Fig. 2. Plot of muscle mitochondrial volume density against body mass in athletic (filled circles) and sedentary terrestrial mammals (small dots) and pinnipeds (open circles). The linear relationship [y=6.75–1.34(log x), 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) [terrestrial data from Hoppeler (Hoppeler et al., 1987)]. In contrast to the adult Weddell seals, whose mitochondrial volume densities are similar to that predicted for a sedentary terrestrial mammal of comparable size, juveniles and pups have mitochondrial volume densities similar to those of athletic terrestrial mammals and shorter-duration divers of similar size.