Curious Nature: Where do the chipmunks and bears go during winter?
Walking Mountains Science Center
Do chipmunks and bears go to sleep during the wintertime? They do not sleep through the winter; their body goes through a form of torpor, which looks different for each of them.
Torpor is a state where metabolism is depressed to less than 5% of normal. Why do each of these animals go into torpor during the winter? Is it more efficient to be awake and forage, or is it more efficient for a bear or chipmunk to drop its metabolism and be in a lowered state of consciousness to minimize fuel consumption?
Animals hibernate in both warm and cold climates. There are tropical hibernators that may do so to beat the heat or even in response to food shortages. For instance, echidnas in Australia, which are often known as spiny anteaters, will hibernate after fires as they wait for food resources to reboot.
Little guys need to save energy
The reason that many small mammals need to hibernate is because their little bodies have a high surface area to volume ratio, making it more taxing to stay warm in cold weather. They need seasonal energy savings more than larger animals. Here in Colorado, the small mammalian hibernators include the jumping mouse, little brown bat, eastern chipmunk and some species of ground squirrels.
The body temperature of chipmunks decreases to near freezing and their metabolism greatly reduces. These small animals are not in this state all the time — they go through intermittent periods of waking and dormancy. They must awaken every few days to raise their body temperature, move around, urinate, and eat. They need to have food that they have harvested during the fall readily available. During the fall they build up a cache area so they have food readily stored away for the winter months when they wake up for short periods of time.
Black bears are called super hibernators. During torpor, or what’s often referred to as denning, respirations in bears decrease from six to 10 breaths per minute to one breath every 45 seconds. They experience a drop in heart rate from 40 to 50 beats per minute during the summer to eight to 19 beats per minute during their dormancy. But unlike true hibernators, their body temperature decreases only about 12 degrees, going from 100 to 101 to 88 degrees.
What does this mean for you during the winter while exploring areas in the wild? Bears can easily come out of their denning if they hear a loud noise or feel like their life is in danger. Bears can awaken from super hibernation quite suddenly if danger is sensed. This behavior is also beneficial to female bears because bear cubs are usually born in winter. The cubs do not hibernate or go into torpor. While their mother is sleeping, they nurse and sleep, too. However, the mother awakes every now and again to care for her cubs and ensure they are safe and warm.
Another major difference between dormancy in bears and the chipmunk is the waking cycle. Bears do not usually wake up to urinate, defecate, and eat. The urea produced from fat metabolism is broken down and the resulting nitrogen is used by the bear to build protein, which allows them to maintain muscle mass and organ tissues.
So next time you are asked about bear hibernation, you can say, actually bears go into super hibernation or denning which is similar to what chipmunks do, but a little different.
Sonya Hueftle is a naturalist at Walking Mountains Science Center. Come talk about animals with her during EarthKeepers on Tuesdays and Fridays at 10 a.m.
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