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When one reads about fish and marine mammals, it is always said that for example blubber of marine mammals helps them with buoyancy because it is so low density and similarly for example sharks have large low density liver (possibly up to 30% of their body mass) and mainly cartilage that is about half the density of a normal bone (and it is implied this is necessary for buoyancy).

However, many terrestrial mammals like humans are already positively buoyant when having lungs filled with air (most humans in their normal weight), so it is not obvious why any kind of adaptions for marine animals are necessary as their body density composition could a priori be essentially just that of humans. There thus has to be a factor which makes it beneficial for e.g. marine mammals to have more something denser in relation to humans (like bigger muscles in relation to lower density tissue) which then needs to be compensated with such things as blubber. I am looking for what that factor is exactly.

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The answer is that air in the lungs of a diving mammal is compressed, meaning that it takes up less volume than air at the surface. This phenomenon is called "Thoracic squeeze". The deeper the dive, the smaller volume the lungs and air occupy.

Because the air takes up less volume, the buoyancy of the lungs is decreased once the diver reaches a certain depth below the surface. You can experience this for yourself in a diving pool - take a deep breath and pointing your toes to the bottom, submerge yourself to about 1-2 m (3-7 feet) below the surface and you will start to sink as the volume of your lungs has decreased to a point where their buoyancy is counteracted by your density.

Positive buoyancy in a diving animal is a plus - it means that they don't need to expend (as much, if any) energy to reach the surface, they can rely on their body to take them up, no matter what.

I have no idea why sharks and fish that don't rely on lungs need low density tissues, other than to make them neutral buoyancy so that they don't need to expend energy to maintain their place in the water column.

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