COMPARATIVE SKELETAL-MUSCLE FIBER MORPHOMETRY AMONG WILD BIRDS WITH DIFFERENT LOCOMOTOR BEHAVIOR

Six muscles of the mallard duck (Anas platyrhynchos), the common coot (Fulica atra) and the yellow-legged gull (Larus cachinnans) were analy sed morphometrically, with special emphasis on their functional implic ations and physiological needs. Oxidative fibres always had significan tly smaller size than anaerobic fibres, although no differences in the number of capillaries per fibre were found. This resulted in greater capillary counts per unit of fibre area and perimeter in oxidative tha n anaerobic fibres, which indicates that the greater demand for oxygen supply may be achieved by decreasing the size of the muscle fibre rat her than by increasing the number of associated capillaries. Fast oxid ative fibres of the pectoralis and the triceps of the gull had greater sizes than the fast oxidative fibres of the mallard and the coot, whi ch correlates with the difference in energetic demands between flappin g and gliding flight. Greater fibre cross-sectional areas and perimete rs seem suited to afford the long-lasting activity with low metabolic demands required during gliding. By contrast, mallards and coots attai n a high oxidative metabolism, during sustained flapping flight, by re ducing fibre size at the expense of a diminished ability for force gen eration. Between-species comparisons of the hindlimb muscles only yiel ded differences for the anaerobic fibres of the gastrocnemius, as an i mportant adaptive response to force generation during burst locomotion . The need to manage sustained swimming abilities effectively may resu lt in similar FOG fibre morphometry of the hindlimb muscles studied, i ndicating that a compromise between the oxygen flux to the muscle cell and the development of power is highly optimised in oxidative fibres of the bird species studied.