Why a hibernator's metabolism falls further than its temperature alone would explain
When an animal cools down, its chemical reactions slow, so metabolism falls. But hibernators go further: they actively inhibit metabolism, meaning they suppress cellular activity beyond what the temperature drop alone would produce. The comparative analysis suggests that this metabolic inhibition is a major tool for hibernators, which rely on it extensively in addition to the effect of low body temperature [1]. Daily heterotherms—species that enter torpor only for part of a day—appear to rely mostly on the fall in body temperature to reduce metabolism, with possible exceptions in very small species and at high torpor body temperatures [1].
The balance between these two levers depends on body size and on the pattern of torpor. In small hibernators, both metabolic inhibition and a large drop in body temperature are used together to maximize energy savings. In large hibernators, metabolic inhibition seems to help trigger the fall in metabolism and body temperature at the start of a torpor bout [1]. This means two hibernators of different sizes can reach similarly low metabolic rates through somewhat different routes.
How low body temperature goes, and what sets the floor
The lowest body temperature a hibernator tolerates is not the same across species. In a study of 13 free-living bat species, the minimum defended temperature—the temperature below which the animal starts increasing its metabolism again to prevent further cooling—ranged from 8 °C down to below 2 °C [4]. That range means some bats routinely let their bodies approach freezing, while others defend a body temperature several degrees warmer.
What limits the floor? The evidence points to a combination of factors rather than a single hard boundary. The temperature at which metabolism is lowest varied among bat species even though their minimum metabolic rates were similar, suggesting that species differ in the body temperature they are built to tolerate or defend [4]. Water loss also differs: bat species clustered into high and low evaporative water loss groups, and the high-loss species include those that have suffered population declines from white-nose syndrome [4]. That pattern is consistent with water balance playing a role in how long and how deeply an animal can hibernate, though the study does not establish cause.
Birds and hedgehogs show the same principles in different settings
Hibernation is not limited to mammals. A comparison of hibernation in birds and mammals describes metabolic adjustments and body temperature changes across species, noting that some birds reduce body temperature and metabolic rate substantially during inactive periods [2]. The comparison supports the broader picture that the same physiological levers—metabolic suppression and controlled cooling—appear across distant animal groups.
Hedgehogs from semi-arid and temperate habitats have also been studied for their hibernation patterns, body temperatures, and oxygen consumption, showing that energy expenditure and temperature regulation during hibernation can differ between species living in different environments [3]. These comparisons reinforce that hibernation depth is tuned to local conditions, not fixed for all hibernators.
Sources used in this answer
Metabolic Rate and Body Temperature Reduction During Hibernation and Daily Torpor
Hibernators rely extensively on metabolic inhibition in addition to body temperature reduction, while daily heterotherms rely mostly on the fall in body temperature, with the balance depending on torpor pattern, torpor state, and body mass.
Hibernation in birds: comparisons with mammals
Hibernation in birds and mammals involves metabolic adjustments and body temperature changes, with some species reducing body temperature and metabolic rate during inactive periods.
Hibernation patterns and energy expenditure in hedgehogs from semi-arid and temperate habitats
Hibernation patterns, body temperatures, and metabolic rates were measured in two hedgehog species from semi-arid and temperate habitats, showing species differences in hibernation physiology.
Interspecific variation in evaporative water loss and temperature response, but not metabolic rate, among hibernating bats
Among 13 bat species, minimum torpid metabolic rates were similar, but the temperature associated with minimum metabolic rate and the minimum defended temperature varied, with minimum defended temperatures ranging from 8 °C to below 2 °C; evaporative water loss also varied and clustered into high and low groups.