The science behind a long winter's nap.
By Tom Clynes
Take a solitary walk through any of America’s national parks in winter, and you may get the impression that you’re the sole wakeful presence in an otherwise sleeping world. For the most part, you’d be right. Although some predators are able to find food in wintertime, animals that don’t migrate generally spend the coldest months in a deep slumber, conserving energy.
The seasonal state of inactivity we call hibernation is characterized by decreased metabolism, lowered body temperatures, and slowed breathing rates. Once food becomes sparse, hibernating mammals seek out or excavate den sites that are, preferably, below the frost line and predator-proof.
Mammals can delay or even forego hibernation to adapt to local conditions. But reptiles have no control over the timing of their winter inactive phase, called brumation.
“Because they can’t generate their own heat, reptiles are completely at the mercy of the elements,” says Joseph T. Collins, director of the Center for North American Herpetology. “When the temperature outside decreases, their internal temperatures decrease and they involuntarily enter brumation.”
In desert parks such as southern Arizona’s Organ Pipe Cactus National Monument, snakes and lizards retreat underground to seek shelter from low temperatures. Some species brumate in groups, forming “hibernation balls” consisting of dozens of individuals. In mountainous eastern parks such as Great Smoky Mountains National Park, timber rattlesnakes form dens of up to 200 individuals, usually in deep crevices on rocky, south-facing slopes.
In autumn, as temperatures fall and sunlight dwindles, most frog species descend to the bottoms of ponds and lakes. The moist, porous film on their skin allows them to take in oxygen and release carbon dioxide—so it’s crucial to find a body of water that has plenty of dissolved oxygen and enough depth so that the bottom will not freeze.
But there are exceptions. In Rocky Mountain National Park, wood frogs spend winters on the forest floor, under a blanket of leaves and debris. When temperatures dip below freezing, water in their bodies freezes, too, making the frog appear dead and rocksolid; the frog doesn’t breathe, and its heart doesn’t beat. Yet in the tissues in and around the frog’s internal organs, an intricate metabolic process generates a mix of sugars and sugar alcohols that act like antifreeze, protecting the cell structure of the delicate organs. When temperatures warm, the frog thaws and awakens to spring.
Mammals don’t go through as dramatic a transformation, but they still have the ability to drastically lower their metabolism. Small animals like chipmunks and ground squirrels cycle in and out of hibernation, sporadically waking to eat stored food and relieve themselves. Yet because their body temperature drops so drastically, their awakening can take several hours. “You could pull a marmot out of its den and use it as a football for a long time before it would wake up,” says Kerry Gunther, a bear-management biologist at Yellowstone National Park.
Of course, Gunther would never try that with the brown and black bears he studies. A hibernating bear’s body temperature remains within 12 degrees of its normal temperature, allowing a bear to react quickly to threats. That can prove challenging when Gunther and other biologists visit dens during hibernation season to refresh batteries in the bears’ radio collars. “We’ve got less then two minutes from the time we reach the den opening to hit the bear with the jab stick and get it drugged,” says Gunther.
For years, some people didn’t consider bears to be true hibernators, because their body temperatures drop so little and they wake so easily. But many scientists now know better. “Bears are best hibernators of all,” says carnivore ecologist Jim Halfpenny, author of Yellowstone Bears in the Wild. “There’s no other animal that can go five or six months without eating, drinking, defecating, or urinating. They even give birth and nurse their cubs during hibernation.” Bear hibernation varies widely by subspecies, geography, and even individual. In Yellowstone, some brown bears will hibernate half the year, and others might get up for a mid-winter walk. On Kodiak Island, adjacent to Alaska’s Katmai National Park, male bears with access to abundant food sometimes forego hibernation completely.
Farther south, warm weather and prospects of a meal bring bears and other mammals out of their dens each spring. Frogs and turtles swim to the top of ice-free ponds, and snakes slither out into the sun, flattening their bodies to increase the surface area exposed to solar warmth. Once food and warmth are plentiful, these animals become active again. “At that point,” says Collins, “their thoughts turn to mating.”