Humans aren’t the only ones who head south for the winter. Some other animals migrate to warmer places where the food supply is better. After nesting in the North Pole, Arctic Terns fly all the way south to Antarctica. Monarch butterflies go to Mexico in the fall. Little brown bats may travel almost 500 miles between summer and winter roosts (called hibernacula.)
|In the far north, Marmots (a relative of the Woodchuck) spend two-thirds of each year hibernating. They stay in an underground burrow with an entrance plugged with grass, mud and poop. This is a Yellow-bellied Martmot, photo by Jon Sullivan
It’s harder for animals without wings. Some stay put and are somewhat active all winter long. Bobcats and deer grow a thicker coat of fur. They are just two examples of mammals whose tracks you might see in the snow. Most fish keep swimming around and eating underneath a layer of ice, which is why people go ice fishing. Some insects die, and only their eggs, larva or pupa survive the cold weather. However, there are a few insects like snow fleas and crane flies that are normally active in the winter.
When temperatures drop, keeping warm uses a lot of energy. Food is in short supply. Hungry predators are also on the prowl. To survive, some other mammals have adapted by going into hibernation.
Hibernation is different from sleep. Instead, the animals become dormant. Their body functions (metabolism) slow way down. During hibernation, their internal body temperature drops (except for bears because they are so big.) A woodchuck’s body temperature can go from 98 F to as low as 38 F. Their breathing and heartbeat slow. In the summer, a woodchuck’s heart beats 80 times a minute. During hibernation, it only beats 4 times a minute. (Count that out to get a sense of the difference.) The animal may have no detectable brain waves. Hibernation can last for days, weeks or even months. Some animals pig out on food before going into hibernation. Bears might gain 30 pounds in a week. But then they do not eat, drink or go to the bathroom for as long as 100 days. Even though they can lose 15-40% of their weight during hibernation, 99% of black bears survive the winter curled up in a cave.
Scientists are very interested in unraveling the mysteries of hibernation. When humans go into a coma (becoming unconscious for a long time, sometimes after a severe head injury), they lose bone and muscle mass. But that doesn’t happen in bears. Even though their cholesterol levels get really high, a bear’s arteries don’t harden and they don’t get gallstones.
Bears can go months without going to the bathroom, somehow recycling their waste in their own bodies. If we could figure how they do it, we might be able to come up with better treatments for conditions like osteoporosis and kidney disease. Experimenters recently caused mice to go into hibernation “on demand,” by having them breathe air that had hydrogen sulfide (a common gas that smells like a rotten egg) in it. While the mice were out, their body temperature and oxygen use dropped by about half. They took only 10 breaths a minute instead of 120. When researchers turned off the gas after about 6 hours, the mice woke up and seemed fine. If we could do something like that for a person on their way to the hospital after a heart attack, it might save their life. Maybe we could even learn how to put people into suspended animation for space travel.
By the way, some biologists argue a lot about what qualifies as “true hibernation.” Some say it only applies to warm blooded animals, and then only to a few species such as hedgehogs, dormice (the sleepy rodent in Alice in Wonderland), marmots and certain bats. Other creatures like skunks are easily awakened and may roam around and eat if the weather gets mild. Chipmunks wake up periodically (called arousal), grab a snack from their stored stash, take a bathroom break and then konk out again. Some birds like chickadees slip into torpor just at night, when their heartbeat, temperature and activity slow way down. Cold-blooded reptiles like turtles and snakes stay in a winter shelter (which may be a hole in the dirt) for the whole winter, and won’t rouse until the outside temperature warms up again. A few bizarre creatures like the Wood Frog actually freeze solid during the wintertime, and then thaw out in the spring.
Joke: Why do bears hibernate for so long? Because no one has the nerve to wake them up!