While some northern animals migrate, others stay put through the cold months by hibernating.
Window on Nature
By Lowell & Kaye Christie, F47246
Cold weather and shorter days often mean spending more time indoors, choosing warmer clothing and, for those of us fortunate enough to travel year-round, migrating to warmer climates. The animals we watch face similar problems, and in some cases escape the cold the same ways. Some migrate each year, while many grow a warmer coat. But some have another way to escape frigid temperatures and decreasing food supplies. They hibernate.
Hibernation can be difficult to define. It’s not so much an extended sleep as it is the way an animal’s body reacts to the environment. An extreme drop in body temperature and metabolism lowers the energy requirements needed to sustain life. Sometimes this change extends over weeks or months, but it also can happen during a single night. (The word torpor typically is used for shorter periods of inactivity.)
Hummingbirds are one example of a short-term “hibernator.” Smaller creatures have a difficult time maintaining their body temperature, requiring that they spend most of the day consuming calories just to survive. This helicopter of the bird world may eat three times its own weight in nectar each day.
Watch hummingbirds carefully, and you’ll find they spend three-quarters of their day perched on a branch. The small bird’s digestive system can’t process food as fast as it can eat, so the hummer conserves energy by waiting for its body to catch up.
A hummer’s metabolism ranks among the highest of all animals “” approximately 30 times that of a human. During normal flight, their wings beat more than 60 times a second and their heart rate may climb to 1,200 beats per minute. If a hummer quit feeding, it would soon starve.
Although the hummingbird is a migratory species, frigid weather isn’t tied to a schedule. So how does this little critter make it through cold nights? Rather than just sleep, the hummingbird goes into a state of torpor. Its body temperature drops by as much as 50 degrees. Its resting heart rate, normally around 500 beats per minute, may decrease by as much as 90 percent.
In the morning, the bird vibrates its wings to produce body heat, bringing its temperature back above 100 degrees. Then it’s time to look for food and start the cycle all over again.
Hummingbirds do move south as the seasons change, but their method of escaping those chilly nights is shared by animals that don’t migrate. Some examples of long-term hibernators are raccoons, skunks, chipmunks, and groundhogs.
A groundhog, or woodchuck, fattens up and stores food in autumn in preparation for winter, but the critter spends much of its time in hibernation once the days grow shorter. Every few weeks it will wake for a midnight snack, but as it goes back to sleep, its heart rate drops from a normal 80 beats a minute to as low as five beats a minute. Its body temperature follows the same pattern, decreasing from a normal 98 degrees Fahrenheit down to 38 degrees.
Perhaps the best-known hibernator is the bear, but not everyone agrees. Some argue that bears don’t hibernate. Others consider bears to be super-hibernators. It’s all a matter of the animal’s temperature variation, and how easily it can wake from its winter sleep. We’ll let you decide how to describe an animal that can nap in its den for as long as 100 days at a time.
American black bears can doze for months at a body temperature of around 88 degrees, just 12 degrees below that of its active summer state. That’s very different from the smaller mammals, such as chipmunks and ground squirrels. These critters must wake periodically (after days or weeks) to raise their body temperature; have a snack; visit the latrine; and then return to hibernation. Bears just sleep on.
That is, unless they are disturbed. The low-temperature hibernators take a long time to regain awareness of their surroundings, but according to longtime researcher Lynn Rogers, it’s almost impossible to approach a black bear in its den without arousing it.
Although the bear is an exception, the average hibernator’s temperature falls to approximately 43 degrees Fahrenheit, and its heart rate to between five and 10 beats per minute. Respiration also falls dramatically. Some animals can stop breathing for an hour or more. But the record for bodily extremes has to go to the wood frog.
Cold-blooded critters such as snakes, lizards, and frogs depend entirely on their environment to keep warm. Snakes, for example, will bask in the sun to absorb warmth, but during the winter they retreat to dens where the temperature remains above 32 degrees. If they stayed above ground, they would freeze. And that’s exactly what the wood frog does.
When the first winter ice appears, the wood frog responds by forcing water outward from the center of its body, so the internal organs are surrounded by water. Then the water freezes. Bodily functions stop. Breathing stops. The heart stops. There may be no heartbeat for days or weeks. As the exterior of the frog freezes, blood sugar is distributed through the circulatory system and acts as an antifreeze to keep the internal organs from freezing. This keeps the water inside the frog’s cells in a liquid state, even when the temperature goes far below 32 degrees. But the frog itself becomes as hard as a rock or, more appropriately, an ice cube.
The wood frog remains frozen until spring arrives. It then starts thawing from the inside out, a process that takes as much as a day. After spending winter in what amounts to suspended animation, the frog doesn’t waste time searching for food. Instead, it starts looking for a mate.
From a single night to an entire season, hibernation allows animals to survive in what would otherwise be fatal surroundings. And when our spring travels take us back north, the wild creatures that spent the winter sleeping will be alive and healthy to welcome us.