primates, mammals, birds, reptiles, amphibians, fish, and insects. And of course, there are animals populating every other continent and the seas that surround them. Among the social animals — those living in groups — the common form of aggression is one-on-one, and the context is typically competition over food, a place to rest, or access to a mate. Sometimes the aggression is initiated as an attack and sometimes it is in self-defense. Sometimes it is highly ritualized and planned, and sometimes it is areactive free-for-all. Sometimes it occurs within the group and sometimes between. Severe injuries arise, but deaths are rare. The aim is to resolve a competitive dispute by means of non-lethal aggression, and if possible, non-physical contact. If someone dies it is because an injury leaves them incapacitated or vulnerable to disease. It is not because their opponent aimed to kill. The ubiquity of non-lethal aggression points to a suite of common biological ingredients, a core set of neurobiological, hormonal and psychological adaptations that constrain how animals fight. It all starts with one individual perceiving a valuable resource that is within reaching distance of a competitor. What launches a first move and subsequently guides the process to its completion with a winner and a loser? In some species there are rules of thumb that deflate any aggressive instincts before they are launched, even though there are clear competitive interests. For example, in territorial lizards and birds, if an emigrating individual lands in an area and sees or hears another individual vigorously displaying — push-ups with colorfully flashing neck sacs in lizards, vocal arias in birds — they move on. The rule: territory owners win, no questions asked. Another rule of thumb arises in species organized around either permanent or breeding-only harems: one male and many females. Two classic cases are the well-studied hamadryas baboons of Ethiopia and the elephant seals of California. I