[chris (OP)]

Hippos can walk along a river bottom, or float along close to the surface. How do they control their specific gravity to achieve this?

Chris

[blakestyger]

Like some other aquatic animals the Hippo' breathes out before it goes under water; this gives it 'negative buoyancy' - ie, it sinks!

They get around very niftily on the river bed by using a modified land gait.

[RD]

Like some other aquatic animals the Hippo' breathes out before it goes under water; this gives it 'negative buoyancy' - ie, it sinks!

They get around very niftily on the river bed by using a modified land gait.

Having achieved 'negative buoyancy' by exhaling, surely they must have to swim or walk* back to the surface to inhale.

[* along the bottom towards the shallower shore]

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[blakestyger]

I'm sure you're right - that's all I knew though. I expect someone will come along soon and fill in the gaps.

[Ethos]

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I'm thankful I don't have to swim in the wake of these fellows. Can you imagine what it would be like if he decided to pass gas?

[blakestyger]

It's a girl! []

[SquarishTriangle]

Many of the shallow aquatic species, such as the hippo, achieve neutral or slightly negative buoyancy by means of a static buoyancy control mechanism. Hippos have evolved relatively dense bone structures, particularly in their limbs, to increase their overall specific gravity and and reduce the amount of effort and energy required to rise and fall in the water column. The marrow cavities of their limb bones are filled in with course cancellous bone which merges with the surrounding thick compact bone layer. This thickening is less apparent in the ribs of the hippo but can be seen in those of manatees and dugongs - possibly a reflection of their more permanently aquatic lifestyle and the modified/reduced nature of their back 'legs'.

The deep diving mammals, such as whales, dolphins and seals instead tend to employ a dynamic mechanism of buoyancy control in which they expel gas from their lungs prior to diving to increase their overall density (as blakestyger eluded to). These animals, excluding some seals, also have secondarily REDUCED bone densities to maximise their swimming efficiency.