What is the function in the vagina in a rat?

Answer 1

The vagina on any mammal, including rats, is involved in reproduction ("making babies").

The male of the mammal species in question, rats in this case, inserts its penis into the female's vagina and releases sperm cells in the form of semen. The sperm cells travel to the uterus or the Fallopian tubes and individual sperm cells can join with any egg cells present and cause pregnancy.

If pregnancy occurs, then the vagina also functions as the birth canal so any offspring ("babies") can be removed from the female's body. The body gives off chemicals to cause the vagina and cervix to relax, and oxytocin causes the uterus to contract and push out the offspring.

For some mammals such as human females, the vagina is also used to push out any material that is leftover from not getting pregnant. In species that menstruate, the uterus grows a blood-filled lining that is suitable for growing a fetus (unformed "baby"). If pregnancy doesn't occur, then that lining, the dead egg cell(s), and the blood inside that lining all have to come out.