As humans, would we ever think of giving up eating, drinking, or even staying awake for months on end to survive a harsh cold winter? We may not even adopt this lifestyle, but some species in the animal kingdom have perfected the art and science of slumber, called ‘hibernation’. To define it in simple terms, hibernation is a state of minimal activity and metabolic depression. It is a seasonal heterothermy characterised by low body temperature, slow breathing and heart rate, and low metabolic rate. Some species adapt to this state during extreme winters.
Hibernation is considered an evolutionary adaptation that allows species like bears, bats, hedgehogs, and even some amphibians and reptiles, to survive the inhospitably cold climate without active foraging. It is a fascinating survival strategy that allows animals to survive harsh climatic conditions, or when food is scarce. Hibernation is a state of psychological and physical adaptation that helps the animals conserve energy and survive for long periods without food. It is an elongated process that has various stages.
STEPS IN HIBERNATION
Preparation: Before hibernation begins, animals go through many physical and psychological changes.
Storing fat: Animals consume large amounts of food for weeks, and sometimes even months, to store fat to survive the harsh winter. Besides, the fat also provides them with energy to sustain themselves during dormancy.
Building shelters: Before hibernating, animals seek or create a safe space such as dens, burrows or caves that will keep them warm during the freezing temperature.
Entering a state of torpor: Hibernation involves a state called ‘torpor’. A phase where animals significantly reduce their metabolic activities to conserve energy. It is a behavioural adaptation that helps animals survive cold weather and food scarcity. During this phase, they go through many behavioural changes --
Drop in body temperature: Hibernating animals experience a lowered body temperature, sometimes to as low as 0 degrees Celsius.
Drop in heart rate: During hibernation, the animal’s heart rate decreases from 110-150 beats per minute, to 5-70 beats per minute.
Breathing and movement: During this phase, the animals’ breathing becomes shallow, they move very little, and are barely conscious.
HOW LONG DOES IT LAST?
The duration of hibernation varies depending on the species, climatic conditions and environmental factors. For instance, bears are long-term hibernators, and one of the species that can hibernate for up to 7 months without eating, drinking, or excreting. Some of the studies conducted on black bears and grizzly bears also show that during hibernation, they reduce their metabolic rate by 75%, their heart rate is reduced to 8-10 beats per minute, and they solely rely on fat reserves for their energy. But, despite being immobile for months, these creatures do not suffer from muscle atrophy or bone loss, and this provides insights into preserving human health for a prolonged period; For eg: Astronauts in space.
A different study conducted on wood frogs in North America revealed that these creatures can freeze themselves during solid winters, where 70% of their body turns into ice. During this phenomenon, their body is protected by the antifreeze chemicals like glucose and urea that are present within. Today, scientists are trying to study the mechanisms behind this process, and its potential application in human cryopreservation and medical emergencies like preserving organs for transplantation.
Then there is another concept, called ‘periodic torpor’. Some animals like bats and ground squirrels wake up periodically during their hibernating phase to drink water and change their positions.
Bats like the ‘little brown bats’, experience torpor - a state of reduced psychological activity. These cycles are necessary to maintain immune functions and survive harsh winters. Some of the key findings from experiments also revealed that energy expenditure during arousal is a critical factor, as environmental disruptions can break this cycle, which in turn increases their mortality rates. Similarly, among squirrels, the arctic ground squirrels can supercool their bodies during hibernation, with their core body temperature dropping to as low as -3 degrees Celsius. This extreme hibernation helps them survive harsh winters with limited food. One of the key findings also revealed that their ability to suppress brain activity and protect cells from freezing, offers potential for medical cryopreservation and organ transplants. Meanwhile, yellow-bellied marmots that emerge sooner due to early snowmelt may have an increased chance of survival and reproduction, while Alpine marmots that emerge earlier may have a decrease in litter size and juvenile survival.
SURVIVING THROUGH EXTENDED PERIODS
Various research studies conducted on hibernation revealed that various psychological, molecular and ecological mechanisms allow the animals to survive this prolonged period. These are some of the key findings that were revealed from different studies.
Metabolic adaptations: Hibernating mammals exhibit significant depression in their metabolic rates to conserve energy during this period, when there is a lack of food to survive. They switch primary fuel sources from carbohydrates to lipids, especially in smaller mammals like ground squirrels, which experience dramatic body temperature reductions during torpor phases.
Molecular mechanisms: Studies on the presence of proteins like myosin in hibernating mammals revealed that adaptations such as phosphorylation enhance energy efficiency and stability in skeletal muscles during hibernation, which helps the muscles from degrading, despite prolonged inactivity.
Thermoregulation: Large mammals like bears can maintain the core body temperature when compared to smaller animals. This is because of the difference in ATP consumption rates in muscle tissues
Neurological and psychological insights: Research shows that specific brain regions regulate the induction and maintenance of hibernation. Techniques such as optogenetics have been used to manipulate these processes, suggesting potential applications in human medicine.
ENVIRONMENTAL IMPACT
Hibernation has many positive effects, such as helping the animals survive harsh conditions. But, this has its negative side as well. Hibernation can make animals more vulnerable to environmental changes that are caused by climatic change. Sarah Mohr, a graduate student in neuroscience at Yale University, says, “The truest, most extreme form of hibernator is called an obligatory hibernator. These animals enter hibernation on a strict seasonal cycle, but cannot be induced to enter the process by changing their environmental conditions or their food and water availability.”
Hibernation also impacts climate change
Earlier emergence: Climate change can cause animals to emerge from hibernation, earlier than they should, and this leaves them stranded in search of food and water.
Habitat destruction: Human activities such as factory farming can make it harder for animals to find suitable habitats to hibernate in.
Susceptible to climate change: Hibernating species that live at high elevations and latitudes are thought to be especially susceptible to climate change. These seasonal responses are further complicated by demography, since phenological and fitness responses to climate can vary across age classes (Rezouki et al., 2016) and sex (Sheriff, Richter, Buck, & Barnes, 2013)
