Why small bodies get the most out of cooling down
Torpor is a controlled drop in body temperature and metabolism used to ride out cold or food shortage. The energy payoff depends heavily on surface area relative to volume: a small body has proportionally more skin exposed to the air, so it sheds heat fast and would burn a lot of fuel staying warm. Letting its temperature fall therefore saves a large share of its normal energy budget [1]. This is why small insectivorous bats, with their high surface-area-to-volume ratios, are strong candidates for daily torpor [1].
The same geometry that makes cooling cheap makes rewarming costly. During rewarming the animal must generate heat quickly, and that phase is energetically expensive and involves heat loss to the surroundings, so it eats into the savings from the cold period [1][2]. The cost of rewarming scales with body size through its effect on specific metabolic rate, meaning the per-gram price of heating back up differs across sizes [2]. In practice, an animal only benefits from torpor if the energy saved while cold exceeds the energy spent getting warm again.
Rewarming is the bill that comes due
Because rewarming is the costly part, animals appear to weigh it against conditions and their own reserves. In eastern long-eared bats, lower ambient temperatures were the main driver of nightly torpor use, but rain, wind, humidity, falling barometric pressure and bright moonlight also increased time spent torpid, consistent with bats integrating several cues about energy benefits, prey availability and predation risk [3]. Interactions with body mass suggested that individual size and energy reserves partly shaped these nightly decisions [3].
Rewarming speed itself is not simply a matter of size. Across seven insectivorous bat species of differing body mass along an altitude gradient in South Africa, rewarming rates did not vary significantly between species or altitudes, even though body mass and life history differed [1]. Males rewarmed faster than females at low temperatures, possibly because females were reluctant to start active rewarming in the cold [1]. There was also some support for cooler high-altitude conditions producing lower initial surface temperatures during cold exposure, which would reduce heat loss through conductance to the air, an advantage for small bats [1]. So body size sets the broad energy arithmetic, but behavior and sex can shift the details.
Where the upper size limit for hibernation sits
The logic points to a ceiling: as an animal gets larger, its relative heat loss falls, so the energy saved by cooling shrinks, while the absolute cost of rewarming does not disappear. At some size the balance tips and deep hibernation stops being worth it. The marsupial review ties body size to how deep and how long torpor can go, framing torpor as one energy-demanding process among several such as growth and reproduction [4]. That is consistent with size constraining torpor depth and duration rather than with hibernation being available to any mammal.
None of these studies pins down a precise body-mass cutoff for hibernation. The bat work addresses rewarming energy and does not examine an upper size limit [1]; the hibernation review addresses body size and rewarming costs but not a ceiling [2]; the nightly-torpor study addresses environmental and individual drivers of torpor use but not a size limit [3]; and the marsupial review addresses size effects on torpor depth and duration but not an upper bound [4]. The honest answer is that the ceiling is set by the shifting balance between heat saved and heat repaid, and the exact mass where that balance fails is not established by this evidence.
Sources used in this answer
Rewarming rates of seven insectivorous bat species along an altitudinal gradient in South Africa
Across seven insectivorous bat species along an altitude gradient, rewarming rates did not differ significantly by species or altitude despite body-mass differences, though males rewarmed faster than females at low temperatures and cooler high-altitude conditions gave some support for lower initial surface temperatures that reduce heat loss.
Mammalian hibernation
In mammalian hibernation, body size affects specific metabolic rate and thus the cost of rewarming from torpor, which shapes the energy feasibility of the strategy.
Nightly torpor use in response to weather conditions and individual state in an insectivorous bat
In free-ranging eastern long-eared bats, lower ambient temperature was the main driver of nightly torpor use, with rain, wind, humidity, falling barometric pressure and bright moonlight also increasing time spent torpid, and body-mass interactions indicating that size and energy reserves partly shaped torpor decisions.
Torpor in marsupials: recent advances
In marsupials, body size affects torpor depth and duration, and rewarming is among the energy-demanding processes that shape torpor use.