Even the Cal Anderson mouse came out to play. pic.twitter.com/9AbLSJLsfs

— Caean Couto 🤨 (@CaeanCouto) February 9, 2019

As I write this, we’re getting ready for another round of snow, and while I don’t know how much will fall, I do know that it’s hard to be outside in cold weather. The warm-blooded wild creatures that live on the hill have to continue to find food, or at least not waste energy, despite the conditions. Unlike some species, which can shut their entire bodies down, even to the point of freezing solid, most birds and mammals need to maintain bodily functions through the coldest months. Birds don’t exactly have it easy, they can’t layer on fat with high metabolisms and the need to be light enough to fly, but they do have the benefit of being able to migrate away.

What do squirrels, raccoons, or even rats do in a cold spell?

Let’s get this out of the way in case you have any delusions, the real answer is lots of them die. Winter mortality is a part of the population dynamics of practically every species that deals with cold weather. Studies of eastern cottontail rabbits (Sylvilagus floridanus) alone report only around a 30% winter survival rate in the total population. This is the time of year when populations thin. The youngest and least experienced don’t know all the tricks. The old and less fit are at risk because their bodies can’t tackle all the hurdles. It’s hard out there.

Unlike those puny skinny birds, robust mammals are great at storing up energy (I have a lot of personal experience in doing this). Mammals need to be able to move, but they don’t have to fly. Many of us are familiar with the concept of mammals loading on fat in preparation for winter. One study of suburban raccoons (Procyon lotor) in Ohio found a 36% increase in summer/fall weight of adults and 121% in juveniles. Fat is energy, and energy is heat. The fat you really want to stock up on if you are enduring cold winters is brown adipose tissue fat, which actually generates heat through the metabolic process. Brown fat is found in all mammals, but is well represented in ones that hibernate and are exposed to cold temperatures. People have some, infants more than adults, and we have less and less as we grow older.

Another tactic familiar to us is hibernation. However, most of the urban mammals we are familiar with do not hibernate, at least not in the way you’re thinking. Squirrels, rats, raccoons, opossums, and many others definitely slow down and sleep longer. They may even go into short-term torpor overnight, where their bodily functions grind to a halt and their heart rate decreases (which is how hummingbirds famously survive nights). Hibernation really has little to do with the sleep itself, it’s about the reduction of the metabolic rate that is achieved with this inactivity.

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Changes in behavior are often key to survival in the winter. That may seem a no brainer, but when you sleep alone during much of the year as a squirrel, having several new sleeping partners might not be that enjoyable. However, it might be all that keeps you alive through the night. Likewise, it’s probably uncomfortable for raccoons to go roaming about during the day as nocturnal animals, but when it’s cold they can avoid the worst temperatures by altering their behavior.

Rodents are well known for their hoarding, and both rats and squirrels create stores of food, particularly in preparation for colder months. Rats may store huge amounts inside so they don’t have to go outside at all (and which is why reports of infestations also increase in winter). Squirrels tend to make small food caches all over the place in an effort to increase their chances of losing all their food to thieves or in some other unfortunate event. They do forget some, which is why I invariably find peanuts in my vegetable garden every spring.

There’s lots of other little tricks mammals have for cold weather. Eastern gray squirrels (Sciurus carolinensis) are really good at shivering, which produces heat. Opossums know to line their winter dens with grass and leaves, which they need because they have little fur. Deer have hollow hairs, which is excellent insulation, and many populations migrate to avoid winter conditions like deep snow (yes, we have urban deer in Seattle). Many animals change their diet, or eat things they’d never eat during the rest of the year.

Overall, and I’ve said this before, you have to admire creatures who can’t opt to go inside and sit by a nice warm fire. This weekend I might turn a blind eye to squirrels raiding my bird feeder; they need it as much as anyone else out there.