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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.

Remote time–depth recorders (TDR) were deployed on six gravid leatherbacks nesting on Sandy Point, St. Croix. Dive behavior was monitored continuously for each turtle during internesting intervals ranging from 9 to 11 days. Dive duration averaged 9.9 min/dive (SD = 5.3, n = 5096); mean depth was 61.6 m (SD = 59.1, n = 5096). One turtle dived twice beyond the range of her TDR to depths we estimate >1000 m. Postdive surfacing intervals averaged 4.9 min/dive (SD = 13.1, n = 5090). Differences in mean dive depth, dive duration, and surface intervals among turtles were not attributable to differences in body size (length or mass). Distinct diel periodicity was observed in dive behavior; submergence intervals were longest at dawn, declined throughout the day, and were shortest at dusk. Night dives (19:00–04:59) were shorter, shallower, and more frequent than day dives (05:00–18:59). Dive depth was less variable at night than during the day. The dive pattern suggests nocturnal foraging within the deep scattering layer, a hypothesis that is corroborated by seasonal weight loss data.

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Radiograms of the upper portion of the respiratory system were obtained at pressures up to 31.6 atmospheres absolute in the Weddell seal, Leptonychotes weddelli, and the northern elephant seal, Mirounga angustirostris. The trachea was considerably compressed but not fully collapsed at the highest pressures. No measurable change in the size of the bronchioles and smaller bronchi was observed. Measurements of total lung volume obtained simultaneously showed that the seals consistently dived with a small volume.