Analysis of the sternotrachealis muscle fibers in some Anseriformes: histochemistry and sex differences.

Histochemical characteristics and sizes of the fibers of the sternotrachealis (ST) muscle have been investigated in some Anseriformes (mallard, Pekin duck, Muscovy duck, and goose) of both sexes. A sexual dimorphism has been shown in the muscle of the species examined. In the mallard and Pekin duck, the male ST muscle shows type IIIA fibers in addition to the type I, IIA, and IIB fibers observed also in the female. In the Muscovy duck, the male muscle has only type I and IIA fibers, whereas the female muscle presents type I fibers and both types IIA and IIB fibers. Moreover, the mean frequencies for each fiber type were significantly different between males and females. In the goose, both male and female muscles present only type I and IIA fibers. In all the species examined, the mean areas of each fiber type are significantly different between male and female, being always larger in the male muscles. The anatomical sexual dimorphism observed in the ST muscle is discussed in relation to function.