It has previously been suggested that tissues with slow time
constants could help buffer PN2 at the beginning of a dive bout,
but they would be a liability after a long bout and possibly lead to
dive bout terminations (Fahlman et al., 2007). Consequently, these
models require sufficient data for the N2 levels to reach equilibrium
so that appropriate conclusions can be made. With this dataset, it
took Mesoplodon 4h(Md78), Ziphius 8h(Zc34) and Hyperoodon 13 h
(Ha28) to reach steady state values for the slow tissues (Fig. 1). The
lower mass specific Q˙ tot in larger whales appears to increase the
time to equilibrium and emphasizes the need for longer dive traces
from larger species. To avoid such a bias others have increased the
tissue PN2 at the start of the dive trace (Zimmer and Tyack, 2007).
However, it is unclear without the analysis presented in Fig. 1, what
starting value should be chosen and we therefore do not advocate
this approach.