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Why would the shape help you float? Shape has no effect on buoyancy, only density.
@mike805 Yea I can totally see their shape helping them be better swimmers by being more aerodynamic. Thats not the same as "floating" though.
@freemo well shape is a factor in surface tension which lets things denser than water float.
While that is true for a few special circumstances the effect here would 1) be extremely small and 2) would be made worse by having a large central body anyway. A decentralized shape (one with very high surface area) would see effects towards this end, the opposite of what is stated.
@freemo stability while afloat is a function of shape. Being less dense than water is no good to the poor capybara if it has to spend tons of energy trying to avoid rolling to a nose-down position. Hippos are similarly barrel-shaped.
@khird While i do agree that the shape can effect orientation in the water, I would disagree that is due to barrel shape. A true barrel shape is symmetric and thus would not allow for better stability.
Barrel shapes are known to be optimally aerodynamic shapes (see planes, and blimps) and thus would be more suited to making swimming more efficient in that sense I would imagine. That is also the reason, I suspect, a hippo is barrel shaped, for aerodynamics in the water.
@freemo it's only laterally symmetric, but longitudinally it can still provide stability. Suppose the legs are enough ballast to keep it from rolling - the barrel shape just has to provide enough resistance to pitching to counteract the weight of the head and stop the animal from tipping forward.
Also airplane fuselages are just cylindrical because that's the easiest shape to design a pressure vessel in. It would certainly be convenient to have a rectangular cross section from the perspective of volumetric efficiency, but it'd have to be far stiffer and thus heavier. Planes that place a higher premium on aerodynamic efficiency - fighter jets, aerobatic planes, gliders - don't generally bear much as resemblance to a barrel.
@khird Ahh your talking about forward tipping, I was thinking of rolling... Ok yea that makes more sense, elongation allows for less equal distribution on that axis.
@freemo Well they do have most of the buoyancy between the front and rear propulsion units, so that might help them move through the water more efficiently. They share the water with large constrictor snakes.