Do your plastic tubs have any edges they can get a hold of? I've been using the cement mixing tubs which are smooth and had no problem. Maybe it's when keepers use sweater boxes with indentions in the bottom that they chew out. I do keep wood blocks in the tubs and they chew the heck out of them.

As far as low temp being a possible cause of baby death it sounds like that isn't proven. Is it generally agreed that they don't breed well under 70 deg F? My babies could have died from any number of reasons but since they didn't have any marks I was thinking too cool for newborns. I wonder if I hadn't found them when I did if they would have been eaten later. In some cases it might be hard to be sure if the parents killed them or just recycled them after they are dead.

I would think they would need a pretty constant milk supply as newborns so might be susceptible to any stress that interrupts that and with a first time mom there might also be timing issues with the start of milk production. Also, coming from warm areas the parents might not be as good at keeping babies warm as cooler temp rodents.