SOME LOCOMOTORY ADAPTATIONS IN MAMMALS.

Abstract

Summary. The pectoral girdle and fore limb of Equus and Dasypus are compared. It is shown that the shoulder muscles of the horse have a small mechanical advantage (1/13 for the m. teres major) and are therefore adapted to produce rapid movements of the limb; these muscles in the armadillo have a larger mechanical advantage (1/4 for the m. teres) to produce slower movements, while exerting a greater force. The broad scapulae and short legs of fossorial mammals are adaptations producing powerful movements of the foot, usually with strong abductor action. Aquatic mammals show similar modifications, though without the emphasis on abductor movements. The high, narrow scapulae and long legs of cursorial mammals are explained as adaptations to speed, though the detailed arrangements by which this is achieved differ in cursorial ungulates and carnivores. The sharp angles at the ends of the vertebral border of the scapula in cursorial ungulates provide pints of insertion for two parts of the m. serratus, which act as antagonists in the fore-and-aft movement of the shoulder. Similarly in the dog, and probably all cursorial carnivores and lagomorphs, the sharp vertebro-axillary angle affords a point of insertion for the posterior digitations of the m. serratus. The mechanical adaptations of the olecranon process and the m. triceps are described. The structure of the vertebral column and axial musculature is shown to depend upon gait. Two types of gallop, the horse gallop and leaping gallop (e.g. dog) are analysed and the association between gait and structure described. The mechanics of gaits are discussed in an appendix. The extensor muscles from the pelvis to the femur fall into two groups, an iliac group adapted to rapid extension of the thigh, and an ischio-pubic group adapted to slower but more powerful thigh extension. The relative proportions of the two regions of the pelvis vary according to the gaits adopted. In cursorial ungulates the ilium is long and the ischio-pubic region short, whereas in aquatic mammals the latter region predominates. The lengths and cross-sectional areas of the bones of the hind limb are shown to be correlated with gait in aquatic, cursorial (leaping gallop and horse gallop), saltatorial and graviportal mammals.