Avian Forelimb Muscles and Nonsteady Flight: Can Birds Fly without Using the Muscles in Their Wings?

AlsTRACT.--Intensity patterns of electromyographic (EMG) signals from selected muscles of the wing were studied during different modes of flight in trained Rock doves (Columba livia). Shoulder muscles exhibited a stereotypic pattern producing maximal EMG intensity during the deceleration phases of the upstroke and the downstroke, whereas the muscles of the brachium and antebrachium acted primarily as joint stabilizers during level flapping flight. During nonsteady flight (e.g. takeoff, landing, vertical ascending flight), the distal forelimb muscles exhibited maximal EMG intensity; their primary function appears to be associated with changing the camber and planform of the wing during rapid oscillation. During steady flight, an automatic linkage system consisting of forelimb skeletal elements and ligamentous attachments is thought to permit proper excursion of the wing as a result of forces generated solely by proximal muscles of the wing. To test this hypothesis, the medianoulnaris and radialis nerves were cut in five animals, thus eliminating the contribution of the forearm muscles, and flight tests were performed. Even though forearm muscles were incapable of contracting, the birds were capable of sustained level flapping flight. They were unable to take off independently or perform controlled landings. Received 3 October 1991, accepted 29 March 1992. DESPITE THE large number of bird species, the wide range of wing shapes (Savile 1957), and variation in flight styles or wing-beat gaits (Ray- net 1988), natural selection has acted to retain the basic musculoskeletal design of the avian forelimb. Few data exist on the functional re- lationship between a species' flying capabilities and its forelimb musculoskeletal architecture. Previous studies of the musculoskeletal system document structural variation, but few studies (see Brown 1948, Fisher 1946, Sy 1936) address the functional aspects of forelimb components. Compared with terrestrial locomotion, flying is metabolically efficient per unit distance trav- elled, but energetically expensive per unit time (Tucker 1968, Schmidt-Nielsen 1984); this is due to the muscular demands associated with gen- erating lift using a rapidly oscillating append- age. Consequently, the musculoskeletal appa- ratus of the avian forelimb should be subject to considerable selective pressures. One way to minimize the moment of inertia of a rapidly moving appendage is to distribute the mass closer to the pivot (Hildebrand 1988). This phe- nomenon is evident among birds as the bulk of the wing's mass is positioned proximally. In