How Five Hibernators Chill Out During Winter

Mammals have their quirks, and that extends to how they hibernate during the cold season.

A ground squirrel stands on its hind legs in gravel with greenery in the background.
Like the ground squirrel pictured above, lots of mammals hibernate. They do so by dropping their metabolism and heart rate to such low levels that they appear to be asleep. Hibernation is a strategy for surviving harsh winters when food sources are low. Mary Lewandowski, National Parks Service

In the dead of winter, it’s standard to grab a cozy blanket, accompanying hot beverage and relaxing form of entertainment. But then, sleep takes over, transforming any plans into a fully-fledged nap.

For some mammals, these winter naps last for months at a time and can involve intense physiological changes. Here are how five mammals put a unique spin on hibernation during the cold weather months.

Fat-friendly fat-tailed dwarf lemurs

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 species of primate known to hibernate, and it has a special strategy for doing so—it stores fat in its tail. This lemurs’ tails can hold up to 40% of their total body fat. They metabolize or “burn” this fat during hibernation, which in this species can last up to seven months.

During hibernation, this little lemur’s heartrate drops by as much as 95 percent and its body temperature follows suit. This state is called “torpor.” But once every week or two, its metabolism goes back up and its heartrate elevates to rewarm the body. This is known as an “arousal period”. After a length of time, the lemur’s heart and temperature drop again and it goes back into torpor.

Fungus-fearing bats

Cave roosting bats, like this little brown bat, can contract a deadly white fungus in the cool and dark locations where they hibernate, also called hibernacula.   National Parks Service

Like the fat-tailed dwarf lemur, the little brown bat also goes through periods of torpor and metabolic arousal when hibernating. This species and its relatives’ can reduce their metabolic energy costs by 98% by dropping their body temperatures to near freezing. The total hibernation period can last for over 6 months, while they wait for the emergence of the insects they eat in spring.

But hibernation comes with a special danger for bats. The dark, humid and cool places where bats hibernate, called hibernacula, are often home to a deadly white-nose syndrome fungus. When bats are in torpor during hibernation, their immune system is drastically reduced. The fungus infects bats' noses, ears, and the exposed skin of their wings while they hibernate.

Architectural groundhogs

The groundhog builds burrows more than 40 feet in length, with specialized bathroom and bedroom chambers. National Parks Service

The groundhog, or woodchuck, is a rodent found throughout much of the northern and eastern parts of North America. It is a member of the squirrel family, but only rarely climbs trees.

When groundhogs hibernate, they do so in style. These rodents build incredibly long burrows. The boutique borrows, which can have multiple “floors” and can measure as much as 66 feet long in some cases, have specialized chambers for eating, sleeping and even going to the bathroom. Groundhogs head into their burrows in the fall, and they can have as many as 10-20 metabolic arousals from torpor throughout their entire 3-month-long hibernation.

Frozen Arctic ground squirrels

The arctic ground squirrel can cool its body temperature to below 0 degrees Fahrenheit when in torpor. Mary Lewandowski, National Parks Service

The arctic ground squirrel may not have quite as snazzy a 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 supercooling its body to below freezing, the ground squirrel slows its metabolism and minimizes its fat consumption so that it can sustain it through seven to eight months of hibernation.

Once every few weeks, arctic ground squirrels must arouse from their torpor. They start shivering—and this shivering can last for as much as twelve hours—as they slowly heat themselves back up to 98 degrees Fahrenheit. Once the arctic ground squirrel is warm again, it cools right back down to extreme torpor.

Abnormally warm bears

Most bears hibernate in colder climates. Because of their size, these extreme sleepers must accumulate tremendous layers of fat to sustain them during hibernation. Chris Carson, National Parks Service

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

True hibernation involves periods when animals’ body temperatures drop drastically low due to slowing metabolic processes. But bears’ bodies stay unusually warm at around 88 degrees Fahrenheit when hibernating.

Researchers realized this because bear pelts are highly insulating and prevent excessive loss of body heat. So, the bears still drop their metabolisms by over 50%, but without the rapid cool down. Because their body temperature stays fairly high, the bears don’t need to periodically arouse form their torpor like other hibernating mammals. They don’t wake up to eat, drink or use the bathroom. The chilliest fact of all is that they can even give birth while hibernating.

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