Fact 1: Threatening behavior by predators causes a porcupine to extend its quills.

Fact 2: Porcupines only inflate and extend their spines as a defense mechanism.

Given the two facts above, answer the question "porcupine can use its quills as a what" with the following options: 
- encoded information
 - threatened
 - defense mechanism
 - cushion
 - protective bony armor
 - cleaning mechanism
 - if humans disturb bears
 - brush


 

   defense mechanism      