SKELETAL-MUSCLE GROWTH, ENZYME-ACTIVITIES, AND THE DEVELOPMENT OF THERMOGENESIS - A COMPARISON BETWEEN ALTRICIAL AND PRECOCIAL BIRDS

We examined activities of enzymes indicative of aerobic capacity (citrate synthase [CS]) and glycolytic capacity (pyruvate kinase [PK]) in pectoral and leg muscles of chicks of the European starling, northern bobwhite, and Japanese quail. The starling exhibits altricial development; the nestlings are highly dependent on their parents for food and maintenance of a high body temperature. In contrast, the quail species exhibit precocial development; at hatching, the chicks can feed themselves and thermoregulate under mild cold stress. In all three species, the proportion of muscle in the body increased with age; in addition, the proportion of pectoral muscle increased relative to that of leg muscle. Pyruvate kinase activity increased in all muscles, but especially in the pectoral muscles of the quail (from approximately 50 to 800 mumol/min . g wet mass [IU/g] through day 21). In contrast, CS increased rapidly in the pectoral muscle of the starling (from 10 to 150 IU/g between hatching and 16 d) but not in quail. Citrate synthase activity initially was high in the leg muscle of quail (40-70 IU/g) but decreased with age; in starling leg muscle, CS activity increased through day 8 and then remained constant at about 45-50 IU/g. In the bobwhite, lactate dehydrogenase activity, which is associated with glycolytic metabolism, closely paralleled that of PK. Muscle-mass-specific metabolic scope, estimated from peak oxygen consumption under cold stress, was higher in neonates of both quail species than in starling neonates, but within 2 wk it increased to a higher level in the starling (30-38 mL/b . g muscle). In general, total metabolic scope per individual varied in proportion to the total CS activity, not PK activity, of the pectoral and leg muscles, regardless of species and age. However, increases in the activities of both CS and PK activities in the starling during the first week were not accompanied by development of thermogenic capacity.