First, let me say it’s been a while since I’ve read the book, so I might have just forgotten the answer to the question.
Anyway, to refresh everyone’s memory, Mrs. Frisby is a mouse who lives in a field in a farm with her children. Every spring, just before planting, they move somewhere else so they’ll be safe from the plows that tear up the field. Then one year, her son is seriously ill, and can’t be moved. To save him, Mrs. Frisby goes to the colony of rats who live in a rosebush on the farm. These aren’t ordinary rats, though. They’re lab rats that have escaped from a NIMH lab, where experiments were conducted on them to give them human level intelligence, and they live in fear that the government will find out about them. The rats owe Mrs. Frisby a favor because her, now dead, husband, was also given superintelligence by NIMH, and was with the rats when they escaped. Of course, by the end of the book, the rats figure out how to save Mrs. Frisby’s family and convince the humans that they’re dead, and it all ends happily.
So here’s my question. The rats, we’re told in the book, have human intelligence because of the experiments done on them. This also explains the intelligence level of the (now deceased) Mr. Frisby, along with another mouse, named Mr. Ages, who is the one to tell Mrs. Frisby to go to the rats. Mr. Ages was also an escaped experiment. The other animals in the book, including Mrs. Frisby, we assume, have ordinary intelligence for their species.
Except, of course, they don’t, because if they did, it would be a really boring book. Mrs. Frisby has to be smart enough, in the book, to realize ahead of time when the plows are going to come, has to realize her son is too sick to travel, has to convince the rats to help her, and so on. These are all things that a human being would know and plan for, but not that an average mouse would.
Normally in a book like this, there wouldn’t be a problem. A lot of fantasy books have anthropomorphic animals, and you just have to accept that the animals of Redwall go on adventures and fight evil weasels, or that the rabbits of Watership Down have their own religion and myths. Accepting that is a natural precondition to the story.
But in this case, where intelligence plays a major part of the story, is it ever explained just how Mrs. Frisby got so smart?