There is wide diversity in the animals that dive to depth and in the distribution of their body oxygen stores. A hallmark of animals diving to depth is a substantial elevation of muscle myoglobin concentration. In deep divers, more than 80% of the oxygen store is in the blood and muscles. How these oxygen stores are managed, particularly within muscle, is unclear. The aerobic endurance of four species has now been measured. These measurements provide a standard for other species in which the limits cannot be measured. Diving to depth requires several adaptations to the effects of pressure. In mammals, one adaptation is lung collapse at shallow depths, which limits absorption of nitrogen. Blood N2 levels remain below the threshold for decompression sickness. No such adaptive model is known for birds. There appear to be two diving strategies used by animals that dive to depth. Seals, for example, seldom rely on anaerobic metabolism. Birds, on the other hand, frequently rely on anaerobic metabolism to exploit prey-rich depths otherwise unavailable to them.

THE PHYSIOLOGICAL BASIS OF DIVING TO DEPTH: Birds and Mammals

The mean aerobic dive limit (ADL) for Weddell seals was calculated from data collected on diving metabolic rates ($\dot{Vo_{2}}$) and blood and muscle O₂ stores. Mean diving $\dot{V}o_{2}$ of adult seals during predominantly exploratory dive patterns was 4.5 mL $O_{2} kg^{-1} min^{-1}$; mean $\dot{V}o_{2}$ of a subadult seal engaged in foraging dive bouts was 8.5 mL $O_{2} kg^{-1} min^{-1}$. The adult value was 30% greater than that used in past ADL calculations. Mean plasma volume was 7% body mass (BM); blood volume calculated with the highest hematocrit (Hct) observed (66) was 21% BM. Hemoglobin concentration at such an Hct was 26% by weight. End tidal $Po_{2}$ (pre-and postdive) justified the use of 95% and 20% arterial O₂, saturations in the blood O₂ store calculation. Total blood O₂ stores were 50% greater than those used in past ADL calculations. Mean myoglobin concentration (5.4% by weight) and more recent anatomical estimates of muscle mass yielded a 35% increase in muscle O₂ stores. The mean esti...

Determinants of the Aerobic Dive Limit of Weddell Seals: Analysis of Diving Metabolic Rates, Postdive End Tidal Po2's, and Blood and Muscle Oxygen Stores

The diving behaviour of 14 adult southern elephant seals was investigated using time depth recorders. Each of the seals performed some dives that were longer than its theoretical aerobic dive limit...

Physiological implications of continuous, prolonged, and deep dives of the southern elephant seal (Mirounga leonina)

Respiratory adaptations in diving mammals.

The spleen of the Weddell seal (Leptonychotes weddelli) may contract and inject red blood cells (RBCs) into the peripheral circulation during diving, but evidence for this hypothesis is indirect. Accordingly, we measured splenic dimensions by ultrasonography, plasma catecholamine concentrations, hemoglobin concentration, and hematocrit in five Weddell seals before and after intravenous epinephrine during halothane anesthesia and while awake at the surface after voluntary dives. Spleen size was reduced immediately after epinephrine injection or after the seal surfaced. Within the first 2 min after the seal surfaced, cephalocaudal splenic length was 71 +/- 2% (mean +/- SD; P < 0.05) and splenic thickness was 71 +/- 4% (P < 0.05) of the maximal resting values. Splenic size increased (half-time = 6-9 min) after the seal surfaced and was inversely correlated with plasma epinephrine and norepinephrine concentrations. Hemoglobin concentration increased from 17.5 +/- 5.3 g/dl (measured during general anesthesia) to 21.9 +/- 3.7 g/dl (measured in the first 2 min after surfacing). At these same times, the hematocrit increased from 44 +/- 12 to 55 +/- 8%. These values decreased (half-time = 12-16 min) after the seal surfaced. We estimate 20.1 liters of RBCs were sequestered at rest, presumably in the spleen, and released either on epinephrine injection or during diving. Catecholamine release and splenic contraction appear to be an integral part of the voluntary diving response of Weddell seals.

G.H.K. Lim

Papers

Blood parameters of the southern elephant seal (mirounga leonina, Linn.) in relation to diving