Fig. 2. Model, as in Fig. 1, incorporating a biphasic oxygen uptake curve (Walton et al., 1998). Due to the initial rapid resaturation of oxygen into the respiratory oxygen stores upon surfacing, a biphasic oxygen resaturation curve means that a range of dive depths (d₁-d₃), associated with travel time (t_T), have identical optimal surface times (t_s^*).