“Rat Girl”, Rat Reproduction, and the Scary Side of the Secret of Nimh

First: she’s 43. Surely “Rat Lady” or “Rat Woman” is more appropriate now? Still… brrrrrrr:

The whole news article is here.

That 15,000 rats from one breeding pair in a single year sound impressive till you grasp that it’s asimple logarithmic progress. One breeding pair become a bunch of breeding pairs which then breed a ton of breeding pairs which end up being 15,000 in a year.

This is going to sound weird, but, okay, whatever… Rats of a special kind play a role in my current writing project, and my time at the Launch Pad Astronomy Workshop instilled in me a sense of the value of sitting down and doing some quick back-of-the-envelope calculations when working on fiction. (Mike Brotherton will be glad to hear that, I hope!)

It’d be nice if I could figure out how many of those 15,000-per-breeding-pair would be running around one month, and two months, into that year. But rather than working backwards from the given total for a single pair, I’d rather extrapolate forward: how many of the rats would be born, per breeding pair, in a month?

There’s some useful information here. I’m going to assume that Debbie “Rat Lady” Ducommun knows what she’s talking about, since the data seems to match what I’ve seen for Norway rats in scattered sources.

Notable details:

  • Unless pregnant or nursing, sexually mature female rats come into heat every 4-5 days, year-round. 
  • The gestation period is normally 22 days, but can vary from 21 to 23 (and maybe even to 25).
  • The average litter of rats is 10-12 babies (“pups”).
  • Rats reach sexual maturity at 5 weeks of age, and indeed will mate with sisters or mothers at that age if they are not weaned by that point.

That means in a single month, a single breeding pair is likely to produce no sexually mature rats–not even if they conceived the rat on the first day–and no more than 10-12 offspring in total (assuming conception occurs in the first week). Of course, it’s more complicated than that in an environment where there are surplus female rats: a super-rat could breed with untold numbers of females, but let’s just say we need two super-rats to make baby super-rats.


Given all that, 15,000 seems pretty high as an end product in a year’s time. It takes eight weeks to get from conception to the first generation being sexually mature, but let’s add a couple of weeks extra since nobody’s weaning the rats. So at week 10, if you’re very lucky and you get an equal sex-distribution of rats (you won’t; if your rats are in a heavy breeding cycle–and producing offspring as quickly as they can, meaning less recovery time after a pregnancy–then the sex ratio will be biased towards more female offspring, which will jack the numbers up,  but let’s ignore that for a moment), you end up with seven viable breeding pairs (including the original). Every ten weeks thereafter, you’ll theoretically get another 7 breeding pairs at most from each breeding pair, so we’re not talking about an exponential progression, just simple multiplication by 7 at every step. The progression thus follows:

  • Week 20: 49 breeding pairs (maximum)
  • Week 30: 343 breeding pairs (max)
  • Week 40: 2,401 breeding pairs (max)
  • Week 50: 16,807 breeding pairs (max)

Of course, that’s ignoring the kinds of losses you get from unsuccessful births (uncommon for rats, but they do occur) and the cases where a mother eats her litter. Neither case is all that pertinent to my story; think of these rats as being like the ones in The Secret of Nimh, smart enough to do basic medical treatment on a fellow rat, and smart enough not to eat their babies…

The rats in my story don’t wear clothes, or ribbons in their hair. But it strikes me there is a bit of Nimh in my project, all the same.

… except maybe as a means of population control; but I’ll also assume they’re still nonhuman enough a certain amount of inbreeding will occur…though let’s just assume that such inbred pairings will mostly produce rats that are viable enough to survive and take part in the rats’ collective plans, even if they’re not, er, perfect specimens.

Once more, that’s assuming your rats’ sex ratio is equal. If there’s a bias toward more female rat offspring (which you will get in a rapid-breeding program like this), then your numbers are likely to rise even higher. Given all that, 15,000 still sounds high for a wild rat population, and the breeding rate isn’t that impression on the short term.

Which also raises the question of why the rats in The Secret of Nimh–who had attained human-level intelligence, or so it seemed–weren’t raising themselves a rat army to overthrow humanity?

I mean, by the end of a second year at this rate, you end up with 1.76 billion rats… or, assuming some amount of attrition along the way, sure, 1.3 or 1.4 billion. Okay, okay, the math actually needs to be more complicated: female rats don’t remain fertile forever, and their litters get smaller as they get older, so the numbers wouldn’t be quite that high, and I can’t be bothered to work it out in detail, but I’m pretty sure that after only three years, unless they consciously controlled their rate of breeding, you’d have broken into anywhere upward of trillions of rats. Sure, intelligent rats would probably hold back a bit, because if they hit trillions, then soon they’d hit quadrillions and then you get mass starvation and chaos. And of course, along the way, where in the hell do you hide a few billion rats? People would notice something was up, you can be sure.

Still, with only seven billion of us, an intelligent rat army would stand a definite chance at winning whatever fight it chose to pick, at a number of possible scales it might choose. These are brainy rats, after all, and they can chew through wiring, hide inside our houses (in distressingly large numbers, if they’re that smart), purposefully spread diseases, and so on. Even with WMD, I suspect we’d still be horrifically outnumbered to the point where all our systems would break down once they seriously attacked.

It’d be worse than a zombie plague, because they’re intelligent. That is, of course, assuming they perceive us as competition and decide to fight us, instead of just undermining us, or deciding to live peaceably. Or, maybe, the (hopefully) surprising thing they do in my story. (Ahem.) No, it won’t be worse than a zombie plague in my story… I promise.


2 thoughts on ““Rat Girl”, Rat Reproduction, and the Scary Side of the Secret of Nimh

  1. Myself, I am potentially lethally allergic to rats. That is, currently they only cause discomfort and coughing blood, but it seems to get worse over time. I live next ot an urban wildlife sanctuary on a coastal wetland area, and my house gets invaded by rats several times a year. I have to do the opposite of what this woman does, unfortunately, though I do respect the rat’s intelligence, adaptability social qualities, and beauty. I do have to defend myself, though.

    1. Yikes, that sounds… not fun. The having one’s home invaded by rats (with regularity!) but also the allergy on top of it. Your need to defend your turf is understandable. I assume all the repellent measures (growing mint and spraying peppermint oil on their usual points of ingress, and so on) simply don’t work sufficiently, if the invasions are that regular? *Shudder* I’ve only ever been in one house that ever had a rat problem, and the problem was (thank goodnes) a one-time occurrence due to doors being left open that shouldn’t have been.

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