Characteristics of male fallow deer muscle at a time of sex-related muscle growth.

Muscle characteristics of male fallow deer undergoing neck muscle enlargement as the mating season (rut) approached were studied. Five commercially-raised males were slaughtered prerut and five were slaughtered 11 weeks later, just before the rut began. During this period, increases in individual muscle weight were not the same: two of the three neck muscles studied grew more rapidly than the average, whereas the back and two hind leg muscles grew more slowly. The splenius, the neck muscle that grew the most apparently in response to a rise in plasma testosterone, was studied. Cryosections were cut and stained for myofibrillar ATPase so that the muscle fiber classes--slow oxidative (type I), fast oxidative-glycolytic (type IIA) and fast glycolytic (type IIB)--could be distinguished. The IIB class was absent from the splenius and the occurrence of I and IIA fibers did not change during the period of splenius growth. However, the splenius of fallow deer increased in activity of NADH-tetrazolium oxidoreductase at the fiber peripheries as the rut approached. Further, fiber areas increased markedly during the period of growth, with type I fibers doubling in area and type IIA nearly trebling. Thus, the endocrinal and/or neural changes associated with the rut differentially affected these fiber types, and since type IIA fibers outnumbered type I by nearly 2 to 1, muscle enlargement is clearly dominated by the former class. Fiber areas were normally distributed for both fiber types, prerut and at the start of the rut, and coefficients of variation were similar. These results suggest that all fibers within a type are equally liable to grow during the growth period.