Correlation between duration of courtship and litter size in outbred NMRI mice.

When group-housed female mice are exposed to male mice, male pheromones induce female ovarian cyclicity resulting in a majority of the females entering estrus and spontaneous ovulation on the third night following introduction of the male--the so-called Whitten effect. We previously demonstrated that females mating on the third night after being housed with a male mouse ovulated significantly more eggs than did females mated on the first, second and fourth night, and that this effect was mediated by follicle stimulating hormone (FSH). The present study demonstrated that female mice, which were mated early after being housed with a male mouse (1-2 days), delivered significantly smaller litters than did females mated after spending 3-4 days with the male prior to mating. The implications both from practical animal production perspective, as well as from a fundamental biological perspective may be significant. In addition, the findings may have a practical research value making scientists in reproductive research aware of the natural variation in litter size, which may be an important confounding variable in all research in which litter size is a variable or a parameter used as a measure of the impact of an experimental treatment.