Biology
Torpor Bout Duration and Rewarming Costs in Hibernating Ground Squirrels
Quick fact
A hibernating ground squirrel's body temperature can plummet to near freezing (as low as -2°C), yet it arouses every 1–2 weeks, during which rewarming can consume up to 90% of the energy saved during the entire torpor bout.
Why this is interesting
Imagine sleeping for weeks at a time, but every few weeks you have to wake up shivering for a day—why not just stay asleep? Ground squirrels face this exact puzzle, and their answer is a delicate energy balance.
Read the full explanation
Understanding Torpor Bout Duration and Rewarming Costs in Hibernating Ground Squirrels
Hibernation isn't one long sleep; it's a series of torpor bouts. Each bout lasts from days to a couple of weeks. During a bout, the squirrel's metabolism drops to a tiny fraction of normal, and body temperature falls close to ambient. But every so often it wakes up, shivers severely, and warms back to ~37°C for a brief period (a few hours) before diving back into torpor. This pattern is like a savings account where you deposit energy slowly but make a big withdrawal each time you visit the bank. The withdrawals are the rewarming costs. If rewarming were free, the squirrel would just stay cold until spring, but it must restore its body functions, clear waste products, and perhaps replenish immune defenses. So the question becomes: how long can it stretch the intervals between costly rewarmings?
A deeper explanation
The duration of a torpor bout is not random; it's controlled by the hibernator's 'thermostat' in the brain. At the start of hibernation, the set point for body temperature is actively lowered, so the squirrel can let its Tb drop. But over time, the set point gradually rises, and eventually the squirrel reaches a threshold where it 'decides' to rewarm. This rewarming is fueled by heat generated mostly via non-shivering thermogenesis in brown adipose tissue (BAT) and shivering. The energy cost of this arousal is substantial because the animal must reheat a large body mass and restart all cellular processes. Interestingly, bout length is not fixed: it is longest in mid-winter when ambient temperatures are coldest and metabolic rate is lowest, and shortens in late winter and early spring as the animal prepares to emerge. The trade-off is that longer bouts save more energy but may accumulate waste products or increase the risk of oxidative damage, whereas more frequent arousals are safer but use more energy. This mechanism explains the regular cycling and highlights a fundamental trade-off in hibernation: the deeper and longer the torpor, the greater the potential savings, but the larger the cost of each rewarming event.