Hibernation
The cusp of spring seems like a logical time for Groundhog Day. By this time of year, most of us are itching in our so-dry-it’s-scaly skin for the paradise of a sunny day that drags itself all the way into the shorts-and-flip-flops territory of the mid-40s. The urge for nicer weather isn’t the only reason we celebrate Punxsutawney Phil in February though.
This is also around the time groundhogs start emerging from their burrows, about 25% lighter in bodyweight than they were when they went in. They’ve spent the past four to six months in hibernation, undergoing miraculous anatomical feats to save energy throughout winter. During this dormancy, their body temperatures dropped from about 99 degrees Fahrenheit to 37. Their heartbeats slowed from 100 beats per minute to a plodding five. You’d be forgiven for thinking they stopped breathing altogether, because at a glacial two breaths a minute, it would be easy to miss.
To say hibernation is incredible might be underselling it. “Miraculous” is a better fit, because it’s genuinely one of nature’s miracles—and a topic that’s unfortunately slept on by most.
Apropos of that, let’s establish this from the get-go: hibernation is not sleep, though it’s an understandable misconception the two are one in the same given their shared similarities, including a decrease in physiological processes like breathing and temperature. However, this decrease is much less dramatic in sleep, and whereas hibernation is mostly defined by these physiological changes, sleep is driven by a change in brain activity.
The brain waves of hibernating animals, though slightly suppressed, show the same patterns as those they display when awake. In fact, sleep deprivation is a real risk for hibernating animals. Most spend about 80% of their saved energy sporadically waking up during their months of dormancy in part to catch up on their Zs.
So we know what hibernation is not. Let’s talk about what it is. (Murky, as we we’ll see.)
Scientists love to quibble over definitions, and hibernation is no different. The strictest definition categorizes it as a voluntary state wherein a mammal’s metabolic rate experiences a long-term drop to conserve energy during cold, food-scarce periods. A key element of this categorization is the voluntary nature and preparation involved. Animals don’t just suddenly fall into a coma-like state one particularly chilly day in November. They rely on cues from the environment, such as shorter daylight hours and falling temperatures, as well as their circannual rhythms to know when to start building up their brown fat and/or storing food.
However, this understanding leaves a lot out. Black bears, the poster children of hibernation, aren’t by this definition true hibernators at all. Their winter dormancy, categorized under the umbrella term “torpor,” is more akin to sleep in that it’s involuntary and less physiologically extreme; their body temperature only decreases by about 10 degrees, allowing them to waken much more quickly and to less stimulus. Many species of birds experience daily torpor, a sort of mini-hibernation that allows them save energy short-term.
Then there’s brumation—the hibernation of cold-blooded animals. Wood frogs practice one mind-blowing form of this. Unlike many other amphibians who seek protection from the extreme cold by overwintering deep underwater, wood frogs remain aboveground, toughing it out in the paltry shelter offered by forest floor debris. Here, they perform a feat resembling something pulled from a sci-fi novel: they freeze. Their body temperature drops to match the outside temperature, their hearts stop beating, and they stop breathing. They look about as alive as a block of ice.
And yet, come spring, wood frogs will gradually unthaw from this state of suspended animation and go about their business unscathed. How do they manage this sleight of hand against nature? Their bodies produce a combination of glucose and urea that acts as a sort of antifreeze, protecting their cells even as their tissue freezes.
Estivation is another hibernation-adjacent state, though it’s practiced not by overwintering animals but those of deserts and tropical climates who have to endure periods of extreme heat and drought. Take the lungfish of Africa, Australia, and South America. When their lakes dry out, lungfish burrow into the mud and secrete a mucus that envelops their bodies and allows them to stay moist. Thanks to these mucus sacs (gross, I know), lungfish can survive an astonishing three years without water.
However you choose to define it, hibernation remains a phenomenon that deserves far more recognition than it usually gets. As we toe our tentative way into spring and appreciate anew the warming weather, let’s also take a moment to remember winter—and the incredible ways our local wildlife survives it.
This is also around the time groundhogs start emerging from their burrows, about 25% lighter in bodyweight than they were when they went in. They’ve spent the past four to six months in hibernation, undergoing miraculous anatomical feats to save energy throughout winter. During this dormancy, their body temperatures dropped from about 99 degrees Fahrenheit to 37. Their heartbeats slowed from 100 beats per minute to a plodding five. You’d be forgiven for thinking they stopped breathing altogether, because at a glacial two breaths a minute, it would be easy to miss.
To say hibernation is incredible might be underselling it. “Miraculous” is a better fit, because it’s genuinely one of nature’s miracles—and a topic that’s unfortunately slept on by most.
Apropos of that, let’s establish this from the get-go: hibernation is not sleep, though it’s an understandable misconception the two are one in the same given their shared similarities, including a decrease in physiological processes like breathing and temperature. However, this decrease is much less dramatic in sleep, and whereas hibernation is mostly defined by these physiological changes, sleep is driven by a change in brain activity.
The brain waves of hibernating animals, though slightly suppressed, show the same patterns as those they display when awake. In fact, sleep deprivation is a real risk for hibernating animals. Most spend about 80% of their saved energy sporadically waking up during their months of dormancy in part to catch up on their Zs.
So we know what hibernation is not. Let’s talk about what it is. (Murky, as we we’ll see.)
Scientists love to quibble over definitions, and hibernation is no different. The strictest definition categorizes it as a voluntary state wherein a mammal’s metabolic rate experiences a long-term drop to conserve energy during cold, food-scarce periods. A key element of this categorization is the voluntary nature and preparation involved. Animals don’t just suddenly fall into a coma-like state one particularly chilly day in November. They rely on cues from the environment, such as shorter daylight hours and falling temperatures, as well as their circannual rhythms to know when to start building up their brown fat and/or storing food.
However, this understanding leaves a lot out. Black bears, the poster children of hibernation, aren’t by this definition true hibernators at all. Their winter dormancy, categorized under the umbrella term “torpor,” is more akin to sleep in that it’s involuntary and less physiologically extreme; their body temperature only decreases by about 10 degrees, allowing them to waken much more quickly and to less stimulus. Many species of birds experience daily torpor, a sort of mini-hibernation that allows them save energy short-term.
Then there’s brumation—the hibernation of cold-blooded animals. Wood frogs practice one mind-blowing form of this. Unlike many other amphibians who seek protection from the extreme cold by overwintering deep underwater, wood frogs remain aboveground, toughing it out in the paltry shelter offered by forest floor debris. Here, they perform a feat resembling something pulled from a sci-fi novel: they freeze. Their body temperature drops to match the outside temperature, their hearts stop beating, and they stop breathing. They look about as alive as a block of ice.
And yet, come spring, wood frogs will gradually unthaw from this state of suspended animation and go about their business unscathed. How do they manage this sleight of hand against nature? Their bodies produce a combination of glucose and urea that acts as a sort of antifreeze, protecting their cells even as their tissue freezes.
Estivation is another hibernation-adjacent state, though it’s practiced not by overwintering animals but those of deserts and tropical climates who have to endure periods of extreme heat and drought. Take the lungfish of Africa, Australia, and South America. When their lakes dry out, lungfish burrow into the mud and secrete a mucus that envelops their bodies and allows them to stay moist. Thanks to these mucus sacs (gross, I know), lungfish can survive an astonishing three years without water.
However you choose to define it, hibernation remains a phenomenon that deserves far more recognition than it usually gets. As we toe our tentative way into spring and appreciate anew the warming weather, let’s also take a moment to remember winter—and the incredible ways our local wildlife survives it.
First Published in the Pioneer Tribune on February 27, 2025