Animals hibernate to conserve energy, avoid competition for limited food resources, and escape predators. Hibernation is common in mammals, some birds, and invertebrates. In order to survive the long winter months of little food and cold temperatures, many species of animals employ the strategy of hibernation. The need to find adequate shelter before winter sets in often forces animals into a life or death decision between remaining active at a time when finding sufficient food is difficult or risking migration only to be overtaken by winter near its end.
How do they know that hibernation is necessary?
Many species of animals exhibit seasonal changes in physiology and behavior during the long winter months. Some species, like bears and ground squirrels, hibernate by entering an inactive state or ‘hibernation’ while others enter a state of reduced metabolism called ‘inactivity’ or ‘torpor.’ There are many different methods for inducing a state resembling hibernation in a mammal; however, most involve a reduction in body temperature and an accompanying decrease in bodily functions such as respiration, heart rate, digestion, etc.
Hibernation is effective
Hibernation is one of the most effective physiological techniques for conserving energy when food is scarce. Although external environmental temperatures can vary greatly from species throughout the year, in areas where winter months are long and severe (such as polar regions), it is not uncommon for regional averages for daily minimum and maximum temperatures during winter months to be extremely high low.
Hibernation conserves energy
When factoring in the cooling effects of wind and reduced solar radiation, the daily fluctuations in temperature are often minimal. In addition, the animals must deal with low food availability due to short days and snow cover. Therefore, hibernators can conserve up to 88% of their normal energy usage. For example, a 10-pound (4.5 kg) brown bear may only need to consume approximately 150 calories per day during hibernation, compared with 3,000 calories per day while active.
Additionally, many animals will experience a reduction in heart and respiratory rates while dormant since these rates are dependent upon core body temperature and vary with the activity level of the animal, a decrease in energy usage results. The respiratory rate for hibernators can be lower by approximately 50% of the normal resting rate.
The heart rate will also be significantly reduced, posing no problem for some species that hibernate underground during the winter months. For example, brown rats spend winters underground in their burrows or igloos and suffer from little or no disturbance from weather conditions. In addition to a reduction in metabolic activities, hibernating species will experience a reduction in the stress hormone cortisol and an increase in growth hormone levels due to hypomethylation of DNA. Changes like these help to ensure that the animal will have adequate energy reserves for the long winter months.
Saving energy and hibernation
Hibernation is a time when an animal is isolated from its surrounding environment and, in many cases, has no access to food unless it wakes up and actively goes out to hunt. Energy conservation may not be the only evolutionary reason for hibernating during the winter months; however, it is one of the most important. Without sufficient energy stores, the animal will have a difficult time surviving these long winter months. In fact, some animals that do not exhibit torpor or hibernation will die during winters with low energy reserves.
Hibernation and winter
In certain species, such as chickens, lack of food during the winter months does not affect the animal’s growth. This is the result of high-fat reserves in egg yolks and meat. In addition to producing energy through metabolism and stored fat, the hormones thyroxine and triiodothyronine are secreted by ovaries and testes from October until February. These hormones help regulate body temperature through feedback loops caused by a reduction in exercise. Therefore, without food during the winter months, physiological changes will also be minimalized as long as the animal keeps its metabolic functioning at a low level during hibernation.
Risks and benefits of hibernation
Although hibernation is more beneficial in many species than hunting, there are some lethal risks that must be overcome while a hibernator remains awake. Some predators will become very hungry and aggressive during the winter months. The most dangerous of these predators include other carnivores such as bobcats, lynx, and foxes. These animals are often larger than prey species such as mice and rabbits and may be able to catch them with little effort.
If an animal is hibernating in a subarctic environment, as many animals do, it will have all the resources (such as food) needed for survival during hibernation season. However, if an animal is hibernating in a temperate zone (such as many birds), then it will likely have increased hunting pressures during the winter months. Some species that exhibit seasonal changes in physiology and behavior can successfully deal with this problem by storing fat reserves during the summer months when food is most abundant. Moreover, some species will eat or supplement their diet with stored fats when food availability is low during the winter months.
Hibernation can be challenging
In addition to the physiological risks that are common in hibernators, seasonal changes in physiology and behavior can also lead to increased mortality rates. In some cases, this is because of the animals’ reliance on energy reserves (such as fat) which may be insufficient for long periods of hibernation. However, in many cases, temperatures increase during active seasons (such as summer), causing desert-dwelling animals to expel internal water stores.
This loss can lead to dehydration and death if no water is consumed. Hibernating species that inhabit colder regions must deal with low temperatures without losing internal water stores or suffering from dehydration. Additional factors that may lead to negative outcomes for hibernators include winter storms, prolonged periods of darkness, and winter starvation.
Survival of species during winter
Hibernation is not the only way animals can survive during the winter months. Some species will migrate to higher elevations where temperatures are much warmer. Others, with larger populations or a more localized habitat, may spend the winter in an area where food is plentiful. These animals may not benefit from hibernation but will survive because they have access to food during the active seasons. For example, migratory birds such as swallows are known to travel thousands of miles between their nesting and feeding grounds in the winter months.
In addition to the migration and hibernation strategies, winter-dwelling animals must also be able to survive the cold temperatures they experience. If an animal cannot maintain a normal body temperature during adverse weather conditions, it may die from hypothermia. This occurs when the body loses heat faster than it can produce heat or store it in fats. However, many animals are equipped with fur coats and other insulation to help them stay warm during long periods of idleness.
Can hibernation in animals cause death?
Freezing of internal water stores is one consequence of hibernation that can lead to death in small mammals like rodents and bats. This occurrence is common in torpid animals that inhabit cold climates because of the combination of low metabolisms, the production of urine, and the storage of water by muscle tissues. This can occur when an animal dies from prolonged exposure to cold weather and water freezes inside its tissues.
Another death-causing condition common to hibernating mammals is starvation due to overfeeding or lack of food. When there are no external factors or predators that lead to their deaths, some animals may die due to their own actions while hibernating.
Although some carnivores will survive the winter by eating their own body weight in food, they are still at risk if they do not eat enough during this short period of time. Insectivores such as owls and shrews typically feed on small prey items like seeds and insects that can sustain them during the winter months.
These species have relatively small bodies that can sustain cold temperatures without shutting down or overheating themselves for long periods of time. The extra energy liberated from digesting large quantities of food is used for thermogenesis. In addition, small carnivores like shrews have a higher level of glucose in the blood and tissues, which can be converted into heat via the Krebs cycle.
Maintaining body heat during hibernation
In order to survive hibernation, animals must protect their body heat from escaping. Mammals have a complex network of surface blood vessels that help their bodies to maintain ideal temperatures during the winter months. The circulatory system transports warm blood from the body’s core throughout the body, which aids in maintaining proper body temperature. To prevent costly energy expenditure, these mammals allow their muscles and organs to rest while they are inactive.
Cold-blooded animals have the ability to maintain body temperatures through the control of their metabolism. In the case of hibernating mammals, these animals can reduce their metabolic rate by as much as 30 % in order to reduce energy expenditure and lower their body temperatures. However, animals are more susceptible to freezing when less energy is used because they do not generate heat with minimal activities performed by tissues.