Threatening behavior by predators causes a porcupine to extend its quills.
Aggression is active, harmful and offensive, threatens all and benefits none.
Aggression by predators causes porcupine to extend its quills.
What by predators cause porcupines to extend their quills?

1) hypothyroidism
2) killing insects
3) kisses
4) aggression
5) love
6) hyperthyroidism
7) kinetic energy
8) cohabitating