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Warm and Cold
Article #355 Febuary 2024
By Bill Cook

Are you a homeotherm or a poikilotherm?  For most of us, we’re the former but with an arguable dose of the latter.  Keep your woollies handy!

    OK, these terms are biological jargon for “warm-blooded” and “cold-blooded”.  Humans are, of course, homeotherms.  This means that we’re among that pool of organisms whose metabolisms work to maintain body temperature at a constant level, about 98.6 degrees Fahrenheit for our species.
    Cold-blooded animals cannot perform this function.  They rely on behavior and micro-environments to keep their body temperatures within a zone where the catalysts and hormones function well.  A painted turtle sunning atop an emergent riverside log is an example of this sort of behavior.  It’s also why so many snakes get run over by cars in the summer. 
    However, it seems to me that we warm-blooded critters use behavior to help keep ourselves “in the zone”, too.  Sitting in the shade sipping an iced tea in the hot of summer might qualify.  Or, donning that fluffy down parka on a subzero winter day. 
    Wildlife have a gazillion ways to stay alive during extreme weather.  These adaptations make for an interesting study, for those so inclined. 
    In winter, most mammals have specialized hairs, in one way or another, to help trap insulating air.  Birds may have more down feathers.  Caches of souped-up high energy brown fat, different from “regular” white fat, helps provide the fuel to keep these metabolic engines humming along, as well as help fuel migrations. 
    Animals of similar species tend to be larger in size and possess shorter appendages and have stockier bodies at more northern latitudes.  This geometrical consolidation of mass exposes less surface area vulnerable to heat loss.  Polar bears are big.  Arctic hares have much shorter ears.  Even populations of the same species can be bigger the father north they go, such as deer. 
    Ruffed grouse will bury themselves under the snow, if there is enough, to help insulate themselves against subzero temperatures.  They can remain there for days, if necessary.  Snowshoers and skiers beware!  A grouse exploding out of its snow roost can knock you over with shock. 
    In summer, shedding excess heat can be difficult for some.  Evaporating sweat helps a lot, but most animals cannot do this.  Dogs sitting in the shade, panting ferociously, provide a good example.  Camels might own the gold medal for advanced methods to maintain a cool attitude in water-starved desert environments.  While not common in the Lake States, camels make for an eye-popping study in animal physiology.
    Speaking about thermoregulatory techniques, hibernation often comes to mind.  Many of us tend to use the term hibernation as a synonym for dormancy.  That’s OK, I suppose, but it’s not biologically correct.  Hibernation, by definition, is an internally regulated reduction in metabolism, maintaining a very low rate of breathing and heart rate.  A true hibernator does not easily “awaken” and, in fact, early arousal can be fatal.  Only certain mammals can hibernate. 
    If you find a thirteen-lined ground squirrel it can be rolled around on the kitchen table for a while before it eventually grinds to wakefulness.  Groundhogs and chipmunks are also among the true Lake States hibernators.  Some might argue about chipmunks, but the ability to hibernate is on a bit of a sliding scale.
    Stress of hibernation and arousal is part of the reason why certain bat species are imperiled, such as the northern long-eared bat.  They are roused in the winter, often by white-nose fungus, but haven’t enough energy juice to support a second round of hibernation.  So, they starve to death. 
    Incidentally, some animals “hibernate” in tropical and subtropical climates to avoid the extreme hot and dry season.  This is called “estivation”.  All these terms are variations on the theme of dormancy. 
    Black bears do not hibernate.  Should you find an occupied bear den in the dead of winter, crawling inside to get a closer look might leave you surprised.  Skunks are this way, too.  They might provide a different sort of surprise.  This style of dormancy is called “torpor”. 
    The poikilotherms depend on some rather miraculous biochemistry to survive winters, and extremely hot summers in other parts of the world.  In the Lake States, they must find hidey-holes that don’t freeze, or undergo some miraculous chemistry so that they can freeze.  Some bury themselves in pond-bottom mud.  Their need for oxygen is much reduced but how they manage their oxygen-carbon dioxide balance is rather amazing.  Turtles can use their shells to help maintain tolerable blood chemistry.  Deeper, insulating snows improve the survival of these species.  Penetrating frost can be deadly. 
    Wildlife in cold regions of the world, which lack sufficient amounts of food, have three basic survival strategies.  They can migrate, they can sleep it off, or they can tough it out.  Nature has provided toolboxes loaded with handy adaptions for each of these strategies. 
    Trees also have specialized adaptations to survive the cold and drought of winter, as well as limb-shattering heavy snowloads.  The structures and chemistry vary between conifers and broad-leafed trees.  One of the common strategies is to dehydrate cells.  Water freezing between cells is OK but water freezing inside a cell is death. 
    No matter who you are in the northern climates, winter extends a survival gauntlet.  The miraculous ways that nature has provided for that challenge is stunning. 

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TRAILER- This website was created by a consortium of forestry groups to help streamline information about forestry and coordinate forestry activities designed to benefit the family forest owner and various publics that make up our Michigan citizenry.  This website is maintained by Bill Cook, Retired Michigan State Extension Forester/Biologist.  Direct comments to cookwi@msu.edu.