Fig. 1. The optimal breathing model (Kramer, 1988). The abscissa shows time spent travelling to and from the foraging site to the left of the ordinate, and time at the surface to the right. The ordinate shows the amount of oxygen consumed during travel and gained during surface periods. The value t_s^* denotes the optimal surface time for the diver, according to the model, in terms of maximising the proportion of time at the foraging site.