I have to say I haven't seen a staggering drop off in litters. I live in Florida and my rats are outside. During the winter, they came in when it got below 55. When the peak of summer hits, I am going to have a fan on them and they always have a white sheet over them to keep them in the shade.

Am I recording litter size per female? per bin? No. But I have had no shortage.

Don't take my word as the Rat Breeding Bible, but don't let temperature scare you away from breeding and definitely get new rats if you haven't had breeding success yet.

A Note- I love my rats and just like in the winter, if it gets too hot in the summer, they will come indoors as well.