CAPILLARITY AND FIBER 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 s upply characteristics of the skeletal muscle of a species with a marke d pattern of gliding flight are established, thus contributing to a be tter understanding of the physiology of a kind of flight with low ener getic requirements. The four wing muscles studied (scapulotriceps, pec toralis, scapulohumeralis, and extensor metacarpi) exhibited higher pe rcentages of fast oxidative glycolytic fibres (>70%) and lower percent ages of slow oxidative fibres (<16%) than the muscles involved in nonf light locomotion (gastrocnemius and iliotibialis). Capillary densities ranged from 816 to 1,233 capillaries mm(-2), having the highest value in the pectoralis. In this muscle, the fast oxidative glycolytic fibr es had moderate staining for succinate dehydrogenase and relatively la rge fibre sizes, as deduced from the low fibre densities (589-665 fibr es mm(-2)). All these findings are seen as an adaptive response for gl iding, when the wing is held outstretched by isometric contractions. T he leg muscles studied included a considerable population of slow oxid ative fibres (>14% in many regions), which suggests that they are adap ted to postural activities. Regional variations in the relative distri butions of fibre types in muscle gastrocnemius may reflect different f unctional demands placed on this muscle during terrestrial and aquatic locomotion. The predominance of oxidative fibres and capillary densit ies under 1,000 capillaries mm-2 in leg muscles is probably a conseque nce of an adaptation for slow swimming and maintenance of the posture on land rather than for other locomotory capabilities, such as enduran ce or sprint activities.