Locomotion and Appendicular Anatomy in Three Soricoid Insectivores

Abstract

The shrews (Soricidae) and moles (Talpidae) comprise the Soricoidea (Insectivora: Mammalia); the moles have evolved from the shrews and differ fundamentally by having acquired a joint between the lateral end of the clavicle and the greater tuberosity of the humerus. This brace of humerus against clavicle allows the moles to burrow by lateral thrusts of the forelimb, whereas other mammals that burrow do so by antero-posterior scratching movements. The following genera, as representative stages in soricoid differentiation, were picked for analysis of behavior and anatomy: Sorex, a small, non-fossorial, semi-cursorial, generalized shrew; Neurotrichus, a primitive, semi-fossorial, semi-cursorial mole; and Scapanus, a specialized fossorial mole. Detailed dissections were made of the limbs; dimensions of each element of the appendicular skeleton and of the major parts of the axial skeleton were analyzed statistically with reference to body length; and the muscles of the forelimb of Scapanus and Neurotrichus were measured volumetrically. Correlation of these data with observed behavior of living animals has revealed the manner and degree of the locomotor adaptations in each genus. Locomotion in Sorex is of a generalized type; the limbs are delicate, are held beneath the body, and move in a fore-and-aft plane; the appendicular skeleton is of an unspecialized monodelphian pattern and the feet are digitigrade. Important fossorial adaptations in moles are as follows: the pectoral girdle has migrated anteriorly, the manubrium has lengthened, the clavicle has shortened and thickened, the humerus projects laterally (Neurotrichus) or dorsolaterally (Scapanus), the shoulder-joint between scapula and humerus has been retained but a new joint between clavicle and humerus has evolved and become specialized, the humerus is broadened, the shafts of the radius and ulna have undergone a 90[degree] supination due to torsion with the result that the plane of the manus coincides with the plane of forearm extension, a flexor check-ligament has developed between humerus and digits, the wrist has become a hinge-joint, and the manus has enlarged into a digging tool. In all fossorial characters Scapanus is more specialized than the more primitive Neurotrichus. The latter has retained a long antibrachium for rapid running and can depress the humerus, placing the palm upon the ground. Scapanus can place only the medial edge of the manus upon the ground and is awkward out of its burrow. Rotation of the true medial side of the talpid humerus posteriorly by the mm. teres major, latissimus dorsi, sub-scapularis, and pectoralis posticus, all greatly hypertrophied, swings the forearm and hand posteriorly through an arc. This rotary movement of the humerus is the basic talpid locomotor action, both for digging and running. The parts of the pelvic girdle of Scapanus are more firmly ankylosed, the hind limb is stouter, and the tibia, fibula, and pes are shorter and broader than in Neurotrichus or Sorex; these adaptations of Scapanus are for more powerful action of the pelvic limb in the burrowing type. It is the author''s opinion that the Talpidae did not necessarily develop from an aquatic ancestor, as has been sometimes thought.