Effect of sex on muscular development of Muscovy ducks

Muscovy ducks display marked sexual dimorphism. The aim of our study was to analyse the consequences of dimorphism on muscular growth and, particularl y, on the myofibrillar typology of the Pectoralis major and Sartorius muscl es. In the Pectoralis muscle, we only found two fibre types: red fast-twitc h oxido-glycolytic fibres (about 90 %) and white fast-twitch glycolytic fib res. In the Sartorius, the innermost part contained both white (30 %) and r ed (55 %) fast fibres and red slow-twitch oxidative fibres (15 %). For both muscles, neither sex nor age had a significant effect on the percentage of each fibre type. The cross-sectional areas of fibres increased with age. T he difference in muscle weight observed between sexes could be explained by a higher size and/or total fibre number in the male muscles. (C) Inra/Else vier, Paris.