Do animals sleep when they hibernate?

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Photo Description: Squirrel, a brown bear, a dormouse, a groundhog and a hedgehog (clockwise from top left)

Photo Source: Irena Wanot, Modified from http://babyland-irena.blogspot.in/2012_02_01_archive.html

Animals spend energy in various bodily functions like walking, breathing, digesting food, thinking and even while sleeping. For instance, in warm-blooded animals, an enormous amount of energy is utilized to maintain body temperature within certain limits (thermoregulation). The body works fine when enough food is consumed to drive these processes. But what do animals do in extreme environmental conditions when food becomes scarce?

One survival strategy to conserve energy during periods of food scarcity is by slowing down their metabolic rate (metabolism), resulting in a decrease in body temperature and in some cases 0°C or below, viz. Arctic ground squirrels. This long-term state of inactivity during winter is defined as hibernation, which can last several days, weeks or months depending on the species and the surrounding environment. Many members of the three mammalian subclasses hibernate (Monotremes, Marsupials, Placentals) but the only avian/bird species known to hibernate is the common poorwill. Some mammals like female polar bears enter this state while they are pregnant and give birth to young ones during hibernation. In reptilians, this state of dormancy is called brumation.

The parallel of hibernation during summer (hot and dry conditions) is aestivation. It is more common in invertebrates like land-snails and insects but occurs in vertebrates also, viz. African lungfish, some amphibians, and Malagasy fat-tailed dwarf lemur. On the other hand, a majority of birds and some mammals undergo “light hibernation” or torpor, during which the period of inactivity lasts only for hours rather than days or weeks and is often interrupted by activity and feeding. It can occur anytime throughout the year.

But how do animals sense that it is time to hibernate or to wake up? Different animals have different ways of knowing, which the outside temperature mostly influences. Animals prepare for hibernation in several ways. Ground squirrels and lemurs make a den (also known as a hibernaculum), polar bears dig tunnels in the snow and other bears might spend the winter in a hollow beside a tree or a shallow cave and bats are well known for wintering in caves or attics. Hence, they have to store adequate energy before entering hibernation. Larger animals do so by eating huge amounts of food and becoming fat whereas smaller ones prefer to store non-perishable food items in their dens (also known as food caching).

Hibernation is a game of hormones. Glands such as thyroid and pituitary tinker the amounts of hormones released in our body and control just every physiological aspect. During this period, animals stop eating and in a few cases even stop excreting, such as the large carnivore American black bear that recycles urea by breaking it down into amino acids. These animals keep their body hydrated by burning fat stored in their fat deposits.

So, is hibernation an extended nap? No. Animals actually aren’t asleep the whole time they hibernate. Rather they undergo drastic physiological changes. Sleep and hibernation are physiologically dissimilar. Hibernation is a survival tactic whereas sleep is a recurring state of mind and body. Sleep occurs in phases i.e., non-REM (deep sleep) and REM (light sleep) (rapid eye movement), a specific phenomenon majorly involving the brain. Although hibernation begins with a non-REM stage like in sleep, measurement of brain activity in placental mammals has revealed that the brain patterns are different as compared to a normal daily/circadian sleep. Sleep is influenced by circadian rhythms (resulting from earth’s daily rotation around its axis) whereas hibernation is circannual (a biological activity that recurs annually). Sleep is easily reversible or breakable but hibernating animals cannot be aroused easily, try and you might effectively kill it or (if it’s a grumpy bear) kill yourself. Basically, they will utilize so much energy in waking up that it would leave them with little chance of making till spring.

When an animal comes out of hibernation, it experiences “sleep debt”. They don’t wake up refreshed and full of energy, rather they stay lethargic for some time since they lost half or more of their body-weights during hibernation. However, they cannot afford to stay lethargic for long because preparation for the next winter has to start right away. So, while their metabolism gears up, some go searching for a good meal for themselves and their young ones, while others get ready for reproduction – until the cycle starts again.

 

Article by:

Shagufta Khan

Senior Research Fellow (PhD)

CSIR-Centre for Cellular and Molecular Biology

Hyderabad – 500007

Email: shagufta@ccmb.res.in