Q. Dear Twig: Why don't porcupines stick to things? All those pointy quills and all ...
A. First, the quills ("kwills") on a porcupine mostly point backward. So when, for example, a porky climbs a tree, its quills don't aim forward and poke things by mistake. (Like branches. Tree trunks. The back ends of bears.)
Second, besides pointing backward, the quills also usually lay flat, near the body. This cuts the chance of accidents, too.
The quills don't lay flat when a porky feels threatened. Maybe a wolf or a dog starts to bother it. Then the porky will stick out its quills. This turns them into a weapon. (Special muscles around each quill pull tight and make the quills stand up.)
Third, if a stuck-out quill ends up stuck into something, like the nose of a dog or the mouth of a wolf, the quill comes loose from the porky very easily. (Much more easily than a flat, relaxed quill.) This lets the porcupine get away — not stay stuck — while the predator deals with a snootful of needles!
P.S. Porcupines can't throw their quills. Nor shoot them. And the quills aren't poisonous.
Notes: The quills of the North American porcupine are barbed, hollow, modified hairs. A porcupine has about 30,000 of them. Scientists call the muscles that make quills stand up "piloerectors," while a special quill-holding skin part called the "spool" lets stuck-out, poked-into-something quills pull loose much more easily than relaxed, unstuck ones do. Sources: "A Facilitated Release Mechanism for Quills of the North American Porcupine" by Uldis Roze, Queens College, New York, in Journal of Mammalogy, 2002; University of Michigan Museum of Zoology Animal Diversity Web, http://animaldiversity.ummz.umich.edu; and "Smart Weapons," also by Roze, in Natural History, 2006.
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