|MadSci Network: Zoology|
Porcupines don't actually "use" their quills for defense. A common misconception is that they throw or shoot their quills. They don't. They can swing their tail and use that as a weapon, and if they hit the offending animal, the quills can stick in that animal's skin. Because of the small barbs along their length, the quills are very painful to pull out and often just work themselves in deeper.
Porcupine quill has a nice picture of a quill, showing the barbs. The Hinterland Who's Who -- Porcupine has a nice picture of a porcupine and a lot more information about them.
The quills are similar to human hair -- in fact, they are just modified hairs. They're hollow at the base and much stiffer than a normal hair, as well as having the barbs I mentioned above. Once the quills have been used, they are often retrieved by people (perhaps from an unfortunate dog) and used in art (see Preparation of Porcupine quills for a description of that).
They grow back at about the same rate as a normal hair. I don't know exactly how long that is, but most likely it's a few weeks, perhaps a month or so.
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