If, despite all your aromatic notices, someone invades your territory, it might be enough to make threatening gestures, or swoop down on him, or bare your teeth and growl. Clearly, claw-to-claw or talon-to-talon mortal combat each time there’s a minor jurisdictional dispute is too costly for everybody—winner and loser. It’s much better to disperse the population through bluff, deception, feints, and a vivid pantomime of what violence you will visit on the intruder should he persist in ignoring your restrained and reasonable warnings. Deterrence is the way these matters are arranged, by and large, on the planet Earth. Real violence lies at the extreme end of the spectrum of aggressive possibilities, a last resort, as Hobbes said. Nature almost always settles somewhere short of that.
To avoid misunderstandings, it’s important to have evolved unambiguous conventions not only for what constitutes aggression, but also for what constitutes submission. Typical submissive gestures in mammals are the opposite of typical aggressive gestures10—averting the eyes so they look anywhere but at the adversary; absolute motionlessness; a kind of bowing in which the forelegs and head are lowered and the rump raised; hiding from view those body parts that are conspicuous in threat displays; and turning jugular vein or belly up, exposing vital organs to the adversary as if inviting evisceration. The pantomime is lucid: “Here is my belly, do with me as you will.” It’s followed almost always by a magnanimous gesture from the victor.* Different species have different hereditary conventions on what constitutes and symbolizes submission. Fighting is transformed into ritual; instead of bloody combat, there is an exchange of data.
Such aggression—most often between males of the same species in disputes over territory or females—is very different from predatory aggression, aggression against members of another species. The two modes share some features in common (baring the teeth, for example), but the one is mainly bluff and the other is in deadly earnest. They engage different parts of the brain. In rivalries of love, cats will hiss, spit, arch their backs, make their hair stand on end, raise their tails high, and dilate their pupils. (Note how many of these postures and gestures make the animal seem larger and more dangerous than it is.) They rarely do each other serious harm, though. A genetic propensity for attacking others of your species, and eliciting attacks from them, has a maladaptive side to it—even if you win every fight, you might be badly injured, or a minor cut might later become infected. Bloodless rituals and symbolic combat are far more practical.
Predatory aggression is just the opposite. Its early object is to come as close as possible to the victim before it realizes what’s up. The cat will slink an inch at a time if it must, ears slicked back, hair tightly following the contours of the body, tail lowered. It stalks in absolute silence. Then the pounce, the kill, and dinner—all done with consummate delicacy and grace. No hissing and spitting here. Intraspecific aggression is almost all show, display, intimidation, coercion, stagecraft. Only rarely does it end in mortal combat. Interspecific aggression, that’s different. That’s business. The prey may get away, but the predator’s intent is murder. Few species systematically confuse the two modes of aggression.
Mock combat is a staple in the theater of intraspecific aggression; both parties go through the motions, but neither is seriously hurt. The deadly, needle-toothed piranha fish of South American rivers fight among themselves, or at least the males do, but never by biting: If there were biting, everyone could get hurt. Instead they push and shove with their tail fins. They want to communicate aggression, but not to bloody the water. It’s as if the combatants walk a fine line between cowardice and murder. Most often—crowded conditions may be another story—the line is walked with astonishing precision. But, as a reminder of how fine the line is, in many species intraspecific fighting is more likely when the animals are hungry. One kind of behavior spills over into the other
The female blue heron hears the love screech of the male. There may be several males calling at once—to the wind, for all they know. She picks her heart’s desire and settles on a branch nearby. The male immediately begins to court her. The moment she indicates interest and approaches him, though, he changes his mind, becomes unpleasant, shoos her away, or even attacks her As soon as the discouraged female flies off, he screeches after her—“frantically,” according to Nikko Tinbergen, the pioneering chronicler of blue heron life. If she gives him another chance and flies back, he may very well attack her again. Gradually, though, should the female’s patience last that long, the fickle male’s grumpiness subsides and he may actually be ready to mate. He is conflicted and ambivalent. Sex and aggression are mixed up in his mind, and the confusion is so profound that, if not for the patience of the female, this species might fail to reproduce itself. If ever there was an avian candidate for psychotherapy, the male blue heron is our nominee. But a similar confusion in the minds especially of males holds for many species, including reptiles, birds, and mammals. Some of the brain’s neural circuitry for aggression seems dangerously cheek by jowl with the neural circuitry for sex. The resulting behavior is strangely familiar. But of course humans are not herons.
Often you can see the ambivalence, the tension between inhibiting and disinhibiting the aggressive machinery in the animal’s behavior. It is literally “of two minds” A fighting cockerel, whose pecks and spurs are deadly, may in the midst of a confrontation turn aside and peck at a pebble on the ground, which after a moment it drops. In human as in animal behavior this is called “displacement.” The aggressive feelings are transferred or displaced to someone or something else, so the passions can be discharged without causing real injury. The cockerel is not angry at the pebble, but the pebble is a handy as well as a safer target.
Some male tropical fish use their vivid coloration to keep other males away, that is, to protect territories and females. The females are, however, similarly decorated. During courtship the female, if attracted to the male, dispenses with her usual indications of submissiveness or readiness for escape and signals her amorous intent by a display to the male—a display, however, which is very similar to the male’s own aggressive posture. In some species, the male becomes enraged (and probably a little confused); he responds by displaying his coloration broadside to her, beating his tail fearsomely, and charging her. But, as noted in a famous study by Konrad Lorenz, he does not actually attack her. (If he did, he would leave fewer offspring.) Instead, narrowly missing the female, he races on and attacks someone else, usually the male in charge of the next territory, who may have been minding his own business, browsing in the algae. Eventually things settle down. Our protagonist no longer attacks his neighbor or charges the female. The species continues. Here, instead of displacing aggression away from a formidable enemy to an inoffensive target, the displacement goes the other way around. This sort of redirection is widespread. Again, gestures, postures, and displays about sex are very close to those about violence. The two can get confused.
One wolf will greet another by placing its mouth around the other’s muzzle. Many other mammals do likewise. Those taming wild animals may be startled when they are at the receiving end of such a greeting. The wolf stands on its hind legs, places its forelegs on the scientist’s shoulders, and places its jaws around the scientist’s head. This is just the wolf’s way of being friendly. If you’re an animal who doesn’t know how to talk, a very clear signal is communicated: “See my teeth? Feel them? I could hurt you, I really could. But I won’t. I like you.” Once more, a very narrow line separates affection from aggression.