Effects of testosterone exposure and fighting experience on the aggressive behavior of female and male mice selectively bred for intermale aggression.

Exposure of female mice to testosterone resulted in aggressive behavior as a function of breeding line in two lines of mice selectively bred for high (Turku Aggressive, TA) and low (Turku Nonaggressive, TNA) levels of aggressiveness. Female TA mice that received a single injection of testosterone propionate (TP) (1 mg in 0.05 ml peanut oil) on the second day of life, and starting at 120 days of age received daily injections for seven consecutive days, displayed aggressive behavior on a level comparable to that of socially isolated males of the same line, whereas control TA females (injected with 0.05 ml peanut oil) and TP-exposed TNA females were totally nonaggressive. The level of aggressiveness was assessed by means of dyadic tests against intact male opponents. Early and adult exposure to TP did not lead to increased aggressiveness in male TNA mice. Fighting experience in combination with prolonged TP treatment changed the aggressiveness of the TP-exposed TA females, leading to decreased aggressiveness in defeated animals. The genetically correlated low level of aggressiveness of TNA male and female mice was unaffected by any combination of TP exposure, learning and social isolation. It is concluded that differences in testosterone reactivity of target organs, other than those which are Y chromosome determined, i.e., the testes, are responsible for the aggressiveness in the strains studied.