Materials and Methods
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In the case of isolation-induced fighting there is reason to suspect hormonal effects in the differences we found between strains. Two kinds of evidence support this hypothesis. 1) The difference in isolation-induced fighting were quite different than those in competitive fighting, i.e. the two strains with the highest levels. Lewis and Fischer, had the lowest levels of competitive fighting. Isolation-induced fighting is powerfully affected by androgens at the level of the analyzers for motivating stimuli, while competitive fighting is less affected by androgens. 2) in mice the extensive work on the behavior genetics of aggression has shown that the differences in isolation-induced fighting are especially linked to genetic factors on the y chromosome which is responsible for androgen effects (Selmanoff and Ginsburg, 1981, Maxson. 1981).
In the case of shock-induced and restraint-induced aggression, there is reason to suspect that the strain differences were due primarily to differences in the analyzers and synthesizers of motivating stimuli for defense. Again the following evidence is offered. 1) The profiles of shock-induced fighting and restraint-induced aggression was not the same across strains which one would expect if the differences lay at the level of the defense motivational mechanism. Thus, for example, the DA strain was low on biting in restraint, but high in shock-induced fighting, while the Fischer strain had the opposite profile. Differences in the functioning of the defense motivational mechanism remain possible in the other three strains, however. 2) The profiles of restraint-induced aggression were similar across all motor patterns, suggesting that there were no differences at the level of motor patterning mechanisms; thus the levels of vocalization, struggling, and biting were correlated across strains and across conditions of social isolation or group housing. 3) The part of the motivational system of defense which has been shown by behavior genetic analysis to be most variable is the neophobia synthesizer (Barnett and Stoddart 1969, Galef, 1970, Smith, 1972).
Since the motivating stimulus analyzers of defense are especially involved in learning effects (see separate chapter on this subject), one may suggest that genetic differences may, especially in the case of defense, interact with learning. In other worlds, rather than postulating that the brain mechanisms of motivating stimulus analyzers for defense are themselves different from one strain to another, we may suggest that the learning mechanisms are more or less sensitive.
Finally, the differences in patrol/marking behaviors such as rub and crawl-over-dish scent-marking may reflect genetic differences in the motor patterning mechanisms. Several kinds of evidence support this. 1) Rather than parallel differences across all the various motor patterns of patrol/marking. the differences tended to be specific to one or another motor pattern. Thus DA rats were low on crawl-over-dish. but higher on rub-against-cage, while Lewis were high on crawl-over-dish and low on rub-against-cage. 2) In a latter study using Lewis and DA rats, we found that all of the genetic variance was in the motor patterns rather than other aspects of patrol/marking (Lee et al. 1984). 3) In comparing muroid rodent species on patrol/marking, it is found that their variability resides in the motor patterning mechanisms of scent-marking which reflects their great variability in anatomical location of secretory glands.
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