1Hominid Paleobiology Doctoral Program, Department of Anthropology, The George Washington University, 2Center for the Advanced Study of Hominid Paleobiology, Department of Anthropology, The George Washington University, 3Human Origins Program, National Museum of Natural History, Smithsonian Institution, 4Center for Functional Anatomy and Evolution, Johns Hopkins University School of Medicine
Friday All day, Plaza Level
Skeletal design among cursorial animals is a compromise between a stable body that can withstand locomotor stress and a light design that is energetically inexpensive to grow, maintain and move. It has been hypothesized that to maintain a balance between safety and energetic cost, cursors have reduced distal musculoskeletal mass. This is due to an exponential increase in energetic demand during oscillation of the distal limb. Experimental research shows that cortical bone in distal limbs experiences higher strains and remodeling rates, apparently maintaining lower mass at the expense of a smaller safety factor. Here, we test the hypothesis that cursors have lower trabecular density in distal relative to proximal limb joints, in order to minimize energetic cost. We use pQCT scanning to measure trabecular density in the lower and upper limbs of humans, chimpanzees, cheetahs and mountain lions.
Our results show that cheetahs exhibit significantly denser trabecular bone in femoral heads than in third metatarsal heads. This result supports the hypothesis that there is proximo-distal reduction in trabecular density among cursorial sprinters. However, chimpanzees (non-cursors) exhibit a similar pattern with denser femoral heads than MT3 heads. Thirdly, contrary to the proximo-distal reduction hypothesis, the upper limbs of cursors (cheetahs-sprinters, mountain lions-distance travellers) and non-cursors (chimpanzees), as well as humans, all exhibit denser third metacarpal heads than humeral heads. These results suggest an overall trend with an increase in trabecular density in the distal elements of the upper limb in both cursors and non-cursors.