By Fran Stoddard © 1984
Issue: December, 1984
Q: Why don't wild animals starve to death while they are hibernating? (From Kitty Piaster, Bassett, Virginia)
If animals just ate a lot and then went to sleep for the winter, but their bodies continued its normal chemical activity (called metabolism), they would probably starve to death. Since that is not the case, what does happen?
Hibernation is not a mere winter sleep. It, is much more complex and not entirely understood by scientists. There are many animals that hibernate, including insects, spiders, turtles, snakes, even fish, but I assume your question is about mammals, the warm blooded animals. With mammals, the animal's temperature decreases greatly, and its metabolism is considerably reduced. Metabolism is the process in which food is changed into energy, new cells, waste products, etc. Also, the mammal's breathing and heart rate slows way down to almost a deathlike condition. In this state very little energy is used. You could liken it to those new wood stoves. They are filled with fuel, then everything is shut up almost completely, so it burns slow and steady for several days. However, when the animal is awake, and normal, it is like a stove with the damper open, it burns up the fuel very quickly.
Hibernation is very practical. It allows the animal to remain completely inactive; dormant, during the winter months when its natural food would be difficult to find. Also, the colder temperature would require them to eat much more just to maintain their normal temperatures and body activities and keep warm.
There are animals which are not true hibernators. They are just winter sleepers. The bear, for example is not a true hibernator. It eats a lot and gets very fat, then if it is in a cold climate, it will den up for the winter, and go to sleep. However, its body temperature does not decrease more than a few degrees, and it maintains a high metabolic rate compared to true hibernators. It can be awakened, so beware!
A large mammal, like the black bear, which has a naturally slow metabolism can lie down in a den and sleep through months of cold weather while it slowly uses up its fat reserves. A small animal though, has such a high metabolic rate that it could not live long on its fat reserves and still maintain its high metabolic rate. It would die in a few days or weeks at the most. However, under hibernation when everything slows down to a small fraction of normal, the animal can survive the winter on the body reserves it has.
At this point the true hibernator is similar to a cold-blooded animal, like a frog. Its body temperature is close to its surroundings, rising and falling with it as long as it remains. above freezing. To keep from freezing to death, the animal's temperature will remain slightly above the surroundings. In case the surroundings go below freezing, then the body begins to produce heat and the animal's temperature rises to prevent tissues from freezing. If it gets too much below freezing it may stimulate it to a full-awakening.
Rodents and bats are two kinds of mammals that hibernate. Though not all rodents and bats do this.
The Woodchuck Or Groundhog
The Woodchuck is one of our true hibernators. By the end of summer, a woodchuck will have a gorged stomach and accumulated layers of fat. It feeds on green vegetation including alfalfa, clover and corn. Sometime in the fall before going underground, for the last time, the woodchuck does a strange thing. It stops eating. It will not eat for 10-14 days. During this time it spends a part of each day just dozing in the autumn sun at the entrance to its burrow. Sometimes it'll wander around, but not too far from home. It may nibble at a leaf or two, but it doesn't really eat.
Its burrow is already dug so it doesn't have to use up valuable energy before going to its hibernation chamber. It dug that earlier and even put dry grass into the chamber during the summer. It now goes into the highest room in its burrow. It usually has several rooms, but this spot is less likely to flood if rains or melting snows should force water into its burrow. The entrance to this chamber is closed with dirt which the woodchuck scrapes from the far end of its room. The door is sealed to prevent unwelcomed visitors like opossums and skunks which are not hibernators but do seek shelter during cold spells where they can sleep for awhile. Rattlesnakes may also come in. Their stay is longer and they will sleep out the cold winter months below the frost lines.
After the entrance is blocked against intruders, the woodchuck curls up into a ball by putting its head down between the short hind legs, its nose against its belly and its tail brought up over its head. It is ready to go to sleep. It closes its eyelids and lips very tightly. Sleep comes gradually. Body temperature falls from almost 97 degrees to less than 40 degrees F. Breathing slows down to about once every 6 minutes, and heartbeat drops from over one hundred beats per minute to about four.
When warm weather comes again the woodchuck awakens. It has lost from 1/3 to 1/2 its weight. You would think the first thing on its mind would be to find something to eat, but that's not so. First, it goes to seek a mate, then it begins to search for food.
Although hibernation varies with the animal, basically it is the same. Mainly, they eat a lot, store up fat or energy reserves, seek a place where the temperature will remain above freezing, even if only one degree, then as they go to sleep, all their vital forces slow down to near deathlike so they will not use up their energy reserves before good weather comes again and they can awaken and search for food. That is why animals don't starve to death when they hibernate.
If there are any questions you have about Blue Ridge Wildlife, or if you have suggestions for this type of articles, Fran welcomes hearing from you. Her address is:
Rt. 5, Box 34
Marion, N.C. 28752