From The Lonely Camp

A nine-banded armadillo can turn its own gut into a life preserver. Facing a river too wide to walk across, it gulps air into its stomach and intestines until they swell to twice their normal size, and the inflated organs lift its armored body just high enough to float. It paddles across riding low in the water, snout up, shell barely breaking the surface.

Then it spends the next three hours slowly letting the air back out. Texas Parks and Wildlife describes it plainly: the animal builds itself a life preserver out of its own digestive tract.

The thing it is solving is a physics problem, and the physics are brutal. The shell is not the light, keratin affair people assume. It is bone. Rows of plates called osteoderms, fused to each other and layered over with a hard outer skin, wrapped across the back and the shoulders and the hips, and it is dead weight in water. An armadillo that falls into a deep pool with an empty stomach does not paddle and struggle and lose. It sinks immediately, like a dropped brick, and it knows this. So before it commits to a wide crossing, it swallows. It takes in air deliberately, gulp after gulp, until its digestive tract is distended into a balloon large enough to cancel out the density of its own armor. It is manufacturing buoyancy it does not naturally have, out of the only material available, on the spot, before it needs it.

And then, for a narrow crossing, it throws all of that out and does the opposite.

Faced with a shallow creek, a drainage ditch, a flooded stretch of road, the armadillo does not inflate anything. It walks down the bank, holds its breath, sinks on purpose, and marches straight across the bottom on its clawed feet with the water closed over its head. It does not swim. It does not float. It uses the weight it spent so much effort canceling out on the wide crossing, letting the shell pin it to the streambed so the current cannot push it around, and it walks, submerged, in the dark, for as long as six minutes on a single held breath, until the bottom rises and it climbs out the far side dripping. The Florida Museum of Natural History has documented armadillos crossing familiar creeks in their own territory this way, on the bottom, as routine.

Which leaves the strange part.

The same animal has two completely opposite solutions to the same problem, and it chooses between them. One method treats the shell as a liability to be overcome. The other treats it as an anchor to be used. Wide water: fill the gut, ride high, paddle over the top. Narrow water: empty the gut, sink low, walk under it. The armadillo is not running one program. It is looking at the water, making some rough assessment of how far the far bank is, and picking the strategy that fits, the way you would decide whether to wade a stream or go find the bridge.

Nobody has pinned down the crossover point. No one can tell you the exact width at which an armadillo stops walking and starts inflating, or what it is measuring when it decides, or whether it ever gets the call wrong and finds itself halfway across water too wide to walk and too far to swim. What is documented is only that both methods exist in the same animal, are used in different situations, and are chosen rather than reflexive. This is an animal most people file somewhere below a possum on the ladder of things that think. It is nearly blind. Its brain is not large. And it stands on a riverbank and solves, correctly, a problem that comes down to buoyancy, distance, and the density of bone.

Then it crosses, in whichever way it decided. And on the far side, if it inflated, it stands in the mud for three hours quietly deflating before it can get on with its evening.