Atrophy of a breast muscle with a single fibre type (M. pectoralis) in fasting willow grouse,Lagopus lagopus (L.)

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Abstract
The willow grouse (Lagopus lagopus) are arctic gallinaceous birds with small fat stores and large muscles. In winter, these birds may starve for periods of several days. It was important to know which energy reserves were utilized during periods of starvation. Body composition of female willow grouse and Bantam hens was studied before and after fasting. Grouse have much larger breast muscles than do Bantams, but reproductive organs are larger in the hens. The relative amounts of adipose tissue are about equal in grouse and Bantams. When the birds had lost about 20% of their initial body weight due to fasting, Bantams had lost as much weight from their reproductive organs as from the adipose tissue, with little loss from the muscles. Grouse lost more weight from the pectoralis muscles alone than from the adipose tissue. Since the major component of muscle is protein, the grouse obtain a larger proportion of caloric needs during fasting from protein than do the Bantams. Grouse breast muscles are dark red, and the pectoralis consists homogeneously of type IIa (oxidative‐glycolytic) fibres, assessed by ATPase and by Sudan Black staining. The supracoracoideus muscle has type II fibres, not resolvable in subtypes. The leg muscle biceps femoris contains the three fibre types I, IIa, and IIb. During fasting, the weight loss of the pectoralis muscle may be accounted for by all fibres losing some material.