisting axis, angular momentum transferred to the somersaulting axis. Owing to the direction of tail-throw, the initiated rotations were partial backward somersaults that brought the hindlimbs forward for landing. This strategy for initiating specific rotations parallels that practiced by human springboard divers.