Sexually receptive female rats normally copulate with several males during estrus, and multiple paternity is common. Sperm competition is therefore likely to occur. One response to competitive mating is to enhance sperm output per ejaculation and another is to augment the number of ejaculations. The latter alternative requires more intense copulatory behavior. In studies in a seminatural environment we observed that male rats did not modify their behavior according to the intensity of competition, whereas observations from standard observation cages suggested that they do so. In order to further evaluate the potential response to competitive mating, we observed male rats copulating in a pair situation, i.e. one male and one female, and in a situation where three males simultaneously copulated with one female. In addition to sexual behavior, social interactions were quantified. It was found that the males in the multiple male condition prolonged mount and intromission latencies, and displayed a reduced number of mounts. There was no change in the number of preejaculatory intromissions or the ejaculation latency. The multiple mating did not affect non-sexual interactions with the female, whereas the female displayed more nose-offs and rejections when copulating with three males. It is concluded that mating competition does alter the initiation of copulation in the male rat, whereas copulatory behavior, i.e. intromission and ejaculation, remains unchanged
