by Sam Archibald, Lead Field Educator

January. A time of cold somber mornings scraping ice from car windshields, days hunkered comfortably indoors or braving the snowy winds for a weekend ski, and cozy evenings by the wood stove. The aspen, willows, and cottonwoods are bare, the songbirds have largely left, and the bears, rodents, and reptiles slumber beneath the snow and dry dead grass. All is quiet.

As we settle into the depths of winter, it is often tempting to think of winter as a season of suspended dormancy. There’s an absence of light and warmth and even life. Many people – though certainly not all – count the days until the blooming warmth of spring. But our perspectives from heated apartments obscure the harsh reality most animals face. For northern temperate ecosystems like Yellowstone, winter is the most powerful force shaping the life, land, and ecosystems around us. It’s the season of survival and death. The conclusion for many species, be they old, sick, injured, or unfortunate enough to be caught in a late spring snowstorm or unusually deep freeze. Winter’s severity determines who can live here and who can’t. Harsh winters create barriers, preventing the spread of invasive species, pests, and pathogens.  Cumulative winters shape the genetic makeup (genotypes) and physical appearances and behaviors (phenotypes) of all who live here. On a landscape scale, winter snowpack determines what life will look like for the rest of the year. Approximately seventy percent of all the streamflow in Yellowstone comes from snowmelt; what happens during these quiet months echo throughout spring and summer.

Black bear at den site in the northern range, YF / Sam Archibald

Every species in Yellowstone has evolved to survive winter. Some species, like wolves, foxes, and otters thrive in the snow, benefiting from easier mobility and weakened or constrained prey. Other species like elk or coyote tolerate winter but often struggle, especially with any thaw-freeze events. Many avoid winter altogether. Most Yellowstone birds fly to warmer regions, sometimes travelling hundreds of miles. Mammals have fewer options, though long-legged ungulates can still migrate to lower elevations. Any form of migration is calorie-intensive and risky though. For less mobile animals, dormancy is often the solution. Many rodents hibernate, drastically lowering their body temperature, heart rate, and respiratory rate as their body shuts down to prevent losing too much heat to the ambient temperature. Reptiles brumate, grouping together and dropping their metabolism to extreme lows while retaining some cognition. Insects shift physics in their favor by voiding their bodies of any particles that might cause ice to form and using cryoprotectants like glucose or glycerol to lower their internal freezing point below 32° Fahrenheit. And some species like Boreal Chorus Frogs actually welcome ice. There’s no way to prevent freezing with permeable skin, but by pumping intracellular spaces full of glucose, these frogs can prevent damage while enduring winter frozen solid.

These various types of dormancies demonstrate the activity of passivity.  Each one is physically and physiologically complex. And fascinating. Take bears for example. Bears defy the easy dichotomy of dormancy. Like true hibernators, their heart and respiratory rates plummet down to around 8-19 heart beats with one breath per minute. Unlike true hibernators however, bears maintain a relatively high body temperature of around 91 degrees, only a few degrees lower than their standard temperature. And unlike rodents they don’t need to periodically rouse themselves to urinate, defecate, and sleep, though they are capable of waking within seconds if threatened. If humans were to try and sleep through the winter, we would quickly die of dehydration and a buildup of toxins. Urination is the body’s way of getting rid of urea and other waste or excesses. Bears have evolved to fully recycle their urea, converting it to usable protein. Beyond the problems of water and waste are the challenges of inactivity. Humans kept in bed all winter would quickly lose bone density and muscle mass due to inactivity and develop bed sores due to restricted blood flow. Worse yet, we would be highly vulnerable to heart attacks due to the hardening of arteries and extremely high cholesterol levels that come from burning fat for months. Bears experience none of that. They emerge from dens healthy and strong with muscle mass intact and only mild loss of body weight (15-30%) due to depleted fat reserves. Though it takes a few days for their metabolisms to ramp back up to normal levels, bears essentially emerge from the depths of winter ready to face a new breeding season.

That’s months away though. For now, the bears, rodents, reptiles, and others slumber – in some form or another – safely beneath the snow. They’re not awake, but they are actively fulfilling a marvelous life strategy that allows them to avoid Yellowstone’s harshest season. All is quiet.