Cold slows muscle before it stops it
In heterothermic bats, whose body temperature swings with hibernation, muscle performance is tied directly to how cold the animal is. The work on these bats examined how motor function holds up at low body temperatures and where it breaks down, and it points to tetanic tension and power output as the capacities that suffer as temperature drops [1]. In plain terms, a cold muscle can still twitch, but it cannot summon the sustained force or the fast, powerful contraction that vigorous movement needs.
That matters because it separates two things people often lump together. An animal can be alive and even responsive at a low temperature while being unable to perform demanding movement. The bat work frames the limit as a property of the muscle itself, not simply of the animal's willingness to move [1].
Some neural jobs keep running near 4°C
The picture is not one of uniform shutdown. In thirteen-lined ground squirrels during torpor, body temperature can fall to about 4°C while the animals reduce metabolism and slow their heart rate [2]. Even so, the brain region that controls release of antidiuretic hormones — the supraoptic nucleus — becomes active early in the transition from torpor back to normal body temperature. Hormone release from the pituitary and rising plasma hormone levels all begin before body temperature climbs back to 10°C [2].
This is a striking contrast with muscle. Processes that are described as prohibitive in non-hibernators are still coordinated at an extremely low body temperature in these squirrels [2]. The finding concerns hormonal and neural control of fluid balance, not movement or heat production, so it should not be read as evidence that cold squirrels can move well. It does show that the temperature floor for function depends on which system you are asking about.
Rewarming is where heat production and movement meet
Arousal from hibernation is an active, energy-hungry process. Work on excitability in mammalian hibernation links brain function at low body temperature to heat production and behavioral arousal during rewarming from hypothermia and during arousal from hibernation [3]. The implication is that the return to normal temperature is not passive warming; the animal's own physiology, including behavior, contributes to generating heat.
Metabolic flexibility in hibernation adds a balancing view: low body temperature helps cut energy expenditure, and the animal's thermal state reflects a balance between reduced metabolic heat production and heat-loss processes [5]. So heat production during rewarming is best understood as something the hibernator ramps up deliberately, against a background of strongly suppressed metabolism.
There is also a hint that the gut can influence this machinery. In mice, which do not naturally hibernate, gut bacteria were reported to trigger a torpor-like state involving thermoregulation and reduced physical activity, with effects on neurons and on heat production in brown fat [4]. Because this is a torpor-like state induced in a non-hibernator, it suggests a possible route by which the body dials heat production and activity down and up, rather than describing what wild hibernators do.
Why zero is a different kind of limit
Below zero, the dominant risk shifts from sluggish physiology to ice. None of these studies directly measures function below 0°C, so the answer there has to be framed carefully. The squirrel work shows that neural and hormonal processes can operate at 4°C, a temperature described as prohibitive for the same processes in non-hibernators [2]. That establishes that cold alone, down to near freezing, is survivable for some systems. It does not establish what happens once body fluids would freeze.
The bat muscle findings suggest the other side of the constraint: even before freezing, the force and power a muscle can deliver fall away as temperature drops [1]. Put together, the evidence is consistent with a floor set by two things — the collapse of muscle power and coordinated movement, and the need to avoid ice forming in tissues — rather than by a single temperature threshold. The exact point where each becomes decisive is not settled by these studies.
Sources used in this answer
Behavior and muscle performance in heterothermic bats
In heterothermic bats, low body temperature impairs muscle performance, with tetanic tension and power output identified as capacities affected by cold.
Neural control of fluid homeostasis is engaged below 10°C in hibernation
In hibernating thirteen-lined ground squirrels, the hypothalamic hormone-release pathway controlling fluid balance becomes active before body temperature rises back to 10°C during arousal from torpor, showing neural and hormonal function at extremely low body temperature.
Excitability in Mammalian Hibernation
Brain function at low body temperature in mammalian hibernation is linked to heat production and behavioral arousal during rewarming from hypothermia and arousal from hibernation.
Intestinal bacteria trigger a hibernation-like state in homotherms via the gut-brain axis
In mice, gut bacteria were reported to trigger a torpor-like state involving thermoregulation, reduced physical activity, neuronal effects, and heat production in brown fat.
Metabolic flexibility in mammalian hibernation
Hibernation thermal state reflects a balance between reduced metabolic heat production and heat-loss processes, with low body temperature contributing to lower energy expenditure.