The Norway Rat's Killing Response To the White Mouse : an Experimental Analysis 1)

1. This paper consists of a descriptive study of the killing-response to the white mouse in both wild and domesticated Norway rats, and of an experimental analysis of some physiological and environmental conditions affecting this response. 2. The rat's behavior toward white mice had been studied under standard environmental conditions : about 70 % of the wild rats but only 12 % of the domesticated animals killed mice spontaneously and with an absolute constancy. no significant difference was observed between male and female rats in their response to white mice. in most cases, both wild and domesticated rats killed by biting through the spinal cord at a high level. in wild rats the delay of the killing-response dropped steadily from several hours, or even a few days, to as little as a few seconds; this delay was almost nil from the first test on in those domesticated rats that killed mice. the average delay between killing the mouse and eating it was of several hours for some animals, of a few minutes only for others. wild rats usually started eating the dead mouse at the very spot where they bit it to death, while domesticated animals showed a marked preference for the mouse's brain. 3. Bilateral electrolytic lesions in the amygdala abolished the killing-response to mice in 14 out of 16 operated rats, and at the same time elicited a marked decrease in the responsiveness of these animals to any kind of emotion-provoking stimulus. 4. Surgical removal in 2 stages of the anterior two thirds of both frontal lobes induced 12 out of 29 operated animals to start killing mice and to go on doing so, and at the same time elicited in these animals a more or less marked hyperexcitability. No behavioral change was observed after the first stage of the operation, a partial frontal lobectomy on the right side. 5. The effect of pregnancy and lactation upon the rat's response to the white mouse was studied in 22 non-killing domesticated females. Two animals began to kill mice shortly before delivery, but they stopped doing so a few days later. All the others displayed a more or less active maternal behavior towards the mice, even if the latter interfered seriously with the survival of the newborn litter. The rat's behavior was essentially the same in a subsequent second or third pregnancy. 6. The effect of starvation was studied on 15 wild and 16 domesticated rats that had never killed a mouse under standard laboratory conditions. A large majority of both the wild and the domesticated animals starved to death without ever showing any hostility towards the mouse living in their cage. They made a clear difference. between a dead mouse, which they ate almost immediately, and a live one, which they did not kill in order to get food. Two out of the four domesticated rats and 3 wild animals which began to kill when starved, went on killing even when given plenty of food. 7. Adaptation to the environment increased the number of mice-killers among wild rats, and reduced the delay of the killing response. On the other hand, this delay was considerably increased when wild rats were transferred from their habitual environment into a new one. 8. Painful stimulation with induction shocks that induced wild rats to fight each other did not induce them to kill mice, if they had not done so under standard environmental conditions. 9. The discussion deals with various hypotheses of why, from a broader behavioral point of view, rats kill mice.