What is the common purpose of a porcupine's quills?

The common purpose of a porcupine's quills is for defense. These quills are sharp and barbed, allowing them to serve as a formidable deterrent against predators. When threatened, a porcupine can raise its quills, making it more difficult for an attacker to approach without sustaining injury. This defensive mechanism helps protect the porcupine from being hunted while it forages for food, as they are relatively slow-moving animals. The quills are an effective adaptation that enhances the porcupine's survival rate in the wild.

Other options do not align with the primary function of quills. For instance, while camouflage is a form of protection, porcupines rely more on their quills for direct defense rather than blending into their environment. Mating displays and locomotion are not relevant uses of quills, as they do not play a role in attracting mates or aiding in movement. Thus, the primary role of a porcupine's quills is indeed for defensive purposes.