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The functional morphology of the pectoral girdle and arm of Artibeus jamaicensis (Chiroptera: Phyllostomidae) was interpreted on the basis of gross dissection, high speed cinematography, and electromyography (EMG). EMG data obtained during flight for 17 muscles elucidate several temporal patterns of activity associated with the wingbeat cycle that contrast with the patterns of flexor, extensor, and bifunctional muscles observed in terrestrial mammals. Abductor muscles exhibit intense activity associated with the early upstroke phase of the wingbeat cycle and include clavotrapezius, acromiotrapezius, latissimus dorsi, and triceps brachii (long and lateral heads). All abductors except for the triceps brachii exhibit a secondary period of low-amplitude activity associated with the early downstroke. Adductor muscles exhibit primary activity immediately prior to and during the early downstroke phase. The adductors include the serratus ventralis thoracis, pectoralis, clavodeltoideus, and biceps brachii (short head). Bifunctional muscles exhibit two periods of activity or activity throughout most of the wingbeat cycle. The bifunctional muscles include the spinotrapezius, supraspinatus, infraspinatus, subscapularis, and biceps brachii (long head). Dissection of all other muscles of the shoulder and antebrachium form the basis of interpreting musculoskeletal movements during flight in Artibeus. The major muscles of support in Artibeus include the serratus ventralis thoracis, pectoralis, and the trapezius group. These muscles support the trunk between the wings during flight or terrestrial walking. Propulsion during flight differs from that observed during stepping in terrestrial mammals. During the wingbeat, the pectoralis provides the major component of thrust, both by adducting and pronating the wing. Although latissimus dorsi is a major propulsive muscle during stepping in terrestrial mammals, its major function in Artibeus is to abduct the wing and reposition the wing prior to the beginning of the downstroke.