Winter and Summer Sleep in Animals, Hibernation, Estivation, Torpor
For the average person, sleeping uninterrupted for 12 hours is an absolute record. It’s hard, then, to imagine that many other mammals (and not only mammals) can spend even several months in a specific kind of sleep. You’ve undoubtedly heard about hibernation and animals’ winter sleep many times. The first association probably goes to the bear, which we’ve been told since childhood falls into winter sleep. Is it the only animal that sleeps through the winter? And besides, does hibernation mean the same thing as winter sleep? What exactly is hibernation? How does it affect an animal’s body? Are only vertebrates capable of entering a state of hibernation? In this article, we will try to answer these and many other questions.
What is Hibernation?
Hibernation is a state of slowed metabolism and a drop in body temperature. During this time, the breathing rate and heart rate also decrease. The term “hibernation” originally referred to a significant cessation of vital functions observed in rodents. Today, however, it is also applied to the winter sleep of bears, which more actively slow their metabolism without the need for a drastic drop in body temperature.
Scientists also believe that the phenomena of torpor and hibernation share similar mechanisms. Torpor is likewise a state of controlled reduction in body temperature and metabolic slowdown. However, it occurs more frequently than hibernation (e.g., many bat species enter a state of torpor every night).
Beyond vertebrates, certain insects can also enter hibernation, such as the wasp species Polistes exclamans, which forms large groups in suitable locations called overwintering sites before falling into this sleep state.
Hibernation and Estivation
A variation of hibernation – winter sleep – is estivation, or summer sleep. Both phenomena are responses to unfavorable environmental conditions, such as extremely high temperatures and an insufficient amount of drinking water.
Why does body temperature decrease during hibernation?
Many states resembling hibernation are associated with a reduction in body temperature. The purpose of this process is to conserve energy during times when food is scarce or entirely unavailable. However, to achieve this state, the metabolic rate must first be slowed down, which directly affects body temperature in warm-blooded animals.
How long can hibernation last?
Winter sleep can last for several days, weeks, or even months, depending on the species, ambient temperature, time of year, and the individual health condition of the animal.
Preparations for sleep
However, before an animal enters this long sleep, it must accumulate enough energy to survive the winter. Larger animals, such as bears, become hyperphagic before entering hibernation – they consume very large amounts of food, which allows them to build energy reserves in the form of subcutaneous fat. Smaller animals, on the other hand, often store food in their dens, which eliminates the need to gorge themselves before entering hibernation.
Hibernation and pregnancy
Some mammal species enter hibernation while pregnant. Their young are often born while the mother is still asleep or has recently awakened.
Female polar bears enter winter sleep during the coldest months. At the same time, their cubs are born. The mother then loses 15-27% of her pre-hibernation body weight, as the stored fat is converted into the energy needed to survive this challenging period.
Pregnant female bears store significantly larger amounts of fat, which is reflected in the weight of their offspring. An adequate amount of fat in female polar bears ensures sufficient warmth necessary for their newborns.
Obligate and Facultative Hibernators
Animals that enter winter (or summer) sleep are called hibernators. There are two groups of hibernators:
Obligate Hibernators
These are animals that must enter a state of hibernation every year, regardless of the season, temperature, or food availability. This group includes many species of ground squirrels and marmots, lemurs, as well as the European hedgehog (Erinaceus europaeus), monotremes, marsupials, and even butterflies such as the small tortoiseshell (Aglais urticae). In this state, their body temperature drops to a level close to the ambient temperature, and their heart rate and breathing slow down dramatically.
During the winter season, these animals interrupt their sleep with periods of arousal, during which their heart rate and body temperature approach average values.
The exact purpose of these arousals, however, remains unclear. It is speculated that the animals need to periodically warm up to be able to fall back asleep. Thus, they wake up to accelerate their metabolism and generate more heat to enable further sleep.
This theory is supported by evidence from observations of the Arctic ground squirrel (Urocitellus parryii). Another hypothesis suggests that the elevated body temperature following arousal enhances immune defense.
Facultative Hibernators
Animals in this group enter hibernation when the amount of available food is too low and/or the ambient temperature is too low or too high.
A good example of a facultative hibernator is the black-tailed prairie dog (Cynomys ludovicianus), also known as the prairie dog. Although it is a close relative of the obligate hibernator, the white-tailed prairie dog (Cynomys leucurus), it does not enter deep hibernation but rather a lighter state of dormancy that lasts for a short duration.
Examples of Animals Entering Winter Sleep
Bears
For many years, the view that bears undergo hibernation was considered controversial because their body temperature drop is relatively small compared to smaller animals that enter typical hibernation.
However, this state is not defined solely by the degree of temperature reduction but also by the suppression of metabolism. In bears entering winter sleep, metabolism slows by approximately 75%, which unequivocally indicates that the state they are in during the winter period can be classified as hibernation.
Nevertheless, it is believed that bear hibernation differs significantly from that of rodents or primates. Regardless of ambient temperature, it involves a suppression of metabolism.
Since a bear can process urine and proteins during winter sleep, this allows it to eliminate the need to urinate and prevents muscle atrophy.
Primates
Hibernation in rodents has been known for years. However, not everyone is aware that some primates can also enter a similar state. This refers primarily to the Madagascan lemurs known as fat-tailed dwarf lemurs (Cheirogaleus medius). These animals spend approximately 7 months of the year asleep in tree hollows.
From December to February, temperatures in Madagascar rise above +30°C (+86°F). The state of dormancy is strongly dependent on the conditions inside the den. If the entrance to the hollow is unsealed, the range of body temperature fluctuates widely. However, if the opening is tightly sealed, the body temperature remains stable, and the animal periodically interrupts its sleep with bouts of arousal.
Birds
Members of this group typically do not enter hibernation. In birds, a slowdown of metabolism and a decrease in body temperature is referred to as torpor. This is observed in young swifts waiting for a parent foraging for their food.
The only known case of multi-week or multi-month torpor in birds occurs among common poorwills (Phalaenoptilus nuttallii).
Fish, Amphibians, and Reptiles
Since most fish are naturally ectothermic (cold-blooded) animals, they cannot freely regulate their body temperature or metabolism, which in turn prevents them from achieving a state of hibernation. Nevertheless, their metabolic rate sometimes noticeably slows, particularly in cold and/or oxygen-deprived aquatic environments.
For much of the 20th century, it was also believed that basking sharks (Cetorhinus maximus) – among the largest sharks in the world – basking in the sun just below the water’s surface, entered a form of hibernation or torpor.
Thanks to research by Dr. David Sims in 2003, this was proven to be an erroneous theory. Instead, Dr. Sims demonstrated that basking sharks travel vast distances throughout the year, searching for areas rich in plankton, their primary food source. It was also observed that another shark species, the epaulette shark (Hemiscyllium ocellatum), can survive extended periods without oxygen, even at temperatures reaching up to +26°C (+78.8°F).
Several other species can also endure long periods without access to oxygen, including the wood frog (Lithobates sylvaticus), the goldfish (Carassius auratus), the red-eared slider turtle (Trachemys scripta elegans), and the bar-headed goose (Anser indicus). However, these abilities are not equivalent to the capabilities of hibernating warm-blooded animals.
Animal Sleep – Do Animals Dream?
An extension of this article is the study Animal Sleep, in which we explore the functions of sleep, answering questions such as how fish sleep, how it is possible that swimming mammals don’t drown during sleep or naps, and addressing many other topics related to animal sleep.
Winter Sleep in Animals – Interesting Facts
- During hibernation, the abdominal temperature of the Arctic ground squirrel averages -2.9°C (-19.2°F) for about 3 weeks, even though the temperature of its head and neck remains at 0°C (32°F) or slightly above.
- In the past, it was believed that, compared to rodents, bears enter a shallow winter sleep. The body temperature of bears drops by 3-5°C (5.4-9°F), while that of rodents decreases by about 32°C (57.6°F) during this time. Today, this theory is no longer considered valid.
- The Roman writer and historian Pliny the Elder believed that swallows enter a state of hibernation, but the 18th-century ornithologist Gilbert White did not confirm this theory.
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