Why mild weather and plentiful food don't simply end torpor
It is tempting to assume that a well-fed animal in a mild winter would abandon torpor entirely. A study of fishing bats (Myotis macropus) in mild Australian winter conditions tested exactly this, predicting that stable water temperatures would make winter foraging more reliable and that the bats would therefore use torpor less than other bats [1][2]. Instead, the bats entered torpor every single day, with bouts lasting a median of about 21 hours and stretching as long as 145 hours [1][2].
That result matters because it shows torpor is not just an emergency response to cold and hunger. Even when food was likely available, the bats kept using it extensively [1][2]. The authors suggest torpor in this setting may function to minimize the risks of mortality caused by activity and to help build body condition for the coming breeding season [1][2]. In other words, staying cold and quiet can be worthwhile even when a meal is possible.
The trade-off: saving energy versus the risks of being active
Torpor is a controlled, reversible drop in body temperature and metabolic rate that delivers substantial energy savings, and it is used more when temperatures are low and prey is scarce [1][2]. But the fishing-bat findings suggest the ledger has a second column: the dangers of being awake and moving. If activity carries real mortality risk, then torpor can be the safer choice even when energy is not the binding constraint [1][2].
A modeling study of hummingbirds frames torpor the same way, as a balance between energy savings and non-energetic risks, and explicitly compares the logic to an economic cost-benefit calculation [3]. The model uses lipid (fat) thresholds and metabolic rates to predict how often and how long hummingbirds enter torpor, and it points to seasonal shifts in when torpor makes sense [3]. Together, these lines of evidence support the idea that animals regulate torpor by weighing savings against costs rather than by responding to temperature alone.
When extra food changes the balance
Direct evidence that food supply shifts torpor use comes from a supplemental feeding trial in the northernmost population of black-tailed prairie dogs [4]. The study examined variation in hibernation expression, including sex differences and the effect of pre-hibernation body condition, and found that animals in different energetic states differed in how much of the winter they spent in torpor and in how long and cold their bouts were [4]. That is the clearest sign in this evidence that improving an animal's energy situation can change its torpor behavior within a species.
The fishing bats add a complementary angle: even in a mild climate with likely food, torpor remained near-universal, and the timing of arousals—just after sunset, tending to be longer on warmer evenings—suggests some winter foraging and possibly mating-related activity was still occurring [1][2]. So extra food or warmth may loosen the grip of torpor, but it does not erase it; other needs, including breeding and safety, keep pulling animals back toward the cold, quiet state.
What this reveals about cost-benefit regulation
Across these studies, torpor looks less like a fixed winter switch and more like a dial tuned to circumstances. Mild conditions and better body condition are associated with changes in torpor use [4], while the hummingbird model shows that a simple rule based on fat reserves and metabolic costs can reproduce patterns of torpor frequency and duration [3]. The fishing bats show the other side of the ledger: when activity is risky, heavy torpor can persist even in gentle weather [1][2].
The practical takeaway is that predicting how a species will respond to a warmer winter or a richer food supply requires knowing both what energy it saves by staying torpid and what it risks by waking up. Where activity is dangerous or breeding demands are high, torpor may hold steady; where energy is the main constraint, extra food can shift the balance toward more activity and less torpor [3][4].
Sources used in this answer
Winter torpor and activity patterns of a fishing bat (<i>Myotis macropus</i>) in a mild climate
Fishing bats in a mild Australian winter used torpor every day, with bouts lasting a median of about 21 hours and up to 145 hours, despite likely food availability, suggesting torpor may minimize activity-related mortality risk and improve body condition for breeding.
Winter torpor and activity patterns of a fishing bat (Myotis macropus) in a mild climate
The same fishing-bat study reports that multiday torpor bouts were more common in females, arousals occurred just after sunset and tended to be longer in males and on warmer evenings, consistent with some winter foraging and possible mating-related activity.
The hummingbird's adipostat: can a simple rule explain torpor frequency and duration in hummingbirds?
A hummingbird model using lipid thresholds and metabolic rates suggests torpor frequency and duration can be explained by balancing energy savings against non-energetic risks, similar to an economic cost-benefit calculation.
Covariates of intraspecific variation in hibernation expression in the northernmost population of black-tailed prairie dogs
In the northernmost black-tailed prairie dog population, a supplemental feeding trial and body-condition measures showed that energetic state was associated with how much of winter animals spent in torpor and how long and cold their bouts were.