What adaptations make seals champion breath-hold divers?

Seals have remarkable abilities to dive deep and long and to spend only minutes at the surface to breathe between dives. In the chapter opener about elephant seals, we posed several questions that apply to all seals: Why aren’t seals crushed by the water pressure at extreme depths? Why don’t they get the bends? What are their O₂ demands? And what are their O₂ reserves? The answers to the first two questions are related. Only air-filled structures of the body are compressible, and the seals exhale at the beginning of a dive. Their residual volume is restricted to their airways, where gas exchanges do not occur. Thus the high pressure of the small amount of gas in their respiratory system does not drive dissolved nitrogen into their tissues. Investigating Life: Seals Are Champion Breath-Hold Divers presented metabolic rate (MR) measurements for Weddell seals resting, sleeping, and diving. No significant differences in MR for various activities were observed, leading to the conclusion that the metabolic costs of diving are quite modest. In fact, MRs during dives were only about 10 percent higher than at rest—not much different than for humans going from sitting to standing.

How is seal activity supported by such a low metabolic expenditure? And given those low MRs, how do seals maintain O₂ reserves sufficient to support them during dives? The calculations made in the work with the data exercise in Investigating Life: Seals Are Champion Breath-Hold Divers showed that the O₂ reserve of a sleeping seal would permit it to not breathe for a maximum of 12.8 minutes, which corresponds well with the maximum episode of sleep observed—13.5 minutes. Using the MR value for all dives, calculations show that the seals’ O₂ reserves would only support dives that were under 10 minutes in duration. This means that additional adaptations enable the seal to remain active underwater for long periods at an average MR similar to what they experience during sleep.

Future directions

Recording devices attached to seals diving in nature reveal adaptations that minimize their O₂ demands during a dive. Because they exhale at the beginning of a dive, they are negatively buoyant, meaning they can drift down to depths like a falling leaf. These recordings also answered another question. Since the seals at sea dive continually day and night, when do they sleep? They spend much of their passive descent time sleeping, thus lowering their MR even more. Yet more metabolic savings are needed to enable long dives. One clue comes from measures of the seals’ heart rates during dives. Their heart rates are very much reduced, a phenomenon called diving bradycardia. We will discuss diving bradycardia in the next chapter and see how it relates to seals’ ability to drastically lower their MR while diving.