Testosterone modulates these brain chemicals, suppressing serotonin in the service of heightening aggression, and ramping up dopamine to add to the already reinforcing properties of testosterone. But like dopamine, testosterone is also directly linked to the reward system. Mice will work a response lever to deliver testosterone, and humans become addicted to it. If you inject testosterone into a mouse while it is moving about, the location associated with the injection becomes tagged as a favorite spot in the landscape, a place to revisit. Drug abusers and gamblers, two personality profiles associated with heightened experience of reward and poor self-control, have elevated levels of testosterone and dopamine.
Twirling inside the brains and bodies of all social animals is a physiological ballet that controls the capacity to harm others. This choreography links harming others with the experience of reward. In
some animals, the link between harm and reward was upgraded to a capacity for lethal aggression.
HARMING OTHERS, version 1.5: upgrade to lethal aggression
All social animals have evolved the capacity for aggression, using it to fight members of their own species for food, land, and sex. For virtually all animals, winning a fight means chasing away or injuring a competitor, but not killing them. There are, however, three situations in which animals kill, two are broadly distributed across the animal kingdom and one is extremely rare. In virtually every taxonomic group of animals — insects, reptiles, amphibians, fish, birds, and mammals — there are predators and prey. Predators are not merely aggressive, but designed to kill prey species for the purpose of survival. Also common is infanticide, situations in which adults kill infants. Infanticide is often committed by males who have recently entered a group with infants sired by other males. By killing these infants, not only does the newcomer obliterate the competition’s fitness, but he effectively reboots the female’s sexual receptivity. Both predation and infanticide entail significant asymmetries in size or weaponry between attacker and victim, making the kill relatively cost-free. Rare in the animal kingdom are cases where attacker and victim are from the same species, both adults, and thus, comparable in size and weaponry. This kind of killing — call it adulticide — only occurs in a small number of species, but the attacks are sufficiently frequent to count as part of the repertoire: ants, lions, wolves, chimpanzees and humans. The rarity of adulticide raises important questions about the evolutionary pressures that favored this upgrade to harming others, as well as the mechanisms that evolved to make it possible.
Battles among ant colonies are notorious for their organized attacks, designed to kill the enemy and minimize costs. Watching ant colonies battle it out piques the imagination, recalling the classic face offs between British and French brigades, each side lined up in strategic formation, divided into ranks, set up to protect the land and royalty. In his book Life in the Woods, the American writer and nature lover
Henry David Thoreau, writes that the ant battles were “deadly combat ... without any noise... I never
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