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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In scent-marking, WAG-Rij rats showed significantly lower levels than other strains in accumulation of marking substance in the Petri dish throughout the five weeks prior to testing, on the test day itself, and on home rat crawl-over-dish and rub-against cage behavior. WAG-Rij intruders did not differ from intruders of other strains, however. In addition to the WAG-Rij deficits in scent-marking, there were statistically significant deficits shown by DA and Lewis rats in specific measures. Lewis rats were normal in observed crawl-over-dish behavior and accumulation of scent-marks in the early weeks prior to testing and on the test days itself, but they had a significantly lower accumulation of scent-marking in the fourth and fifth weeks prior to testing and lower percentage of animals with rub-against-cage during the tests. DA rats were normal in crawl-over-dish and rub-against-cage behavior during the test session, but they showed lower accumulation of scent-marking material in the Petri dish and a low percentage of intruder rats with crawl-over-dish during the test session.

In other behaviors unrelated to aggression, there was only one major difference among the five strains. The percentages of DA and Lewis rats that mounted their opponent during the tests for isolation-induced fighting were significantly greater than those of Irish and Fischer rats (50% and 37% versus 5% and 0% respectively, pooled home and intruder data). WAG-Rij rats were intermediate (26%). Mounting was shown by both home rats and intruders, and within strains there was no correlation between mounting and attack behavior.

Strength of Behaviors in Hybrid and Backcross Generations

Five of the behaviors were tested in hybrid and backcross generations: competitive fighting, isolation-induced fighting, shock-induced fighting, olfactory investigation, and scent-marking behavior. The results are summarized in Figure 2 and Figure 3.

Isolation-induced fighting of hybrids and backcrosses was at intermediate or higher levels than the fighting of the inbred parental levels. In the case of hybrids and backcrosses of Fischer and DA strains, the levels were intermediate. In the case of hybrids and backcrosses of WAG-Rij and DA strains, the levels tended to be as high or higher than the parental levels, which represents heterosis (hybrid vigor).

Shock-induced fighting of hybrids and backcrosses was at intermediate levels between those of the inbred strains involved. This was the case for both the Fischer-DA crosses and the WAG-Rij-DA crosses.

Olfactory investigation showed heterosis (hybrid vigor) in all cases. Hybrids and backcrosses of Fischer and DA strains had higher levels of olfactory investigation, as measured by number of minutes with anogenital sniffing of the opponent, than did the parental animals. The same was true for hybrids and backcrosses of WAG-Rij and DA rats.

Scent-marking results were different for the two sets of hybrids and backcrosses. In the case of DA and Fischer breeding, the levels of scent-marking, as measured by percentage of males with substantial accumulation of scent-marking material, were intermediate between those of the parental strains. In the case of DA and WAG-Rij breeding, the Fl hybrid had higher levels than the parental strains (hybrid vigor), but the subsequent back-crosses had levels that were very low, equivalent to those of the WAG-Rij strain with which they were being mated.

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