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How five hibernators relax during the winter | Smithsonian Voices

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Like the ground squirrel pictured above, many mammals hibernate. They do this by lowering their metabolism and heart rate to levels so low that they seem to fall asleep. Hibernation is a strategy for surviving harsh winters when food sources are low.
Mary Lewandowski, National Park Service

In the dead of winter, it’s common to grab a cozy blanket, accompany a hot drink, and some form of relaxing entertainment. But then, sleep takes over, turning any project into a full-fledged nap.

For some mammals, these winter naps last for several months and can lead to intense physiological changes. Here’s how five mammals give hibernation a unique twist in the cold months.

Big tailed dwarf lemurs

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Fat-tailed dwarf lemurs are aptly named. In preparation for hibernation, these hibernating primates store up to 40% of their total body fat in their tails.

Allan Hopkins, CC BY-NC-ND 2.0

The fat-tailed dwarf lemur is the only primate species known to hibernate, and it has a special strategy for doing so: it stores fat in its tail. This lemur’s tail can contain up to 40% of its total body fat. They metabolize or “burn” this fat during hibernation, which in this species can last up to seven months.

During hibernation, this small lemur’s heart rate drops by up to 95 percent and his body temperature follows suit. This state is called “torpor”. But once a week or two his metabolism goes up and his heart rate rises to warm up the body. This is called a “wakefulness period”. After a while, the lemur’s heart and temperature drop again and it returns to torpor.

Bats fearing fungi

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Cave bats, like this little brown bat, can contract a deadly white fungus in the cool, dark places they hibernate, also called hibernacula.

National park service

Like the fat-tailed dwarf lemur, the little brown bat also goes through periods of torpor and metabolic arousal during hibernation. This species and its relatives can reduce their metabolic energy costs by 98% by lowering their body temperature to near zero. The total hibernation period can last more than 6 months, waiting for the emergence of the insects that they eat in the spring.

But hibernation carries a particular danger for bats. Dark, damp, cool places where bats hibernate, called hibernacula, are often home to a deadly species white nose syndrome mushroom. When bats are numb during hibernation, their immune systems are significantly reduced. The fungus infects the nose, ears and exposed skin of their wings of bats during their hibernation.

Architectural marmots

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The groundhog builds burrows over 40 feet long, with bathrooms and specialized rooms.

National park service

the Marmot, or groundhog, is a rodent found throughout much of northern and eastern North America. It is part of the squirrel family, but rarely climbs trees.

When groundhogs hibernate, they do so in style. These rodents build incredibly long burrows. The borrowing store, which can have multiple “floors” and can be up to 66 feet long in some cases, has specialized rooms for eating, sleeping, and even going to the bathroom. Groundhogs head to their burrows in the fall, and they can have up to 10-20 metabolic arousals from torpor throughout their 3-month hibernation.

Frozen arctic ground squirrels

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The arctic ground squirrel can cool its body temperature to less than 0 degrees Fahrenheit when in torpor.

Mary Lewandowski, National Park Service

The arctic ground squirrel may not have such a stylish burrow as the groundhog, but it’s still cool in its own way. This species has the lowest body temperature ever recorded during torpor for a mammal. By cooling their body below zero, the ground squirrel slows down its metabolism and minimizes its fat intake so that it can maintain it for seven to eight months of hibernation.

Once every few weeks, arctic ground squirrels should come out of their torpor. They start to shake – and this chill can last for up to twelve hours – as they slowly heat up to 98 degrees Fahrenheit. Once the arctic ground squirrel is warm again, it cools down to extreme torpor.

Unusually hot bear

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Most bears hibernate in colder climates. Due to their size, these extreme sleepers have to accumulate huge layers of fat to support them during hibernation.

Chris Carson, National Park Service

Bears are perhaps the most famous hibernators, but for a while their credibility in hibernation was called into question.

True hibernation involves periods when the body temperature of animals drops dramatically due to slowing metabolic processes. But bear bodies stay unusually warm at around 88 degrees Fahrenheit during hibernation.

The researchers understood this because bear skins are very insulating and prevent excessive loss of body heat. Thus, bears still lower their metabolism by more than 50%, but without rapid cooling. Because their body temperature remains fairly high, bears do not need to periodically awaken their torpor like other hibernating mammals. They don’t wake up to eat, drink or use the bathroom. The coldest fact of all is that they can even give birth during hibernation.

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