My Border Collie is extremely aggressive. She has never been beaten or abused in her life and yet she has turned into this barking and snapping nightmare. Dogs do both. The basic way of canine life is hierarchy. Most move about frequently and don't stay long enough to maintain fixed territories.
Circumstances change a bit when dogs begin to establish a fixed home base with cooperative hunting.
They become territorial, usually as a group. Physical Features of Aggression Dogs must gear up, as all animals do, for aggressive encounters. Normally dogs maintain a balance between the two. But at times they listen only to the sympathetic side. In these circumstances, adrenalin pours into the blood, the heart beats faster, blood is transferred from skin to muscles and brain, blood pressure increases, red blood cell production increases, coagulation time decreases, digestion and food storage stops, salivation is restrained, the rectum and bladder do not empty as easily as under normal conditions, stored carbohydrates come out of the liver and flood the blood with sugars, respiratory rate increases dramatically, temperature-regulating mechanisms are activated, hair stands on end, and there is profuse panting.