I have a female rat that I got on 4/1/15. She was sold as a small rat so she was probably around 5-6 weeks old. I got a male on 5/20/15. I wasn't sure on his age but a month later he weighed around 120g and the female (at the time he would have been around 16-17 weeks old) and she weighed 165g. She's now about 19-20 weeks old (last weigh in was 6/30 and she was 208g) and I have not gotten a litter from her yet. They are kept in the same room as my reptiles so the room is around 78-80 during the summer. I cannot move them to another room. Could the hot temp be affecting their ability to reproduce? I would have thought she'd have a litter by now. The male isn't that much younger than her....maybe by a month or so.
I also have a 1.4 group of mice and I suspect at least one is pregnant so this has me concerned that maybe there is something going on with the female.