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Presumably, as a whale or seal dives, its lungs get compressed by the increasing water pressure, and it gets less buoyant.

Under this model, for a given amount of air taken in at the surface, the animal has a very specific depth D at which it is neutrally buoyant. Any deeper, and the lungs shrink, making it negatively buoyant. Any shallower, and the lungs expand, making it positively buoyant.

In other words, until it reaches depth D, the animal is positively buoyant. To get to depth D, the animal would have to do a lot of work swimming down, fighting against its own floatiness.

The amount of air that keeps you neutrally buoyant at 10 meters will inflate to twice the volume once you surface, keeping you pinned there. Speaking as a scuba diver, I can say that it would take a lot of work to dive back down to 10 meters. (We deflate our air vests at the surface and re-inflate at depth from our incompressible tanks, something animals don't have.)

For very deep divers like sperm whales, swimming down against positive buoyancy seems like a huge energy waste, the kind that one might expect wouldn't be tolerated long by evolution.

So do marine mammals do anything to compensate for the effect of fluctuating lung size on their buoyancy? One guess is that they actually exhale fully before diving, thus making lung buoyancy a small player in the overall body buoyancy. Another guess is that they compress the air with their chest muscles at the surface in order to sink. These are just guesses; I'd love to hear the real story.

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From what I can tell, marine mammals can't dynamically control buoyancy during a dive. They ease the beginning of the dive by starting with a small lung volume to reduce buoyancy.

Pinnipeds like seals do this by exhaling half their breath before diving.

Deep-diving whales actually breathe in before diving, but their lungs are small relative to body size to begin with. This makes their surface buoyancy weak enough to swim against without great difficulty (unlike humans).

As they dive their lungs compress rapidly with depth:

V = Vs / (1 + D/10)

V is lung volume, Vs is lung volume at surface, and D is depth in meters.

At 90m, the lungs are already at 10% original volume. Pretty soon they've shrunk enough that their effect on buoyancy is negligible compared to the density of the incompressible tissues. Presumably, these are close to neutrally buoyant (though the fact that dead whales sink suggests that they're slightly negatively buoyant).

Thanks to @souvik-bhattacharya for the ucsc.edu link.

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enter image description here

This scenario is called neutral buoyancy, and it's what marine mammals have, so it doesn't take energy for them to stay still in the water, and it's not particularly hard for them to go up or down when they want to either.

Imagine an animal that is as dense as a rock trying to swim up for air, or one that has low density, like a balloon, trying to dive down to find food; it would take a lot more energy to counteract that net buoyancy-weight force (up or down) than it would if that net buoyancy-weight force was 0. So by having neutral buoyancy, marine mammals save energy.

And how do they achieve neutral buoyancy?

They have more fat in their bodies (fat is less dense than water), It also helps that saltwater is a little more dense than pure water, so the buoyant force is a little stronger.

So...as you predicted...neutral buoyancy helps them, but not by inhaling specific amount of air...instead one can say that's their inherent property...

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  • $\begingroup$ Neutral buoyancy is not an "inherent property" of any soft body that contains a compressible fluid (air). $\endgroup$
    – SuperElectric
    Nov 5, 2014 at 9:49
  • $\begingroup$ @SuperElectric , what I meant is that they don't need the help of any external component like air to maintain their neutral buoyancy....also if I go by technical details...during a dive the body of a mammal virtually becomes incompressible with Air cavities, lined with venous plexuses, filling at depth,along with lungs, thus obliterating the air space. $\endgroup$
    – souvik bhattacharya
    Nov 5, 2014 at 10:57
  • $\begingroup$ Lungs with a volume Vs at the surface will have volume V = Vs/(0.1 D) at depth, where D is the depth in meters. Buoyancy force B is proportional to V. Therefore it's true that buoyancy becomes less sensitive to depth the deeper you get (lim D->inf dB/dD = 0). So for the operating depths of, say, sperm whales it may not matter as much... but only once they've achieved that depth. If they wish to achieve neutral buoyancy at 1000 meters, they will be extremely buoyant at 0 meters, making it quite a chore to swim down to that depth. This seems like a wasteful aerobic excercise. $\endgroup$
    – SuperElectric
    Nov 5, 2014 at 15:44
  • $\begingroup$ One workaround would be to exhale as much as possible before diving, so that lung volume V is a small component of overall buoyancy of the body to begin with. This means relying only on the oxygen reserves dissolved in the blood, which won't compress. That's just one guess, though, and I'd appreciate more expert opinion. $\endgroup$
    – SuperElectric
    Nov 5, 2014 at 15:46
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    $\begingroup$ Ok, figured it out. Whale lungs are small to begin with: catalyststudent.org.uk/dl/… Thanks for the link; will use it in the answer. $\endgroup$
    – SuperElectric
    Nov 7, 2014 at 17:31

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