Capillary supply of heart and skeletal muscle of small bats and non-flying mammals.

Capillary density (CD), fiber cross-section area, capillary-to-fiber ratio and intercapillary distance of papillary and a calf muscle were evaluated in four species of bats (body weight 23-150 g) and six non-flying mammalian species (2.5-170 g). In bats the pectoralis muscle was studied additionally. In both groups CD increased in papillary as well as in calf muscle with decreasing body weight. The papillary muscles showed the highest CD in the 2.5 g shrew Suncus etruscus (7287 cap./mm2) and the 23 g bat Myotis myotis (6748 cap./mm2). The flight muscle of the 38 g bat Molossus ater showed the highest CD (4992 cap./mm2) of all skeletal muscles investigated. The slopes of regressions of CD versus body weight for papillary (calf) muscles were -0.42 (-0.70) for bats and -0.15 (-0.26) for non-flying mammals, indicating that bats have a higher increase of weight-specific metabolic rate with decreasing body weight. This holds for species with a body weight below 30 g.