PORTOCARRERO, R.J.; WUNDERLICH, R.E.*; James Madison University; James Madison University: Arm and tail use during terrestrial bipedalism in Propithecus verreauxi

Leaping primates use arm and tail movements to assist the overall acceleration of the body and to initiate rotations. The extent to which primates use the arms or tail is dependent on body size. Propithecus is a vertical clinger and leaper that uses arm and some tail movements during the aerial phase of leaping. When Propithecus comes to the ground it uses a bipedal gallop in which the trunk is oriented obliquely to the path of movement. One limb leads for 5-7 strides, and then they turn to switch leading limbs. We examined sagittal and frontal plane kinematics of Propithecus arm and tail movement during terrestrial bipedalism with respect to a theoretical model to assess the contribution of the arms and tail to balance, propulsion, and rotation. Propithecus extends the tail at touch down and flexes it during take-off. Mediolateral movements are most prominent during lead limb switching. The contribution of the tail to take-off acceleration, landing deceleration, and rotations during switching is more prominent at higher speeds. The lead arm behaves differently than the trail arm. The trail arm is abducted and extended during the first half of stance, and it externally rotates when the lead hindlimb touches down. The arm remains extended throughout the aerial phase until it flexes slightly at touchdown. The lead arm is flexed during early stance, it extends, abducts and externally rotates during the second half of stance and the aerial phase, and it flexes during touchdown to slow the acceleration of the body into the ground. Whereas the lead arm is contributing to acceleration and deceleration during take-off and landing and assisting with aerial rotations, the trail limb is used for balance. Arm and tail use vary with speed and with age in accordance with ontogenetic variations in bipedal kinematics.