Materials and Methods
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Although there is a rich tradition of behavior genetic analyses of aggression in highly inbred laboratory mice, there has been remarkably little attention to the behavior genetics of aggression in highly inbred laboratory rats. This is despite the fact that for other aspects of the study of aggression they are the most commonly used subject.
In the present study five commonly available strains of inbred rats were obtained from laboratories where they are traditionally used for cancer and immunology research and were given standardized tests for the various kinds of aggressive behavior. Selected hybrids and backcrosses were then bred and tested to explore some of the types of genetic variance involved.
In view of the claim by some investigators (Barnett, 1963) that laboratory rats do not show the full range of aggressive behavior shown by wild rats, the first question was to determine if highly inbred rats show all of the kinds of aggression and all of the motor patterns of aggression that have been described in wild rats and random-bred laboratory rats.
Four test situations were employed, corresponding to four major types of aggressive behavior which have been studied in the rat: competitive fighting, isolation-induced fighting, pain-induced fighting, and reaction to handling and restraint. Competitive fighting, in which deprived (hungry) animals are allowed to fight over an incentive object (food), was tested by a method adapted from that of Fredericson and Birnbaum (1954). This behavior has been shown to be equally likely among both males and females and is characterized by stereotyped offensive postures. Isolation-induced fighting, which is shown only by males and is also characterized by stereotyped offensive postures, was tested by introduction of a strange male intruder into the home care of an isolated male. Pain-induced fighting, which is shown by both sexes and is characterized by stereotyped defensive postures, was tested by delivery of footshock to pairs of rats in a small enclosure after the technique of Ulrich (1966). Reaction to handling and restraint was tested by holding the animal in a gloved hand and touching the face and vibrissae with a pencil. Scent-marking and olfactory investigation were also analyzed across the five strains in view of evidence that these behaviors play a critical role in isolation-induced fighting (Adams, 1976).
Following the initial tests across five strains, a breeding program was instituted to determine the genetic basis of some of the observed differences in aggressive behavior in three of the five strains. Since Lewis rats turned out to be similar to WAG-Rij and since Irish rats turned out to be similar to DA rats, these strains were dropped from the analysis and hybrids were bred of the other three strains. Finally, when it was found that there was an especially interesting genetic difference in competitive fighting among the strains, a breeding program was continued to isolate the genetic factors involved.
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