Which animals hibernate and why

It is a common thing that, generally, we think of hibernation as a long period of time during which an animal shelters itself from adverse conditions and only sleeps. In this reflection there is nowhere any allusion to the species to which we are referring. For example, if we found a stunned snake in winter, we would surely say that it is hibernating. However, there would be a confusion between concepts, since not all animals hibernate, nor are all processes related to lethargy or states in which animals appear sleepy are called as such. To clarify these aspects.

What is hibernation and why do some animals hibernate?

We can define hibernation as the ability of some species to enter a state of prolonged lethargy to cope with the cold weather conditions of winter , during which the energy available in the environment decreases. This state of lethargy, which can last from days to months, resembles a state of hypothermia (the body temperature drops considerably).

What happens during this process is that the metabolism drops to very low levels to better conserve energy, digestion stops, hibernating animals do not need to drink and they dispense with other needs such as urinating or defecating. They also decrease the respiratory rate and their body temperature, so that the heat is 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 lethargy, their weight will have decreased and they will be low on energy. Generally, when animals go into hibernation it is difficult to wake them up, and they can be manipulated. However, there are animals that wake up periodically to consume the food reserves that they have stored in their respective burrows, such as squirrels, which store nuts such as acorns to consume during this period.

However, there is one aspect that we must comment on and on which to emphasize, and that is that hibernation is just one type of a group of natural processes that involve the temporary suspension of the biological activity of the organism. This is called dormancy. By this we mean that there are several animal species that join in these states of lethargy, but not all under the hibernation process, which occurs to a greater degree in homeothermic or endothermic animals, that is, in warm-blooded animals.

Dormancy or dormancy is a period in the biological cycle of an organism in which there is a temporary suspension of activities typical of organisms such as growth, development and physical activity. The goal is for the body to be able to conserve its energy by drastically reducing its metabolic activity. Dormancy is usually closely related to the climatic and environmental conditions of the environment.

In the animal world there are 4 different types of dormancy : hibernation (explained in the previous section), diapause, estivation and brumation.

  • Diapause: it is a process that is genetically predetermined. It is very common in insects, which can suspend their development between winter and spring. However, there is another case, that of the European red deer ( Cervus elaphus ), which presents embryonic diapause. This means that the moment in which the embryo implants in the uterus is delayed until the conditions for the birth of the young are favorable, for example, during the spring.
  • Estivation: it is common in invertebrates such as snails of the genus Helix or earthworms and some fish (such as lungfish). Estivation occurs in response to very hot environmental conditions or in periods of drought, so it could be considered the same or very similar process, but as a response to the opposite situation to hibernation.
  • Brumation: occurs in reptiles (ectotherms, they regulate their temperature from the ambient temperature) and is similar to hibernation. The difference basically resides in the regulation of metabolic processes in each type of dormancy, different from each other. On the other hand, reptiles need to wake up from their lethargy to ingest water and even, although in low quantities, food. They can suffer from sub-freezing in the blood, fluids or cells, which allows them to survive in sub-zero temperatures.

What animals hibernate – list and examples

At this point , which animals are the ones that hibernate? Not all have this ability. Hibernation is presented by animals that inhabit temperate environments with a very marked winter season, which usually occurs at high latitudes in both hemispheres of the planet. Thus, these are the animals that hibernate:

Birds

There are species of birds that go into torpor for a short time, generally at night, but it is not considered a true hibernation. However, there is a species that does present hibernation as we know it: the pachuca nightjar ( Phalaenoptilus nuttallii ), which lives in North America and has nocturnal habits. This bird hibernates in very cold, very warm conditions (estivation) or when food is scarce. In addition, on the occasions when hibernation occurs, it takes advantage of the incubation of its eggs.

brown bone

The most typical case of hibernation known in endotherms is that of the brown bear. The moment of going into hibernation depends on various factors such as the weather, the availability of food or other individual characteristics.

Normally, males, which remain active for a longer time, leave their shelters first, while, in the case of pregnant bears, they are the first to enter hibernation and the last to leave it in the company of their young. This is one of the reasons why some authors consider that not all bear species hibernate, since pregnant bears giving birth during hibernation must “wake up” at some point to attend to their cubs during childbirth.

There are families of bears that do not hibernate or hibernate very briefly. Sometimes hibernation can be more or less energy efficient. There are bears with young that, in mild winters, can benefit by staying active looking for food if it 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 (such as lactating females or developing juveniles) it can pay off to stay active.

Bats

The sense of hibernation in this case is twofold since apart from low temperatures, bats hibernate due to the scarcity of prey (mainly insects). Their hibernation can last up to 183 days during which their body temperature drops and they undergo physiological and metabolic changes. In the case of bats, they can wake up from lethargy about every 10 days to defecate, urinate, or move to safer or more suitable dens.

European common hedgehog

The European Hedgehog may hibernate from weeks to months depending on the weather. In addition to hibernating, a process during which their heart rate decreases by up to 90%, the hedgehog can estivate if it is in hot and dry climates.

Rodents

The squirrels, marmots and prairie dogs (belonging to the family Sciuridae) can hibernate during the coldest half a year or aestivate if the atmosphere is warm. Squirrels accumulate nuts and fruit in their shelters for several weeks leading up to winter. On the other hand, marmots can hibernate for up to 7 months, a capacity from which the famous expression “sleep more than a marmot” derives. Other rodents that also hibernate are dormouses, hamsters or gerbils.

In addition, there are some marsupials that also hibernate, such as possums , and some primates, such as lemurs .

There are animals that do not truly hibernate but that spend these cold periods in a more or less profound torpor or lethargy, such as skunks or badgers, that go through a deep torpor of up to 3 weeks if the weather becomes very cold and the snows very thick. It differs from real hibernation in that the heart rate does not drop extremely low.

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