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 1s 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. In both species, males are much larger than females, with elephant seals providing an extreme case — the harem master can be ten times bigger than the females he mates with. In hamadryas, no one challenges the male over access to the females in his harem. Competition arises in acquiring females into a harem, a process that starts early, with individual males recruiting juvenile females. In elephant seals, either one or a few males completely monopolize the mating among the often hundred or more females within the harem. These males rule. As evidenced by genetic fingerprinting, virtually all of the offspring are sired by 1-3 males. No mating competition. Competition arises when the young turks try to wear down the harem master through repeated challenges over the season. Eventually, often over the course of several mating seasons, the harem master loses a fight and hangs up his gloves.
Dominance hierarchies provide another set of rules or norms that guide competition, and thus aggression. In general, irrespective of the species, high ranking animals outcompete low ranking animals for access to resources. If the spread between two individuals within the hierarchy is large, the subordinate acts like a migrating lizard or bird landing in a resident’s territory: no contest, no competition,
no fighting. If the spread is less, say two individuals who hold adjacent positions within the hierarchy,
Hauser Chapter 1. Nature’s secrets 33
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