You've seen them, if not in real life then on Animal Planet, floating just under the water, only a pair of eyes and nostrils visible above the surface. Alligators possess an astounding ability to control their buoyancy, like submarines, by inflating and deflating their lungs. The adaptation helps them hide, sneak up on prey and conceal their size.
Humans have muscles that contract and relax to move air into and out of the lungs. Alligators have these, as well, but they also have special muscles around the lungs that do more than just move air in and out. Below the lungs, humans' diaphragms flatten to allow the lungs to expand. Alligators, though, have a special muscle that attaches the liver to the hip bones, and this muscle pulls the liver down -- not only allowing the lungs to expand but actually stretching it further back. In addition to this special muscle, alligators also have muscles alongside their lungs that allow them to push their lungs to one side or another.
Dive! Dive! Dive!
An alligator floats the way most animals float, by filling his lungs with air. What's special about an alligator's lungs is that they don't just let him float. They allow the gator to control his floating in special ways. When it's time to go under, an alligator has muscles that move his lungs backward in his body, allowing his head to dip so he can dive smoothly through the water -- where he can remain for up to two hours.
Can you imagine being able to move your lungs from side to side? An alligator can. A gator has more special muscles on the sides of his lungs that push them to one side of his body or the other. Just like pushing the lungs back allows the head to dip, pushing them to one side allows the other side to dip so that the alligator can easily roll in the water, almost like an airplane doing a barrel roll.
Like a Ninja
An alligator on land seems so clumsy and slow that you might wonder how he ever sneaks up on his prey. In the water, though, all of this nifty lung shifting means the alligator can move himself around without disturbing the water very much. Because he can dive, surface, turn, and roll simply by moving his lungs about, he can creep through the water silently and almost unseen until it is too late for dinner to escape.