Quill-withdrawal experiments confirm a hypothesis proposing that North American porcupines (Erethizon dorsatum) give up a quill more easily if the quill's root is first thrust back sharply into the porcupine's skin. The hypothesis was tested in 8 porcupines. When quills are impacted in a way that mimics contact with a predator, the tension required to remove an impacted quill from the porcupine's skin was reduced from 90.3 g ± 16.8 SD by 38.1%. The phenomenon was observed only in quills that had been erected by porcupines. This tension-reducing mechanism allows a porcupine to separate more readily from an attacking predator and adds to the effectiveness of porcupine quills as smart weapons.