Taming the Killer Instinct

In their wild state, rats are natural killers. The mere sight and smell of a mouse seem to trigger an unvarying response: the rat pounces on the mouse, aims for the back of its neck and kills it instantly. In recent experiments at Princeton University, scientists investigating the mechanisms involved in the rat's murderous behavior made a remarkable discovery: by injecting drugs into the rat's brain, they can turn its killer instinct on or off almost at will.

Message Flow. The researchers concentrated their attention on a small area at the base of the brain called the hypothalamus, which plays a part in governing such functions as hunger, sexual behavior, fear and rage--in man as well as in rats. Suspecting that the hypothalamus is also implicated in the rat's aggressiveness, Psychologist Melvyn B. King, then a Princeton graduate student, carefully probed the region with electrodes, until he found one distinct site of the killer trait. Next, King and his colleagues--another graduate student named Douglas E. Smith and their professor, Bartley G. Hoebel--selected several laboratory-bred rats that had no apparent killer instinct; they had coexisted in cages with mice without ever harming them. The scientists implanted tubes into the appropriate point in the hypothalamus of the pacifist rats and fed in small doses of a chemical that promoted the flow of "messages" along the pathway of neurons, or nerve cells, involved in the killing behavior. Such substances, known as cholinomimetics, work by mimicking the action of chemicals found normally in the nervous system.

The reaction of the pacifist rats was untypically violent. Placed inside a cage with a mouse, they quickly became restless. Their hair bristled, some salivated. Within the hour, most struck at the smaller creature, killing it with a single hard bite through the cervical spinal cord. Thus, even though they had never killed before, or even seen a killing, they behaved exactly like wild rats for the duration of the drug's effectiveness.

Instant Pacifists. If such violence could be unleashed chemically, the researchers reasoned, it might also be chemically contained. Repeating the experiment with known killer rats, they used another agent, methyl atropine, which has a different effect on the neurons: it blocks the message pathway. As expected, the killers became almost instant pacifists. They did no more than poke their noses at the mouse, sniff it and follow it peacefully around the cage until the drug wore off.

The Princeton experimenters emphasize that they are still largely ignorant about the basic biochemical mechanisms behind the rat's behavior. They cannot yet explain, for example, why some laboratory rats are born pacifists, while most of their kin are born killers. Nonetheless, the results could lead to similar experiments with other species. If the killer instinct can be chemically controlled in a creature as complex as the rodent, some day such aggressiveness may well be tamed in man. Indeed, among those who are apparently interested in that possibility is one of the sponsors of the Princeton research: the U.S. Arms Control and Disarmament Agency.

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