Heterotypical sexual behavior in male rats: individual difference in lordosis respones

Castrated Wistar male rats were primed with varying amounts of estradiol benzoate (EB) for successive 2 days and progesterone (P) on the third day 6-8 hr prior to the behavioral test. The tests were performed 4 times at 2-3 weeks intervals. As a priming procedure for the first behavioral test, 50 mug EB and 0.5 mg P were given. The quantity of P was kept constant therafter. For the second test, the dose of EB was increased to 100 mug, but decreased again to 50 mug and further to 10 mug for the third and fourth tests, respectively. The inciedence of animals showing lordosis was quite low, and was not significantly changed by increasing or decreasing the dosage of EB during the series of the behavioral tests. Forty-three out of 51 animals never showed lordosis at all 4 behavioral tests. In contrast, 5 out of 8 rats which responded to mountings by the males continued to display lordosis behavior throughout the series of the successive 4 tests. This consistence of individual responeses during the series of the behavioral tests may indicate the possible existence of individual difference in lordosis responde in male rats.