Gonadal influence on agonistic behavior in the male domestic rat

The influence of androgen on agonistic behavior of the laboratory rat was investigated. Castrate males were tested with intact partners at 2-week intervals in neutral observation cages. The castrates were untreated in the first test, testosterone propionate treated in the second, and untreated again in the third test. Testosterone propionate treatment markedly increased the level of interaction between the pairs. Untreated castrate rats not only lacked the tendency to initiate conflict, but also failed to evoke aggressive responses from intact males. Castrate rats fought successfully if such conflict was precipitated, but this was rare. A second experiment tested the interaction between a home cage testing situation and the dependence of agonistic behavior on androgens. Male rats kept for 2 weeks in observation cages, were tested for agonistic behavior after the introduction of intruders; the frequency of occurrence of 15 behavioral acts was recorded. Residents were then castrated, allowed 2 weeks for recovery and retested. After the second test a 10-mg pellet of testosterone propionate was implanted subcutaneously; two additional weeks were allowed and a third test was performed. A third experiment identical to the second, except that testosterone propionate was not administered, was also performed. The expression of agonistic behavior was potentiated by androgen; aggressiveness and dominance were also greatly influenced by residence in the home cage.