Plenty of animals sleep through bad weather. The woodchuck does something far more extreme: it shuts itself down almost completely, for months, in a sealed room it dug for the purpose.
True hibernation, not a long nap
Biologists distinguish between true hibernation and lighter winter dormancy, in which an animal sleeps deeply but wakes readily. The woodchuck is firmly in the first category.
Body temperature falls steeply from its normal level. Breathing slows to a fraction of the waking rate. The heart drops from roughly 80 beats per minute to only a handful. Metabolism runs at a small percentage of normal, and the animal is genuinely difficult to rouse.
The chamber
Hibernation happens in a dedicated chamber, dug below the frost line so that the surrounding soil never freezes. The woodchuck seals itself in, and the earth above does the insulating.
This is a large part of what all that excavation is for. The 700 pounds of displaced soil buys, among other things, a room that stays above freezing while the world above it does not.
Running on fat
Nothing is eaten for the duration. The entire winter is financed by fat accumulated during the autumn binge, which is why late-season feeding is so relentless.
The arithmetic is unforgiving: a woodchuck that fails to build sufficient reserves does not wake up. Emerging animals in spring are noticeably lighter than the ones that went under.
Waking up
Emergence is triggered largely by rising soil temperature at burrow depth rather than by anything happening at the surface, which is why a warm spell in midwinter does not bring woodchucks out.
It is also the grain of truth inside Groundhog Day. A woodchuck surfacing early has detected something real about the season - just not by looking at its shadow.
Frequently asked
Do woodchucks hibernate?
Yes, and they are true hibernators rather than light winter sleepers. Body temperature falls steeply, breathing slows dramatically, and the heart rate drops from roughly 80 beats per minute to only a few.
How long do woodchucks hibernate?
Several months, broadly spanning the coldest part of the year. They remain sealed in a hibernation chamber dug below the frost line and eat nothing for the entire period, living on fat accumulated during autumn.
What wakes a woodchuck from hibernation?
Emergence is driven largely by rising soil temperature at burrow depth rather than by surface conditions, which is why a brief warm spell in midwinter does not bring them out.