The presence of a vaginal plug in a female mouse or rat most likely indicates what?

• A. The female is in estrus
• B. Mating has likely occurred
• C. The female is about to give birth
• D. The female is infertile

Answer: (B)

In female mice and rats, a vaginal plug forms from semen after copulation and can be observed in the vagina. The presence of the plug is taken as evidence that mating has occurred. This is why the best answer is that mating has likely occurred. It doesn’t guarantee pregnancy, indicate that the female is in a current estrus, or imply infertility; it simply shows that mating happened, which is useful for timing subsequent pregnancy checks and studies. The plug can be temporary and may fall out or dissolve, so absence doesn’t necessarily mean mating never occurred.