First 4 males with 4 females is not the best idea, males will spend more times fighting each other and dammaging each other than they will doing the deed.

There are a few factors also are they prime for breeding, did you buy them as adults, if you did they might be past that prime.

With both rats and mice you want to buy young weaned animals and raise them up to breed them.

As for rats it can be stillborn, it can be lactation problem from the mother however usually in that case females will cull them and eat them.



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