Hibernation is a biological strategy where an animal enters a state of metabolic suppression to survive prolonged environmental stress, such as cold temperatures and food scarcity. The capacity for hibernation is entirely inherited, encoded in the species’ genetic makeup. However, the timing and initiation of this genetically fixed ability depend on external environmental cues. This distinction highlights that while an animal cannot learn to hibernate, it must respond to the outside world to activate the complex biological programming it was born with.
The Genetic Blueprint for Hibernation
The ability to become a hibernator is an innate, genetically programmed trait present from birth. This capacity relies on the differential expression and specialized regulation of genes common to all mammals. Scientists have identified specific genetic elements, such as “parallel accelerated regions,” that act as regulatory switches for genes involved in metabolism and fat control. These regulatory regions are often located near the fat mass and obesity-associated (FTO) locus, which is crucial for managing energy storage and utilization.
The genetic programming dictates the complex physiological pathways required to survive hypothermia and metabolic slowdown. For instance, the genes that govern brown adipose tissue (BAT) must be highly regulated for non-shivering thermogenesis, the process that allows for rapid rewarming. An animal that lacks this precise genetic instruction set cannot suppress its metabolism and drop its body temperature without suffering severe damage.
Environmental Cues That Trigger Torpor
While the ability to hibernate is inherited, the initiation of the hibernation season is regulated by external environmental signals. The most important factor is the decreasing photoperiod, or the shortening of daylight hours, which acts as a reliable calendar signal for the approaching winter. Dropping ambient temperatures and reduced food availability also contribute to the decision to enter the dormant state. These external cues are translated into internal biological signals that trigger the animal’s preparation phase.
Hormonal changes act as internal messengers, translating the external cues into physiological action. For example, altered balances of appetite-regulating hormones like leptin and ghrelin signal the body to prepare for the shift. This preparation involves hyperphagia, or excessive eating, to build up the necessary fat reserves that fuel the animal through dormancy. Environmental cues set the inherited biological clock, ensuring the animal enters torpor at the optimal time for survival.
The Physiological State of Deep Hibernation
True deep hibernation, or torpor, is defined by radical and sustained biological changes that dramatically reduce energy expenditure. The animal’s metabolic rate can fall to as little as 1% to 4% of its active rate, a necessary reduction to conserve the limited energy stored as fat. Correspondingly, the core body temperature plummets, often dropping to between 2°C and 10°C, and sometimes even below 0°C. This deep cooling allows the animal to match its energy consumption to the low ambient temperature.
The cardiovascular system slows profoundly, with the heart rate dropping from 200 to 400 beats per minute down to just 3 to 10 beats per minute. This severe suppression of vital functions is interrupted by periodic arousals, brief episodes where the animal rapidly rewarms itself. These interbout arousals are metabolically costly but are necessary for essential biological functions, such as immune system maintenance or DNA repair. The rapid rewarming is accomplished through the activation of specialized brown adipose tissue, which generates heat via non-shivering thermogenesis.
Spectrum of Hibernation Across Animal Species
The genetic basis of dormancy is demonstrated by the wide spectrum of states found across the animal kingdom, each fixed by species. True hibernators, such as groundhogs, marmots, and certain bats, undergo a profound physiological shift into deep, multi-day torpor with a massive drop in body temperature. This deep state is genetically distinct from other forms of dormancy.
Daily Torpor
Small mammals and birds, like hummingbirds and some mice, use “daily torpor.” This is a shallow, short-term reduction in metabolism and body temperature, typically lasting only a few hours overnight to save energy. It is a daily response to a lack of food or cold, not a seasonal event.
Winter Lethargy
Conversely, animals like bears enter a state often called “winter lethargy” or “winter dormancy.” They remain largely inactive, but their body temperature drops only slightly, perhaps by 5°C to 7°C. This slight drop allows them to wake up quickly, underscoring that the specific expression of dormancy is a fixed, species-specific, and inherited trait.