When the cold weather arrives and resources become scarce, a battery of strategies is activated in the animal kingdom that, to put it bluntly and badly, is no small feat. The most famous is hibernation, a state of extreme energy savings that allows us to survive long, harsh winters. Although we usually think of bears, the reality is much broader and more fascinating.
In this guide, we'll unravel what hibernation is, how it differs from torpor, brumation, aestivation, or diapause, and which species practice it. You will see amazing examples (from marmots to frogs that tolerate ice in their blood), the physiological control behind it, and how climate change and human action are altering these cycles.
What is hibernation?
Hibernation is a physiological state of very low metabolic activity which appears, above all, in temperate zone mammals with marked winters. During this period, body temperature drops significantly, the heart beats much slower, and breathing becomes shallow. The animal remains motionless in a den, barely conscious and not moving.
This state can last weeks or months, depending on the latitude and climate of the year: in Arctic regions it can last up to seven months, while in temperate zones it usually lasts two or three. The fuel to get through winter is the fat reserves accumulated in the previous months, which are mobilized to maintain basic needs.
It's not about "sleeping a lot", but about a profound metabolic depression with radical drops in heart and respiratory rates, and internal temperatures close to room temperature in true hibernating species. Even so, many animals undergo periodic micro-awakenings to readjust critical functions.
Hibernation and other states of dormancy
Under the umbrella of dormancy are included several adaptive responses that temporarily suspend growth, development and activity. La hibernation is one of them, but not the only one. It's important to distinguish terms clearly so as not to confuse concepts.
Hibernation (homeothermic mammals)
Typical of animals warm-blooded (homeotherms)It allows the organism to remain alive at a low but controlled internal temperature, with minimal energy expenditure and reliance on fat reserves (including brown fat, which is key for subsequent "rewarming"). It is typical in small mammals (marmots, dormice, and certain bats), although there are some special cases, such as bears.
Lethargy or torpor
Lethargy is a shallower and shorter-lasting torpor This can be daily or seasonal. Some carnivores, such as bears and badgers, enter a "winter sleep" with a marked reduction in heart rate and breathing, but without the drastic temperature drop of a strict hibernation; they can be awakened quickly and deliberately if there is disturbance or threat.
Brumation (reptiles and amphibians)
In animals of cold-blooded (poikilothermic)For reptiles and amphibians, the winter analogue is brumation. Their body temperature follows the ambient temperature, their activity level drops significantly, and they can go for long periods without eating, although they sometimes hydrate or sunbathe if temperatures rise. It's not a typical hibernation: the physiology involved is different.
aestivation
It is the answer to the opposite extreme: heat and droughtSome animals reduce their metabolism and take refuge in humid galleries or chambers during the hottest season, saving water and energy until favorable conditions return.
Diapause
Typical of many arthropods (insects, spiders, some crustaceans) and certain snails. It is a pause in development and metabolism which can occur in different phases (egg, larva, pupa or adult). In wild beesFor example, the young remain underground or in cavities with their metabolism almost stopped until spring.
Why do animals hibernate?
The main reason is the seasonal food shortages coupled with intense cold. In winter, foraging can cost more energy than it provides. Hibernation allows us to "shut down almost everything" and draw on reserves until the environment recovers.
In addition to avoiding risks of adverse weather, hibernation protects against predators (by remaining hidden in stable dens) and, in some species, synchronizes reproduction: female bears giving birth during winter sleep, ensuring that the offspring are born in a relatively safe and temperate environment.
Beware of habitat alteration: deforestation and intensive agriculture open clearings and fragment forests, destroying hibernation sitesCaptivity can also disrupt the natural cycle; some rescued bears, after years of confinement, lose their hibernation pattern and need time to recover in appropriate sanctuaries.
Physiological control: brain, hormones and brown fat
The central “switch” for hibernation lies in the hypothalamus, a structure in the brain that orchestrates the body's automatic functions and maintains connections with the pituitary gland. Injuries to this area prevent hibernation and alter the physiological variations typical of hibernation.
During hibernation the patterns of hypothalamic neurosecretion and the hormonal signals that regulate metabolism, temperature, and use of energy substrates. For progressive awakening, brown fat (thermogenic tissue) "turns on" the animal, producing heat and raising its temperature until it returns to normal activity.
Waking up at the wrong time can be lethal: the energy expenditure of arousal is multiplied, burning reserves rapidlySome small mammals, after long, cold hibernations, need days to recover digestive function before returning to normal feeding.
Emblematic species and cases
Bears: the most famous “winter dream”
Bears are the icon of winter, but theirs is a prolonged torpor rather than a strict hibernation. Their body temperature drops only a few degrees (maintaining above ambient), although their pulse rate can drop from 40 to 8–10 beats per minute, and they can go up to six months without eating, drinking, urinating, or defecating. In particular, Bears They prepare for the period with excessive consumption in autumn to accumulate reserves.
This extreme economy allows females give birth in the den and raise puppies safely. Their preparation includes overeating in the fall to accumulate fat and conditioning the cavity with vegetation for insulation.
Marmots: masters of hibernation
If we think of true hibernation, the marmots They are on the front line. They can pass eight months underground, with heart rates of just 3–4 beats per minute and very low temperatures, alternating episodes of deep sleep with micro-awakenings.
Bats: Savings Specialists
Many bats (especially those in the family Vespertilionidae) hibernate in caves or tree hollows with high humidity and stable temperatureIn winter, they reduce their pulse and breathing to a minimum, hibernating for around six months in cold latitudes.
Torpor is under their control: they can be awakened by danger or a temperature change, but this increases their metabolism and compromises their reserves. Human disturbances (such as entering winter colonies) can trigger arousals and cause subsequent starvation mortality.
In Chile, hibernation of native species is little studied, although there is evidence of daily torpor in Myotis chiloensis and Histiotus magellanicus in cold weather, suggesting a capacity for deep states in winter. A striking finding: hibernation can slow down biological aging in bats, according to recent work.
Hedgehogs: architects of the nest
Before the cold, the hedgehogs They eat conscientiously to accumulate fat and build nests with leaves and straw where they spend weeks or months lethargic. The isolation of the shelter It is as important as the energy buffer to maintain vital functions to a minimum.
Frogs: ice tolerance
Garter Snakes: Community Brumation
Garter snakes perform brumation and unlike others, they do it in large aggregationsHundreds or thousands gather in dens to spend the winter, reducing their metabolism and movement to a minimum, with occasional outings if the temperature rises.
Turtles: Breathe without breathing
In several turtles (such as box turtles) brumation can last from three to five monthsThey slow their pulse rate to 5–10 beats per minute and can suspend pulmonary respiration for long periods, taking advantage of the exchange of oxygen across body surfaces (such as the skin) in cold, well-oxygenated waters.
Snails: epipragma to the rescue
Many snails attach and seal their shell with a layer of hardened mucus, the epipragma, which retains moisture and limits energy loss. It's a lifesaver in both cold and dry periods, and reactivates when better conditions return.
Bees and bumblebees: diapause and cluster
In bumblebees, all males and workers die in the cold, while the hibernating queen (diapause) underground until spring, when a new colony is founded. Among honey bees (Apis mellifera), the colony doesn't "shut down": it forms a thermal cluster that surrounds the queen within the hive, with workers rotating positions and consuming reserves.
wild and solitary bees They follow a different script: the adults die at the end of the season and leave offspring in cavities or soil. The young hibernate in diapause, sometimes as pupae and other times as non-emerging adults, with their metabolism almost halted until the weather permits.
Flat-tailed dwarf lemur: the hibernating primate
Madagascar is home to a unique case: the flat-tailed dwarf lemur, the only known primate It hibernates. It takes advantage of this strategy to cope with the dry season, lowering its temperature and heart rate for long periods when food is scarce.
Squirrels: from storage to supercooling
Arctic ground squirrels challenge physiological limits with a controlled supercooling in galleries under the snow. Other squirrels, on the other hand, choose to store food and alternate between prolonged sleep and occasional outings to feed.
Birds: from torpor to a notable exception
In general, birds do not hibernate: many solve the winter with migration or brief episodes of torpor. The famous exception is the nightjar (Phalaenoptilus nuttallii), capable of prolonged seasonal lethargy resembling hibernation.
Badgers and rabbits: torpor that is not hibernation
Some medium-sized mammals, such as the European badger Rabbits, or rabbits, greatly reduce their activity in winter and take refuge in deep burrows. They don't hibernate strictly, but rather go into torpor, which allows them to react quickly to signs of danger or improving weather.
Little monkey: a living fossil with superpowers
The Chilean marsupial (Dromiciops gliroides), a relict of Chilean history, alternates between seasonal and daily torpor, even in summer. hibernate near 0 ºC and extend the period up to seven or eight months if fat reserves allow it.
It is social: it forms groups of 4–9 individuals in nests of quila and mosses, taking turns to provide heat to the group (the well-known “guatero effect”). If you wake up at an unusual time, your energy expenditure skyrockets and it may take days to recover full digestive function, a serious risk if there is no food available.
Geographical, environmental and climate change factors
The duration and depth of hibernation varies with latitude, altitude, and winter harshness. As winters become milder due to climate change, some species are shortening their dormancy period or adjusting their awakening time to resource availability, impacting reproduction and survival.
Compared to migration, hibernation avoids large travel costs, but requires live off reserves and find very stable shelters. Some species combine both: certain bats migrate to temperate zones and hibernate there, maximizing their options.
Plants and dormancy: the other winter
Plants do not hibernate, but they do go into hibernation. dormancy: They temporarily halt growth and activity to weather frost or drought. This dormancy can be "predictive" (activated before adverse conditions arrive, following photoperiod and temperature) or "consequential" (in direct response to adversity). In greenhouses, artificial light can disrupt this clock and "trick" the plant.
Habitat, conservation and our role
Hibernation requires suitable shelters: crevices, hollows, caves, burrows, and continuous forests. Fragmentation due to deforestation and intensive agriculture eliminates many of these critical sites. Supporting habitat protection, the restoration of ecological corridors, and real animal welfare sanctuaries helps prevent these cycles from being broken.
Hibernation, torpor, brumation, aestivation and diapause constitute a range of strategies and Interesting Facts finely tuned to climates and resources. From bears that don't eat for half a year From frogs that survive with ice in their tissues, to bees that form thermal clusters, and small marsupials that cooperate to keep warm, winter sleep epitomizes an extraordinary evolutionary intelligence that we need to understand and protect today more than ever.
