Body density decreased and buoyancy increased as air sac volume increased with inflation pressure in the three species. Body density decreased and buoyancy increased as air sac volume increased with inflation pressure in the three species. These parameters were calculated on the basis of the known body mass of the birds and the total body volume and air sac volume measured at the different inflation pressures (Fig. 4). The percentage of the body submerged changed from >96% at the lowest mean air sac volume to 74% at the mean air sac volume at 30 cm H2O (2.94 kPa) inflation pressure. Notably, the heaviest emperor penguin (EM0091) was more negatively buoyant at all air sac volumes measured [at as high as 20 cm H2O (1.96 kPa) inflation pressure in that bird]. N values are as in Fig. 2, except that EM0091 is graphed separately in this figure. For comparison, arrows indicate end-of-dive air volume (respiratory air and feather layer air) estimated from buoyancy–velocity calculations in diving penguins (Sato et al., 2002, 2011). P. J. Ponganis et al. J Exp Biol 2015;218:720-730 © 2015. Published by The Company of Biologists Ltd