Capillarity and Fibre Types in Locomotory Muscles of Wild Yellow‐Legged Gulls (Larus cachinnans)

This study analyzes the Capillarity and fibre‐type distribution of six locomotory muscles of gulls. The morphological basis and the oxygen supply characteristics of the skeletal muscle of a species with a marked pattern of gliding flight are established, thus contributing to a better understanding of the physiology of a kind of flight with low energitic requirements. The four with muscles studied (scapulotriceps, pectoralis, scapulohu meralis, and extensor metacrpi) exhibited higher percentages of fast oxidative glycolytic fibres (>70%) and lower percent‐ages of slow oxidative fibres (>16%) than the muscles involved in nonflight locomotion (gastrocnemius and iliotibialis). Capil‐lary densities ranged from 816 to 1,233 capillaries mm‐2, having the highest value in the pectoralis. In this muscle, the fast oxidative glycolytic fibres had moderate staining for succingate dehydrogenase and relatively large fibre sizes, as deduced from the low fibre densities (589‐665 fibres mm‐2). All these findings are seen as an adaptive response for gliding, when the wing is held outstretched by isometric contractions. The leg muscles studied a considerable population of slow oxidative fibres (>14% in many regions),which suggests thatthey are adapted to postural activities. Regional variations in the relative distributions of fibre types in muscle gastrocnemius may reflect different functional demands placed on this muscle during terrestrial and aquatic locomotion. The predominance of oxidative fibres and capillary densities under 1,000 capillaries mm02 in leg muscles is probably a consequence of an adapta‐tion for slow swimming and maintenance of the posture on land rather than for other locomotory capabilities, such as endurance or sprint activities.