Bird bones are hollow. This is a previously known thing, and something that many people know about birds. They have to fly, after all, so weight reduction blah blah blah.
But nature, being what it is, abhors a vacuum, or at least an unused empty space.
Many of these hollow spaces in the bones are actually filled with a number of small air sacs that are directly connected to the bird's respiratory system. When I first heard this, my first thought was that this must mean that when a bird inhales, it could use its bones as auxiliary lungs! As it turns out, it's a bit more complicated than that.
When the bones are being formed, the connection to the respiratory system allows the sacs to inflate, moving bone material out of the way and forming the hollow cavities. Once the bones are fully formed, the sacs retain their connection, but do not have the parabronchi (analagous to the mammalian alveoli) found in the lungs, so they cannot exchange oxygen and carbon dioxide. Instead, they serve strictly to hold and move air around, ultimately giving the bird a greater "lung" capacity and higher efficiency.
And then there are crocodiles, whose (I seem to remember) lungs are pushed around by their livers.
ReplyDeleteOr possibly vice versa. At any rate, yes, crocodilians can rearrange their innards to an extent in order to affect buoyancy characteristics without having to actually move (and thus alert prey).
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