Fig. 4. Final P_O2 and dive duration. Final P_O2 values were recorded within the last 15 s of each dive. (A) Final venous P_O2 and dive duration from nine emperor penguins (key shows individual symbols for each penguin). The variability in final P_O2 values in relation to dive duration is presumably secondary to differences in metabolic rates during dives. In addition, the span of P_O2 values at the aerobic dive limit (ADL) clearly indicates that the blood O₂ store is not depleted at this limit, the dive duration associated with post-dive lactate accumulation. The exponential regression (y=96.416e^–0.1216x, r²=0.94, P<0.001) was constructed from the highest final P_O2 during each 1-min interval of dive duration. This represents the minimum rate at which final P_O2 declines in relation to dive duration. (B) Comparison of final venous P_O2, arterial P_O2 and air sac P_O2 (Stockard et al., 2005) demonstrates that air sac, arterial and venous P_O2 values become indistinguishable in longer dives.