An emergent encoding of aggressive motivation in neurons of the hypothalamus Aggression between members of a species takes many forms, but the rule-book that guides it is ancient. Fighting may help resources and mates go to the fittest members of a species, as Darwin posed ( 1 ). This aggression can also serve as a repelling force to distribute animals evenly over an environment ( 2 ). In social species, aggression creates hierarchies that shape the health and behavior of group members ( 3 ). But picking a fight is costly because even the winner may walk away with injuries. If time allows, it is safer to begin with threat displays and posturing and only escalate to outright attack if neither side will back down ( 4 ).