Even so formidable and elegant an example of hardwiring as a cat hunting a rat, though, depends a great deal on past experience. In a set of classic experiments, the psychologist Z. Y. Kuo6 showed that almost all kittens who witness their mother killing and eating a rodent eventually do so themselves. However, when kittens are raised in the same cage with a rat, never seeing any other rat, and never seeing a cat kill a rat, then they almost never kill rats themselves. When kittens have a rat for a littermate and also witness their mothers killing rats outside the cage, about half of them learn to kill—but they tend to kill only the kinds of rat they had seen their mothers kill, and not the kinds that they grew up with. Finally, when kittens are given an electric shock each time they see a rat, they soon learn not to kill rats—indeed, to run in terror from them.
So even such basic hardwiring as the predation program in cats is malleable. Of course humans are not cats. But we might be tempted nevertheless to guess that childhood experience, education and culture can do much to mitigate even deep inborn proclivities.
Starting with the early microbes, the behavioral machinery for hunting and escaping, and for altering these inclinations according to experience, were developing. Predators slowly evolved into larger, faster, and smarter models, with new options (for example, feinting). Potential prey likewise evolved larger, faster, and smarter models with other options (for example, “playing dead”)—because those who didn’t were more often eaten. Many strategies were devised; the successful ones were retained: protective camouflage, body armor, ink or sprayed noxious liquids to cover an escape, poisonous stings, and exploiting niches where there were as yet no predators—a shallow hole in the ocean floor, perhaps, or a sanctuary in a seashell, or a homestead on an untenanted island or continent. Another strategy was simply to produce so many progeny that at least some survive. Again, no potential prey plans such adaptations; it’s just that after a while the only prey left are the ones who act as if they had planned it all out. No matter how fine your intentions, how benign and contemplative your inclinations, if you’re potential prey you’re forced by natural selection into adopting countermeasures.
By around 600 million years ago, many multicellular animals started walling themselves in, surrounding their soft bodies with shells and carapaces, learning to do small-scale civil engineering, building defenses out of silicate and carbonate rock. Lifestyles of clams, oysters, crabs, lobsters, and many other armored animals, some now extinct, developed then. Since, with rare exceptions, soft parts of dead animals decompose quickly and hard parts or their imprints survive longer—sometimes even long enough to be noticed by paleontologists hundreds of millions of years in the future—the evolution of body armor made these distant creatures knowable to their remote collateral relatives.
The warfare between predator and prey extends to the plant kingdom as well. Plants load themselves with poisons to discourage animals from eating them. The animals evolve detoxification chemistry and special organs—the liver, most prominently—to keep pace with the plants. What we like about coffee, for example, are the toxins that have evolved to deter insects and small mammals from consuming coffee beans.7 But we have sophisticated livers.
Of course, predators need not be bigger than their prey. Disease microbes can be formidable predators—not only attacking and eventually killing the organisms that bear them, but also taking over their hosts, changing their behavior to spread the disease microorganisms to other hosts. One of the most striking examples is the rabies virus. On being injected into the bloodstream of a placid, people-loving dog, they head straight for the limbic system of the dog’s brain, where the control buttons for rage reside. There, they set about converting the poor animal into a marauding, snarling, vicious predator that now bites the hand that feeds it. Rabid animals are afraid of no one. At the same time, other rabies viruses are dispatched to inactivate the nerves for swallowing, to put the saliva-manufacturing machinery into overdrive, and to invade the saliva in huge numbers. The dog is furious, although it has no idea why. A pawn of the viruses within it, it’s helpless to resist the impulse to attack. If the attack is successful, the viruses in the dog’s saliva enter the bloodstream of the victim through the lesion or laceration, and then set about taking over this new host. The process continues.
The rabies virus is a brilliant scenarist. It knows its victims, and how to pull their strings. It circumvents their defenses—infiltrating, outflanking, accomplishing a coup d’état within beings so much larger, you might have thought them invulnerable.*
In influenza or the common cold, it’s not an incidental adjunct of the infection that we cough and sneeze, but rather central to the proliferation of the virus responsible, and under its control. Some other examples of microbes pulling the strings:A toxin produced by the cholera bacterium interferes with reabsorption of liquid from the bowel, thus resulting in profuse diarrhea that spreads the infection … Tobacco mosaic virus causes its host to enlarge cell membrane pores so that the virus can pass through to uninfected cells … A lancet fluke is effectively transmitted from ants to sheep because it induces an infected ant to climb to the top of a blade of grass and grab on, never to let go. A fluke causes snail hosts to crawl to exposed sections of beach where they are easy prey for the gulls that are the next host in the life cycle.8
Over many generations of life-and-death interaction between predator and prey, a kind of permanent arms race is established. For every offensive advance there is a defensive counter, and vice versa. Measure and countermeasure. Rarely does anyone become safer.
Some prey grow up together, swarm together, school together, herd together, flock together. There’s safety in numbers. The strongest can be brought in to intimidate or defend against a large predator. The attacker can be mobbed by the entire group of prey. Lookouts can be posted. Danger calls can be agreed upon and coordinated, escape strategies chosen. If the prey are quick, they can dart before the predator, outrace and confuse it, or draw it away from especially vulnerable members of the group. But there is also a selective advantage for cooperation among the predators—for example, one group flushing prey toward another that lies in ambush. For prey and predator alike, community life may be more rewarding than solitude.
To play the escalating evolutionary game of predator and prey, complex behavioral repertoires are eventually needed. Each must detect the other at a distance, and a high premium is established on supplanting local senses such as touch and taste by more long-range senses such as smell, sight, hearing and echo-location. A talent for remembering the past develops in the heads of small animals. Some simple cases of contingency planning, imagining what your response might be to a variety of circumstances (“I’ll do Z if it does A; I’ll do Y if it does B”) may already have been in the genes; but expanding that talent into more complex branched contingency trees, new logic for future needs, greatly aids survival. Indeed, to find and eat anyone—even organisms that take no evasive action—requires, especially when the supply is sparse, a predator to know a great deal.