There is not (that I I have found) hard core scientific proof that rats have the ability to store sperm. However, there is a lot of information about male rats reaching sexual maturation early. My thoughts are that you are not removing the weanlings early enough and in the absence of an adult male, the largest and/or most dominant male offspring is maturing early to compensate. This is very common in mammals where sperm storage is not so common. Have ALL the females in the tub given birth multiple times without an adult male? The answer to this question is your solution.