A Genetic Analysis of Aggressive Behavior
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Title Page

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Materials and Methods
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Figures 1-3
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Tables 1-3
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Types of Aggression in Inbred Rats

Aggressive behavior, including the full range of acts and postures previously described in outbred laboratory rats, was shown by inbred rats in all four tests for aggressive behavior. In isolation-induced fighting tests, a majority of the individuals of three strains showed offensive sideways posture and a majority of the individuals of two strains showed the bite-and-kick attack. In tests of competitive fighting, pain-induced fighting, and restraint-biting, a majority of the individuals of two different strains showed aggressive behavior. Among the specific acts and postures observed were offensive sideways posture in both isolation-induced fighting and competitive fighting tests, and crawl-under behavior in the isolation- induced fighting tests. These were the two behaviors which Barnett (1963) described in the wild rat, but which he could not observe in laboratory rats.

Phenotypic Differences in Aggression across Strains

The percentage of animals of each of the five inbred strains showing aggressive behavior in the four types of tests have been presented in Table 1 and Figure 1 where it will be observed that statistically significant differences appeared among strains in every test. Irish and DA rats usually showed both competitive fighting and shock-induced fighting, while rats of the other three strains did not. Irish and Fischer rats usually showed biting in the restraint tests, while rats of the other strains did not. In the isolation-induced fighting tests, Lewis, DA, Irish, and Fischer rats showed aggressive behavior while WAG-Rij rats did not. In sum, the Irish rats were aggressive in more tests than any other strain. Wag-Rij were less aggressive in most tests, and the other strains showed intermediate levels of aggression.

An illustrated summary of the strain differences in aggressive behaviors and related behaviors of olfactory investigation and scent-marking is presented in Figure 1.

In the competitive fighting tests, only the DA and the Irish rats showed any stereotyped aggressive acts or postures (statistically significant by Fisher's exact test). The aggressive behavior usually consisted of offensive sideways posture, which was in many cases accompanied by full attack posture, and in some cases by a bite-and-kick attack. Fischer, Lewis, and WAG-Rij rats fed vigorously during the tests, and repeatedly pushed each other away from the food hopper, but no stereotyped aggressive postures were ever observed.

In the shock-induced fighting tests, consistent boxing was shown by a statistically greater percentage of DA and Irish pairs (80% and 90% respectively) than by pairs of rats of other strains. Lewis, WAG-Rij and Fischer rats showed boxing in 20%, 10% and 10% of pairs respectively. In all of the strains there was more boxing by males than by females (difference significant by X² for pooled data). In tests in which no boxing occurred, the animals usually engaged in other competing behaviors such as jumping, flinching, and attempts to escape from the test chamber, the latter being especially evident in females. The thresholds of boxing of DA and Irish rats were similar (0.22 mA and 0.23 mA respectively), while thresholds could not be determined for other strains since most pairs failed to show consistent boxing behavior.

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