A long winter's nap?

Busy day today -- the semester started, and I spent the morning at the eye-doctor, so here's another Colorado shot:

Fox Squirrel (Sciurus niger), Colorado Springs, El Paso Co, CO 1/1/2014

This little guy was quite busy that day, although he appeared rather miffed that I didn't have any nuts for him. Even around here, where the temperature almost reached 10 degrees today, his cousins are out and about all winter.

This little guy, on the other hand, disappears in November, and won't reappear until March:


Thirteen-lined Ground Squirrel (Spermophilus tridecimlineatus),
Illinois Beach State Park, Lake Co, IL  10/21/2006

And this one is fast asleep right now, but if we get a nice February thaw, he'll be out searching out lost acorns:


Eastern Chipmunk (Tamius striatus), Ryerson Conservation Area, Lake Co, IL 3/16/2012

Finally, the odd man out:

Grizzly Bear (Ursus arctos), Cheyenne Mountain Zoo, 12/27/2013

This guy will occasionally wake up on warm days -- if you ever find one active in the winter, keep your distance. He'll probably be hungry!

Unfortunately, I don't have a good shot of a marmot to add.

What's the theme here? Hibernation, of course. Whereas birds migrate instead (with one odd exception), most terrestrial mammals simply can't move far enough fast enough to make it worthwhile. So, if you want to live in an area where you can't get enough food over the winter, and your food doesn't keep well, what do you do? Eat all you can while you can, then spend the winter sleeping it off. And to stretch it a bit further, drop your metabolic rate so you're not burning so many calories. For most species, that means very slow heartbeats, very slow breathing rates, and very low temps -- Arctic Ground Squirrels actually experience below-freezing temps in some parts of their bodies! For a long time, it was thought that bears slept without hibernating, but later work indicates that they show reduced metabolism without noticeable temperature drops, apparently by reducing blood flow to peripheral tissues. (1)

But why so many patterns? Excluding the Fox Squirrel, we have two obligate hibernators (Thirteen-lined Ground Squirrel and the marmots), and two facultative hibernators (which means they only hibernate when they need to -- obligate hibernators enter hibernation whenever the environmental triggers are reached). Food is obviously a big part of the answer, specifically the predictability of winter food supplies. If you know that you're not going to be able to eat when you wake up, why bother? If you've got a good chance of finding a meal on the occasional warm days, it might be worth it.

Of course, waking is only "worth it" if the meal you might find gives you more energy than you're spending warming up. (You can hibernate at low temps, but you can't be active that way.) And the amount of energy it takes to warm up is greatly dependent upon how big you are. Calculations suggest that marmots are just about at the maximum size for a mammal to be able to warm up again on a consistent basis -- anything heavier may well not be able to store enough food to survive the winter and manage to get going again. Although there is a big difference between a marmot and even a Black Bear, the fact that bears hibernate at close to normal body temps does suggest that this idea holds some validity.

(1) Folk Jr, G. Edgar, Jill M. Hunt, and Mary A. Folk. "Further evidence for hibernation of bears." Bears: Their Biology and Management (1980): 43-47.