Materials and Methods
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Genetic variance was found in every measure of aggressive behavior. This finding, made for the first time in highly inbred strains of rats is similar to what has been found in extensive research on highly inbred strains of mice. Thus, Southwick and Clark (1968) reported systematic differences in intermale fighting in 14 homozygous mouse strains and Thiessen (1976) tabulated such differences in 22 such strains. Competitive fighting also differs across highly inbred strains of laboratory mice (Fredricson and Birnbaum, 1954), although it has not been investigated as systematically.
The finding that aggressive behaviors have a great deal of genetic variance in inbred strains simply confirms what we have known from other techniques about the great deal of genetic variance in aggression in wild and outbred strains of rodents. One such technique is selective breeding: mice may be selectively bred to produce either more or less intermale or interfemale fighting (Lagerspetz, 1961, Ebert and Hyde, 1976). Another technique is the genetic analysis of domestication: both mice (Coburn 1923, Dawson, 1932) and rats (Yerkes, 1913, Stone, 1932) are less defensive after domestication, a difference which is, at least in part, due to genetic selection.
Heterosis (hybrid vigor) was found here in the case of isolation-induced fighting, which is similar to what has been found in studies of isolation-induced fighting of mice. Eleftheriou et al, (1974) have reported that hybrids of highly inbred strains of mice have more offense than their inbred parents. Demonstrating the same phenomenon with a different technique, Connor (1975) has shown that inbreeding may reduce the probability of inter-male fighting in originally wild stocks of mice.
As one would expect, those strains that have diverged most recently in the history of inbreeding were also more similar in their profile of aggressive behaviors. Thus, the WAG-Rij and Lewis strains, which are related in their breeding histories, were similar in their lack of competitive fighting and low levels of both shock-induced and restraint-induced aggression. Similarly, the Irish and DA strains, which are also related in their breeding histories, were similar in their competitive fighting, isolation-induced fighting, and shock-induced fighting, differing only in their response to restraint. Finally, the most unique profile of aggressive behaviors was shown by Fischer rats which are also most unique in their inbreeding history.
In the final analysis a genetic difference must be expressed in a phenotypic difference in terms of the structure and functioning of the motivational system as illustrated in the following figure. This figure is a composite of those shown in other chapters of this book, and interested readers are referred to those chapters for details.
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