Preliminary Evaluation of Muscle Fiber Composition in the Middle Gluteal Muscle in Race Mules and Mammoth Donkeys

Research on mule and donkey muscle composition remains limited despite their global importance as working equids. The objective of this study is to identify Mammoth donkey jacks with higher percentage of fast twitch fibers for racing mule production. A total of 33 animals were biopsied; however, only 12 samples were suitable for analysis, including racing mules (n = 7) and male Mammoth donkeys (n = 5). Animals were sedated with detomidine (10 µg/kg body weight) and butorphanol (20 µg/kg body weight). Middle gluteal muscle biopsies were collected using a 6 mm Bergström biopsy needle at a site located 20 cm dorsocaudal to the tuber coxae at a 45° angle to the base of the tail. Collection depth was 7.5 cm in adult mules and 5 cm in donkeys. Samples were prepared aseptically, anesthetized subcutaneously with lidocaine hydrochloride, and frozen in liquid nitrogen. Histochemical analysis included myosin adenosine triphosphatase (ATPase) staining at pH 9.5, 4.6, and 4.3. Fibers were classified as Type I, Type IIA, or Type IIB, and CSA measurements were obtained using NIH ImageJ software. Statistical analysis included group contrasts, summarized as mean ± SD with 95% confidence intervals, while Bayesian ANOVA outputs were presented as exploratory evidence summaries. Type IIA fibers were greater in mules (47.84 ± 7.30%) than donkeys (38.47 ± 4.48%). Results suggest that differences in equid muscle architecture may be associated with variation in Type IIA fiber composition related to work or use.