Crouched posture and high fulcrum, a principle in the locomotion of small mammals: The example of the rock hyrax (Procavia capensis) (Mammalia: Hyracoidea)

Abstract

Abstract The cineradiographic study of the locomotion of the rock hyrax (Procavia capensis) and the functional interpretation of its locomotory system, reveals that the main action of proximal segments is combined with flexed position and low movements of limb joints. This observation can be applied to the locomotion of other small mammals. In the forelimb, scapular rotation and translation account for more than 60% of step length. Effective shoulder joint movements are mostly restricted to less than 20°, and elbow movements range mainly between 20°-50°. The detachment of the shoulder girdle of therian mammals from the axial skeleton, and development of a supraspinous fossa, are correlated with movements at a high scapular fulcrum. Movements at such a high fulcrum are in interdependency with a crouched posture. Only flexed limbs can act as shock absorbers and prevent vertical changes in the center of gravity. Basic differences in forelimb movements exist between larger primates (humeral retraction) and smaller mammals (scapula retraction). In the hyrax, propulsion is due mainly to hip joint movements in symmetrical gaits, but sagittal lumbar spine movements play the dominant role at in-phase gaits. Joint and muscular anatomy, especially of the shoulder region, are discussed in view of the kinematic data.