Fossorial Adaptation of the Shoulder Muscles in the Three Species of Insectivore

The weight ratios of the four forelimb muscles (M. supraspinatus, M. infraspinatus, M. teres major, and M. subscapularis) were compared among the three species of Insectivora, the musk shrew as a terrestrial species, the mole shrew as a fosso-terrestrial species and the lesser Japanese mole as a completely fossorial species, to statistically clarify their adaptive changes in the shoulder region. From the quantitative results of the muscle weight, we demonstrated that M. teres major were statistically larger in the lesser Japanese mole and that it was adapted for the adduction of the upper arm in the digging behavior. In contrast, M. supraspinatus was well-developed in the musk shrew and the mole shrew, and we suggest that it may play an important role to extend the shoulder joint in the terrestrial locomotion of the two species. The completely fossorial species such as the lesser Japanese mole were equipped with the well-developed M. infraspinatus that contributed to the flexion and the abduction of the humerus needed to the digging action, and the larger M. subscapularis that helped M. teres major to adduct the upper arm region.