Respiratory Physiology of Freely Diving Harbor Seals (Phoca vitulina)

Two 2-year-old harbor seals (Phova vitulina) were studied during free diving in a circular tank in which they were forced to come to a one-way valve to breathe. Expired gas volume and composition were used to calculate V̇o2 and V̇co2. End-tidal Pco2, and Po2, were continuously recorded during the surface intervals of a series of spontaneous dives when the animals breathed room air or inspired gas mixtures which had either an increased Pco2, or a decreased Po2. A smaller tank which restricted activity was used to obtain minimal values for V̇o2. The seals' ventilation was characterized by rapid frequency (f), 33-30 breaths/min, with a VT between 0.9 and 1.01 liters. Changing the inspired gas composition did not alter the f and had little effect in increasing the VT. The major change caused by these gases was to shorten the diving time. Assessment of ventilation involved a consideration of the relationship of the time spent on the surface breathing (ts) to the total time between dives (tt). When the seals were breathing room air, the ts/tt was constant over a wide range of diving times and spontaneous activity. Changes in inspired gas composition markedly changed the ts/tt, and it is suggested that this represents a behavioral control of respiration which must be considered in evaluating the responsiveness of diving mammals to Pco2, and Po2. The energy expenditure of these seals at rest was greater than predicted and during swimming was about the same as walking for terrestrial animals of this size. Estimates of V̇co2, between the lung and the blood during the dives were about 20% of those during the surface interval, and the V̇co2, was even less. These differences are thought to reflect the decrease of the cardiac output which is part of the "diving reflex."