Why Does a Hibernating Animal Keep Waking Up? The Most Expensive Part

A hibernating ground squirrel is not simply asleep, since its body temperature falls to within a degree or two of its surroundings, sometimes below freezing, its heart rate drops from hundreds of beats a minute to a handful, its breathing becomes intermittent with long pauses, and its metabolism falls to a small percentage of normal. That state is maintained for days or weeks, and then, without any external trigger that anyone can identify, the animal warms itself back to normal temperature over a few hours, stays there for perhaps twelve to twenty four hours, and then cools down again into another bout. This cycle repeats throughout the season, giving a dozen or more interruptions in a winter.

Rewarming is the dominant expense of the whole season:

• •Heating a whole body from near freezing to normal requires an enormous burst of energy
• •Specialised brown fat exists largely to produce that heat directly
• •Estimates attribute the majority of total winter energy use to these episodes
• •The dormant periods themselves are comparatively cheap
• •An animal that could skip them would save a large fraction of its reserves
• •The fact that none do implies the interruptions are doing something necessary

Several hypotheses compete and the evidence does not clearly favour one. Sleep debt is among the most discussed, on the observation that animals entering these periods show brain activity characteristic of sleep deprivation and spend the warm interval sleeping, suggesting that the cold state is not restorative in the way sleep is. Waste accumulation is another, since processes that clear metabolic products may not function at low temperature. Immune function is a third, because immune responses require normal temperature and dormant animals cannot fight infection. Restoring depleted substances, rebalancing electrolytes, and simply eating or drinking in species that store food all appear in the literature, and the honest position is that the question remains open.

The strategies among animals that survive winter dormancy are more varied than the single word suggests. True deep dormancy with body temperature near ambient is found in ground squirrels, marmots, dormice, hedgehogs, bats and a small number of others. Bears lower their temperature only a few degrees and stay comparatively alert, which is a different physiological state entirely and one where the animal does not eat, drink, urinate or defecate for months while remaining rousable. Daily torpor, a shallow version lasting hours rather than weeks, is used by hummingbirds, some bats and small mammals to survive cold nights. One bird, the common poorwill, enters extended dormancy, which is the only well-documented case among birds. Some reptiles, amphibians and fish enter comparable states with entirely different mechanisms.

The research attracts funding well beyond zoology because the physiology involved would be extremely useful if it could be understood. Animals in this state tolerate body temperatures, oxygen levels and blood flow that would cause severe injury in a human, and they emerge without damage to heart, brain or kidneys, which is directly relevant to stroke, cardiac arrest and organ preservation for transplant. They lose remarkably little muscle and bone despite months of immobility, which is relevant to long-term bed rest and to spaceflight. Their resistance to the damage caused when blood flow is restored to a deprived tissue is studied intensively. And induced dormancy for long-duration space travel remains a serious research interest rather than only a fictional one.

Dormant animals let their temperature fall close to their surroundings, then rewarm completely for a day or so before cooling again, repeating this a dozen or more times a winter. Rewarming consumes most of the energy the season saves, so the interruptions must be doing something necessary. Sleep debt, waste clearance and immune function are the leading explanations and none is settled.