Materials and Methods
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The isolated male home rats were tested for biting under restraint on the day following the isolation-induced fighting test. The animal was grasped by a gloved hand across the back of the shoulders, and the arms were pinned across and beneath the animal's chin so that it could be conveniently held. A wooden pencil was moved near the face, and contact was made with the snout and vibrissae. Biting was recorded only if the bite produced tooth marks on the surface of the pencil or if it penetrated the leather glove of the experimenter. Struggling and vocalization were noted. Following the test under isolation conditions, the males were paired together in groups of four for one month. Then they were tested again to provide data on rats in an unisolated condition since previous investigators have reported that isolated rats respond more aggressively to restraint (Hatch et al, 1963).
Shock-Induced Fighting Tests
Following the competitive fighting and territorial fighting experiments, the females remained as pairs in their living quarters and the male home rats were paired and placed into 16x9 in. cages for one month. Each pair of cagemates was then tested for shock-induced fighting.
The animals were placed into an experimental chamber 8x9 in. with an electrified grid floor. No escape from the chamber was possible. Electric shock, which consisted of half second trains of 60 cycle alternating current, was administered through a grid scrambler. Current variation due to different postures of the animals was minimized by placing a 400 kilohm resistance in series with the circuit. Current level was monitored by a voltmeter reading across a precision resistor in series with the animals. It should be noted that current levels represent the current conducted by both animals in parallel; hence to compute the amount of current conducted by one animal, the intensity must be divided in half.
Each trial consisted of 20 shocks, one half second on, one half second off. Trials were conducted at one minute intervals. Boxing behavior and defensive upright behavior were noted both during and between trials. A test began with trials of shock at a current level of 0.5 milliamps.
If, after 5 trials, the rats had failed to exhibit boxing (defined as upright postures by both rats facing each other and accompanied by sparring movements of the forepaws), the current level was raised to 1.0 milliamps. If, after 10 trials there was no boxing, the current level was raised to 1.5 milliamps. Tests were discontinued at 15 trials if there was not any boxing.
It was found that all pairs of rats that eventually boxed did so by the third trial. They were then given a "titration" schedule of current intensities to determine the boxing threshold defined as the current intensity at which boxing occurred on 50% of the trials. The titration schedule was conducted by raising the intensity one level following a trial without boxing and lowering it one level following a trial with boxing. Current intensity levels were 0.05, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0, and 2.5 mA. The test was completed when 25 trials with boxing had been obtained on the titration schedule. Of the 30 pairs of rats that showed any boxing at all, 21 boxed consistently enough for thresholds to be obtained while the other 9 pairs failed to continue boxing.
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