Ask Sam: Why Are So Many Mice Invading My Basement?

Nov 2, 2018

Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam.”

Pam asks: Thank you for your article about the swimming squirrels. It made me wonder about all of the mice that are coming into our house and our rental, and wondered if you can explain why so many of our friends are finding mice already coming into their homes.  

This might seem like a simple question to answer, but there are layers here, people!

Ok, for starters, Pam referenced the squirrels. (We can’t escape the squirrels. This must be what it feels like to be in a band that has 20 albums but has to play that same radio hit at every concert?) So this is an easy hypothesis for why she is noticing more mice this year than other years: because mice also eat mast crops and last year was a big mast year we can assume that at least at the beginning of the year there were probably a lot of mice kicking around. It could be that Pam is noticing mice “already” coming into the house because there were just more of them around to notice.

But let’s answer an implicit assumption in her question, is it possible that the mice are coming in earlier this year?

To answer that, let’s dig into why do mice come into the house in the first place? Well, because it’s warm in there, of course. (I mean, why do you come into your house when it’s cold outside!) Mice, like all mammals are warm blooded, or in Science-Speak “endotherms.”

“They maintain a certain a body temperature, and they have to defend that body temperature against whatever the ambient temperature is,” says Matina Kalcounis-Rueppell, head of the biology department of the University of North Carolina Greensboro.  

There are lots of ways that mammals keep their body temperatures up — running around, shivering, their roaring metabolism generally — but all of them require a lot of energy, and if they can find a nice, warm, sheltered place to huddle in the winter, it just means they don’t have find nearly as much food to survive the cold months.

This is especially the case for female mice that are nursing pups (yes, baby mice are called pups). Kalcounis-Rueppell has noticed that when mice invade her pantry in the winter, they will choose milk chocolate over dark chocolate. “What they’re looking for is a good source of fat, and they’re looking for that fat because they’re making milk.”

And this is actually true of us as well. Even when not shivering, we keep warm though a process called non-shivering thermogenesis. Anecdotally, if you’ve ever talked to a winter backcountry hut caretakers (which even with the fire going are often in the forty or fifty degree ranges) they will tell you that they eat all winter long just to stay warm, and never seem to gain weight.

This is especially true for mice because smaller animals are more prone to lose body heat in the cold. Simply put, being little is not a good strategy for surviving in cold climates, because small creatures have more surface area relative to their mass, meaning they radiate more of their hard-earned heat out into the world. This observation has led to the famous Bergmann’s Rule, which holds that the farther north an animal lives, the bigger it will get. It has been pointed to as an explanation for why animals may have evolved to be larger during Ice Ages.

Generally speaking, in the winter little creatures are living on a razor’s edge: they are rapidly drawing down their bank account of calories day after day, so anything they can do to make the withdrawals slightly smaller helps them out.

Now, are mice coming indoors especially early this year? I doubt it. Looking back at the recorded high and low temperatures for October (in Concord, sorry Pam, you didn’t say where you’re from!) I found our high temperatures were about 4 degrees below average, but our lows temperatures were actually about 2 degrees warmer. (This, by the way, is on track with recent climate trends, our lows are warming faster than our highs.) It’s really those frigid low temperatures at night that make life hard for wildlife, so if I had to guess, Pam is just noticing more mice because there are more mice this year.

And honestly, if your house is anything other than brand new, good luck keeping them out. A delightful twitter rodent enthusiast — owner of the handle @OrderRodentia, who actually conducted our interview while in a mouse costume for Halloween — pointed out that young mice or mice from smaller species can fit through openings the width of a pencil, and have even been known to swim through the u-bend of a toilet to get into a home. “People always think ‘oh no, my house is so secure, but they find a way,” she said.

So, in my book rather than trapping them the best way to keep mice out would be to seal your house up really well in order to keep them from getting inside. The up-side: if you do a really good job mouse-proofing your house, it will probably have the side-benefit of reducing your energy bills at the same time!

[And the energy nerd steps off his podium.]

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