Different Types of Hibernating Animals

The definition of hibernation in animals is the ability of some species to enter a state of prolonged torpor. The reason why animals hibernate is due to climactic changes, specifically when the temperatures drop during the winter period. This state of torpor requires the animal to slow down their metabolic functions and resembles a state of hypothermia since their body temperature drops considerably. Hibernation can last from a period of days to months.

Bodily processes of hibernating animals need to slow to glacial levels. This is due to a need to conserve their energy and survive. The main changes during hibernation include:

• Metabolism: drops to a very low level. Since the animal doesn't move to any great extent, they do not require much energy, but their internal systems still need to function. It is for this reason animals will eat a lot before hibernating since it provides them with the reserves they need during this period.
  
  
• Digestion: although metabolism continues, the digestive process essentially stops. This means they do not need to urinate or defecate during hibernation.
  
  
• Respiration: the animals will still need to respire to survive, but the process of respiration slows down to almost imperceptible levels. It is for this reason we often think hibernating animals appear to be dead as we cannot see them breathing.
  
  
• Temperature: body temperature drops to very low levels so heat can be concentrated in the vital organs.

For the hibernation process to be successful they must have increased the fat reserves stored in their bodies during the warmer months. When they come out of torpor, their weight will have decreased and they will be low on energy. It is generally difficult to wake animals from a state of hibernation it is difficult to wake them up and they can be manipulated. Some animals will wake up periodically to consume food reserves stored in their dens. When squirrels hibernate in winter, they wake to consume the nuts they have been storing up during warmer months.

It is important to emphasize that hibernation is not one single process. Although all involve the temporary suspension of biological activity, this is a general state known as dormancy. Hibernation is just one type of dormancy in animals. One key factor is whether the animal is homeothermic (mostly endothermic animals), meaning they are warm-blooded. We will explain more in the following sections.

Dormancy is a period in the life cycle of an organism in which there is a temporary suspension of certain biological activities. These include growth, development and physical activity. The objective is for the organism to conserve its energy by drastically reducing its metabolic activity. Dormancy is usually closely related to the climatic and environmental conditions of the animal's environment.

In the animal kingdom there are 4 different types of dormancy: hibernation, diapause, aestivation and brumation:

Hibernation

This type of animal dormancy is explained in the above section.

Diapause

This is a genetically predetermined process. It is very common in insects, which can suspend their development between winter and spring. It is rarely seen in mammals, with the red deer (Cervus elaphus) being an exception as their present embryonic diapause. This means the moment in which the embryo is implanted in the uterus is delayed until the conditions for the birth of the calf are favorable, e.g. during the spring.

Estivation

It is common in invertebrates such as snails of the genus Helix, earthworms and some fish (such as lungfish). Estivation (or aestivation) occurs in response to very hot environmental conditions or in periods of drought. In this way, it can be considered the same or very similar process as hibernation, but as a response to the opposite environmental circumstances.

Brumation

It occurs in reptiles which are ectotherms, i.e. or cold-blooded animals. This means they regulate their temperature based on environmental temperature,. It is similar to hibernation. The difference basically resides in the regulation of metabolic processes in each type of dormancy, which are different from each other. Reptiles need to wake up from their lethargy to ingest water and food, although the latter is in small amounts.

Brumating animals can undergo supercooling the blood, fluids or cells, which allows them to survive in freezing temperatures. This is a process which allows them to remain unfrozen, preventing the cell damage and death that this freezing would incur.

Not all animals have the ability to hibernate. There are many reasons for this. For some, their lifespans are so short, hibernation is not necessary. For others, it is because they live in temperate environments which have neither extreme of temperature. Ecosystems which experience very cold winters are usually those at high latitudes in both hemispheres of the planet. Within these ecosystems, we can find the following hibernating animals:

Birds

There are species of birds that go into torpor for a short time, usually at night. This is not considered true hibernation. However, there is one species that does meet the definition of hibernation, the common poorwill (Phalaenoptilus nuttallii). This bird lives in North America and has nocturnal habits. They hibernate in very cold conditions, very warm conditions (estivation) or when food is scarce. They sometimes take the opportunity to incubate their eggs during hibernation.

Grizzly bears

The animal many of us most associate with hibernation is the bear, specifically the North American brown bear, commonly known as the grizzly bear. The time in which they enter hibernation depends on various factors such as weather, food availability and even their own characteristics.

Males normally remain active for the longest time and are the first to emerge from their dens after hibernation. Pregnant females usually enter dormancy first and are the last to emerge, doing so in the company of their young. It is for this reason some ethologists don't consider bears to be true hibernating animals as they have to wake at some point to deliver their cubs.

Not all types of bears hibernate or do so for long periods of time. This is often because circumstances are not always consistent. During mild winters, there are female bears with cubs that may benefit from being active when food is abundant. This fact depends on the amount of fat they have accumulated prior to the cold season. If they come into winter with low fat stores (as lactating females or developing juveniles) it may pay to keep active.

Bats

The reason for hibernation in bats is twofold. In addition to low temperatures, bats hibernate due to the scaricty of prey, mainly in the form of insects. Their hibernation can last 183 days during which their body temperature drops and they undergo physiological and metabolic changes. Bats can wake up from torpor every 10 days or so to defecate, urinate or move to safer or more suitable roosts.

European hedgehog

The European hedgehog (Erinaceus europaeus) can hibernate for periods ranging from weeks to months, depending on the climate. During hibernation their heart rate decreases by up to 90%, but the hedgehog can also estivate if they live in in hot and dry climates.

Rodents

Squirrels, marmots and prairie dogs (belonging to the family Sciuridae) may hibernate during cooler periods or estivate if the environment is warm. Squirrels accumulate nuts and fruit in their shelters for several weeks leading up to winter.

Marmots can hibernate for up to 7 months, making them one of the longest hibernators. Other rodents that also hibernate are dormice, hamsters or gerbils. Some marsupials also hibernate, including opossums. While it is rare among most primates, some species of lemur are known to hibernate.

There are animals that do not truly hibernate, but that spend cold periods in a dormancy of various degrees. These include skunks and badgers which go through a deep torpor of up to 3 weeks if the weather becomes very cold. This differs from true hibernation as their heart rate and metabolism does not drop as extremely.

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