Biomechanical determinants of reduction of the second ray in Lorisinae

Abstract

Members of the subfamily Lorisinae are characterized by a reduced second ray. When walking on branches, lorises grip the support with the second ray pointing in the direction of movement. This does not contribute to the grip of either the hand or the foot. The finger is hyperextended at the metacarpophalangeal joint, flexed at the proximal interphalangeal joint, and hyperextended at the distal interphalangeal joint. As a result, the palmar (volar) pad proximal to the second finger and the distal pad of the digit are pressed against the underlying support. The toe is sometimes placed on top of the third digit. The morphological and behavioral features have a mechanical basis. The orientation of the second ray in combination with the great stride length characteristic of these animals generates high stress in bones and tendons. Its reduction in length as well as its flexed posture during locomotion are means to avoid over stressing. Moments in the basal joints of fingers and metacarpals that must be counterbalanced by muscle action are also reduced. In the larger-bodied lorisine species bending stresses are higher than in the smaller ones: the reduction of the second ray is therefore more pronounced.