Muscle Fiber Types in the Pectoralis of the White Pelican,a Soaring Bird

The pectoralis muscle (M. pectoralis) of many premier soaring birds contains a smaller, accessory, deep belly in addition to the much larger superficial belly found in all flying birds. Here we describe the muscle fiber types in both the superficial and deep bellies of the pectoralis of one such adept soaring species, the white pelican (Pelecanus erythrorhynchos).Histochemical techniques are used to demonstrate both nicotinamide adenine dinucleotide (reduced) and myofibrillar adenosine triphosphatase activities within the muscle fibers. Immunocytochemical methods employing several monoclonal antibodies, each directed against a different myosin heavy chain epitope of the chicken, are also used to characterize the fibers. While the superficial belly of the muscle consists entirely of fast-twitch oxidative-glycolytic fibers, the deep belly is composed exclusively of slow fibers. These slow fibers are labelled by two different antibodies specific for chicken slow myosin. We suggest that the fibers of the superficial belly are best suited to flapping flight, and that the fibers of the deep belly would be recruited only during soaring flight. Furthermore, we hypothesize that the deep belly found in the pectoralis of soaring species probably evolved from a deep neuromuscular compartment of the superficial belly.