Megan's Nature Nook
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Fall is a time when animals have to get serious about how they’re going to survive winter. Will they hibernate, migrate, adapt, or die? Cooler temperatures, shorter amount of daylight, and dwindling food sources are some factors that contribute to their survival decisions. When you hear the word “hibernation” the pictures that pops into your head is most likely a black bear (Ursus americanus). Although we often think of a bear when thinking of hibernation, bears actually don’t hibernate. They do something else called torpor which is similar to hibernation but not as extreme. Hibernation An example of a true hibernator would be a woodchuck (Marmota monax), also known as groundhog or whistle pig. In September - October, woodchucks begin going into their winter dens for hibernation. Once they are asleep, they stay asleep and don’t eat or drink during their slumber. Since they don’t eat during hibernation, woodchucks have to conserve their energy by slowing down bodily functions. A woodchuck’s usual body temperature is around 99°F, during hibernation they drop their body temperature down to around 37 - 40°F, not much above freezing. They slow their heart rate from 75 - 80 beats per minute in warmer weather down to 4 or 5 beats per minute during hibernation, and their breathing rate slows dramatically. These extreme reductions help to conserve energy during their long sleep so they only loose about one fourth of their body weight during their months of not eating. After a few months of hibernation, the woodchucks will emerge in March or April, depending on their region – which is a bit late for Groundhog’s Day! Although, males will often wake in February to locate females, then head back to sleep for another month or so until it is time to mate. Once they’re out and about for the warmer months, woodchucks will spend their days eating and fattening up - preparing for the next hibernation. A woodchuck can eat up to 1 pound of vegetation in one sitting! Woodchucks are not the only mammals that are true hibernators in Minnesota. Ground squirrels and four species of bats join them in this category. Torpor Now onto torpor. Torpor is similar to hibernation with the slowing down of bodily functions, but not as extreme. You’ve heard the expression “don’t poke the bear” well that is true! During torpor bears can wake up easier due to torpor not being as extreme as true hibernation and will wake up periodically throughout the winter. In October – November bears will find their den and gather leaves and sticks to line it as their final preparations for their long winter slumber. Bears rely on photoperiod (day length) and the availability of food to know when it’s time to head to bed. Temperature doesn’t play much of a part, except it does tend to coincide with shorter days and dwindling food sources. While in torpor bears will not eat, drink, or go to the bathroom for months on end. In order to prepare for this, they must forage for food all summer long to pack on the pounds. Mid-summer to the end of autumn a bear can consume up to 20,000 calories a day! Resulting in putting on 4 - 5 inches of body fat to help them survive a winter of cold weather and no eating.
A bear’s normal body temperature is around 100°F, during torpor they will decrease it about 10-12°F to around 88°F. Their heart rate decreases to 8 - 19 beats per minute, and their breathing rate is one breath every 45 seconds. These decreases are not as extreme as our woodchucks, but still help to conserve energy all winter long. A bear will typically lose 20 – 30% of its body weight during torpor, with some females loosing up to 40% if they had cubs and were nursing during torpor. Come April, the bear will awaken and emerge from its den, searching for food to begin the preparation for its next torpor session. Bears aren’t the only animals that go into torpor. Hummingbirds, raccoons, skunks, and chickadees are just a handful of other animals that do! Animals typically enter torpor because of the low availability of food for a period of time. Each animal has their own form of torpor that best suits their lifestyle and needs. Some will go into torpor for a few hours and for others it may be days or even months. Even in the warm weather of summer, Ruby-throated Hummingbirds lower their body temperature and heart rate during the night to conserve energy. If they didn’t do this, they wouldn’t be able to survive through the night because of their fast metabolisms and the need to feed often. During winter raccoons will come out during warm days to look for food but will sleep more often and spend much of their time cuddled up with their families in dens. During winter skunks they will enter long periods of sleep and wake occasionally, during torpor they slow their metabolism and decrease their body temp by 20°F. Chickadees decrease their body temperature at night to conserve energy in the winter. It is amazing to see the different ways these animals can conserve their energy in order to survive our cold and sometimes quite harsh Minnesota winters! Fun Fact: Bears can’t hibernate because of their large body mass and surface area. There is too much surface area for body heat to be lost. If their resting temperature was close to freezing like true hibernators, they would be more susceptible to their body temperature lowering too much and their cells freezing.
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