Brief Communication: Stand and shuffle: when does it make energetic sense?

Many reasons for the emergence of bipedalism have been proposed, including postural arguments which highlight that a sub-optimal form of bipedalism ("shuffling") might have been used by protohominids to cover short distances between resources that require bipedal standing. Bipedal shuffling may have been employed because it avoids the cost of raising the trunk from the quadrupedal orientation, which we assume is the habitual locomotor stance of protohominids. To date, these postural proposals have not been analytically assessed, a lack we rectify herein. Our model seeks to specify a threshold distance, below which bipedal shuffling uses less energy than quadrupedalism. Parameters for the model include the mechanical cost of transport, the ratio of bipedal to quadrupedal cost, and the cost associated with raising the trunk. We found that, using reasonable model parameters, open distances of approximately 9-16 m support the use of bipedal shuffling. Protohominids may have used shuffling as an energetically effective way to traverse between resource patches.