Fig. 5. Plot of muscle mitochondrial volume density against body mass in athletic (filled circles) and sedentary terrestrial mammals (small dots) and pinnipeds (open symbols). The linear relationship (y=6.75-1.34[logx], r²=0.70) was generated from the volume densities of the vastus medialis, a primary locomotory muscle, from various terrestrial mammals ranging in size from the dwarf mongoose to the steer (Kanatous et al., 1999; and terrestrial data from Hoppeler et al., 1987). By contrast to the shorter-duration divers, the volume density of mitochondria in the M. longissimus dorsi of Weddell seals was similar to that predicted for a sedentary terrestrial mammal of comparable size.