From four legs to two

Abstract

An animal can only stand on an Inclined surface if a vertical line through the centre of gravity passes through the quadrilateral formed by its feet. In practice animals adopt one of two characteristic postures to keep the line of the centre of gravity within this boundary. Either the axes of the limbs are retracted while that of the body is parallel to the slope, or the axis of the body is held horizontal by flexure of either the front or hindlimbs. The ancestors of primates were small tree-shrew-like animals with long bodies and short legs so the centre of gravity fell within the pedal quadrilateral on all but the steepest slopes. Although the gravitational effect on their light bodies was of lesser importance than that on larger animals, the strain on the central part of the back and belly was relieved by loading the body weight on balanced cantilevers. The shape of a tree-shrew conforms well to Gray’s optimal tetrapod form (Figure 4.1).