Consider predators, such as a wolf pack hunting elk. To be effective, the wolves’ motivation to kill must be strong, strong enough to overcome fear of a prey that can lethally kick and horn-gouge, but not so strong as to induce recklessness. A delicate balance. The wolves that keep to the rear of the elk try to rip the leg tendons without getting kicked. Those harassing from the front try to keep clear of the dangerous horns. They aim to rip out the throat. The wolves attack the beast and defend themselves at the same time.
This mix of energy, strong desire, fear, and vigilance is crucial for hunting success. Overpowering the prey means success, and success means food. And that, of course, means pleasure. Maybe the pleasure-hostility link is also, in part, owed to the response of the reward systems to a victory, with the result that on the next occasion, mere anticipation of victory brings pleasure. In other words, the animal gets a dopamine hit after the first prey catching. The brain then associates the predatory action with the pleasure of eating. That value is then attached on the next occasion to the goal itself. Anticipatory pleasure is real pleasure. What are the neurochemicals at work in generating this pleasure? Endogenous opioids? Endocannabinoids? Dopamine? All of the above?
Defense against attackers, too, has its “up” feeling, though defense will also involve fear, possibly overwhelming the thrill of being energized for action. Endogenous opioids are probably released, enabling the animal to keep fighting back despite injury. This would partially explain the feelings of pleasure. It also explains the frequent phenomenon whereby only modest pain is felt during a fight, even when a person is shot or hit hard. Success in defense leaves us awash in joyful energy, any fear having utterly vanished. The pleasure of success in beating off a predator is empowering. I can do it! Failure? Defeat, especially chronic defeat, makes animals depressed and withdrawn.
Aggressive behavior can also emerge in defense of offspring. From the perspective of evolution, mammalian parents who respond with ferocity to threats to their offspring are likely to have more offspring survive than those who timorously abandon their offspring to an early death—other things being equal, of course. Hence, the genes of the ferocious defenders will spread; those of the timorous will not.
The determination of parents is breathtaking. Crows will mob and dive-bomb anyone who comes close to a newly fledged crow; a mother squirrel will hurl herself into the maw of a dog to give her baby a chance to escape. A mother bear, otherwise fairly shy, can be a powerhouse of fury if she perceives her cubs to be threatened. Protecting the young is a massively powerful impulse among mammals and birds. Human parents may be at their most aggressive in trying to get their child into Harvard or Stanford.
Predation and defense are forms of competition. The intended prey is competing against the predator for its life, the predator is competing against the intended prey for its protein. There is also territorial aggression, which is really a proxy for food. A patch of land will support only a limited number of animals, such as bears or barn owls. The bear who has command of that territory will roust others eager to partake of the berries on his turf. It is easy to see how territorial aggression would have been selected for, just as it is easy to see how predators of one kind or another inevitably emerged. Territoriality probably evolved many times, just as color vision and sociality evolved many times.
Could there have been a world without aggressive behavior? Probably not our world anyhow. Natural selection inevitably involves competition for resources. At some point in biological evolution, some creature will have the capacity to kill and eat others. Just as inevitably, some of those others will eventually be born with the capacity to resist, perhaps by camouflage, perhaps by beating a hasty retreat, making a terrible smell, or fighting back. The arms race is on.