PREMIGRATORY HYPERTROPHY OF FLIGHT MUSCLES - AN ULTRASTRUCTURAL-STUDY

Fibre diameters and the proportions of fibre cross-section area comprised of myofibrils, sarcoplasm and mitochondria were measured in the pectoralis muscles of three species of shorebird Calidris alpina, C. alba and C. canutus. Comparisons between the structure of muscles in mid-winter and just before spring migration, after increase in muscle mass, revealed that fibre diameters had increased but the proportion of myofibrils decreased alongside a compensatory increase in mitochondria. The proportion of sarcoplasm did not change. Comparisons between the ultrastructures of muscles of C. canutus on arrival and just before departure from a spring migratory staging post, however, revealed no change in percentage composition accompanying hypertrophy. The functions of flight muscle hypertrophy in shorebirds are only incidentally related to breeding, as no differences between the sexes were noted and other proteinaceous tissues and organs contributed more to the overall increase in lean mass. Hypertrophy does not appear to occur in response to increased body mass; the control systems are probably complex.