Rib Cage Flaring

Humans and apes share in having a transversely broad and dorsoventrally flattened rib cage, which contrasts with the transversely narrow and dorsoventrally deep thorax of monkeys. Hominoids are also characterized by a more ventral position of the vertebral column (such that it lies more within the thorax), which again contrasts with the condition seen in monkeys (in which the vertebral column is on the dorsal aspect of the thorax). However, humans and other great apes differ in the relative mediolateral dimensions of the upper and lower ribs, which leads to a difference in the overall contour of the rib cage (barrel-shaped in humans, conical or funnel-shaped in the great apes, as seen in anterior view). These differences in thoracic shape are largely a function of differences in the size of the superior-most and inferior-most ribs in the series, but also come about from differences in the curvature and torsion of the bodies of all of the ribs. The overall shape of the hominoid thorax may be an adaptation that serves to better dissipate tensile forces in the body during under-branch suspensory behaviors.