The effect of time of day on sperm competition and male reproductive success in laboratory rats

The study was designed to test the hypothesis that siring success of male rats depends on when during the receptivity period the matings occur. In Experiment 1, females were mated with pairs of males differing in coat colour genetic marker. The males copulated with the female successively, either about 1 hour apart at dusk (schedule AA) or about 1 hour apart around midnight (BB) or one at dusk and the other around midnight (AB). The males mating first had lesser siring success under the schedule AB than under the other two schedules. In Experiment 2, females were mated under the same regimen as in Experiment 1. One male of each competing pair had its spermatozoa labeled by 3H-thymidine. The proportions of spermatozoa of the labeled males were identified in samples from uterine horns and oviducts. Balanced contributions of both males were recorded in uterus in all groups. No definitive conclusion concerning oviductal sperm composition could be reached. In both experiments, night-mating males copulated more quickly. It is suggested that copulation at about midnight is advantageous in intermale reproductive competition and that the function of the diurnal variation in the speed of male sexual behaviour might be clarified on this ground.