Influence of female's sexual cycle on aggressiveness in male rats.

Two experiments examined the relationship between the female's stage of estrus and agonistic behavior among males. Socially subordinate male albino rats were exposed to an inaccessible female and then given equal numbers of free-choice and forced-choice trials to a more aggressive "target" male or to an empty compartment. The submissive males were more aggressive following exposure to an estrous female than following exposure to a diestrous female. Furthermore, the probability of approaching the more aggressive male increased. This heightened level of aggressiveness was accompanied by a comparable increase in hostile behaviors by the dominant target males of Experiment 1. However, in the second experiment the increase in submissive-male aggressiveness was sufficient to reduce the social distance between the submissive subjects and their moderately aggressive target males.