Aggression
The most apparent type of aggression is that seen in the interaction between a predator and its prey. An animal defending itself against a predator becomes aggressive in order to survive and predator in order to secure food. Because aggression against a much larger enemy or group of enemies would be nearly certain to lead to the death of an animal, animals have developed a good sense of when they are outnumbered. This ability to gauge the strength of other animals gives animals a "fight or flight" response to predators; depending on how strong they gauge the predator to be, animals will either become aggressive or flee.