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In The Country


Tim Iverson
Seasonal Naturalist
Cunningham Falls State Park

(12/2012) Winter has settled in on us, and I can relate to this month’s cast of characters. I abhor the cold. As far as I’m concerned winter is only good for two things: Christmas and my birthday (it’s in January). That list used to include snow, but as an adult the world doesn’t stop turning just because it snowed it just becomes more inconvenient. If I could I’d migrate south for the winter, or hunker down and take a nice long nap until the spring. I’m talking about hibernating! It’s pretty common in the natural world, and I want in on that action.

Hibernation is an evolutionary adaption that helps mammals and reptiles alike survive the winter months. Every living thing burns energy all the time simply by being alive. It takes energy to walk, sleep, breathe, and even to think! Mammals spend a lot of their energy just regulating body temperature. So in order to get enough energy to do all these things we eat. We eat food in order to put enough gas in the tank so we can continue to do all the stuff we do, like existing. This all works really well when there is enough food to go around. But during the winter plants stop producing fruit, critters become scarcer, and food is all around a lot harder to come by. What’s an animal to do?

Some animals migrate south in search of warmer temperatures and more abundant food supplies. Meanwhile, some animals that stay behind will hunker down and enter a hibernation state. During hibernation metabolic rates essentially come to a grinding halt. Heart rate can drop to as little as 3% of normal rate. For example, a chipmunk will go from 200 to 5 heartbeats per minute during hibernation. Breathing can slow to half (or more) of the usual rate, with some species stopping breathing entirely.

Hibernation, contrary to popular belief, is actually not at all like sleeping. These animals virtually lose all consciousness and are nearly impossible to wake up. When they do eventually come out of hibernation they often exhibit signs of sleep deprivation, and may need to dedicate a substantial portion of time to sleep. The primary difference between sleep and hibernation basically boils down to what the body is doing. During sleep there are minor physiological changes to the body, it’s mostly mental change. It’s also very easy to wake up from sleep, whereas hibernation is nearly impossible making these animals susceptible to predation. Brain activity is actually very similar during hibernation compared with normal active brain activity. Hibernation just brings animals to the lowest possible metabolic rates they can stand so they require nearly no energy.

Animals can’t undergo this process without a lot of work and forethought though. They must spend a great deal of time building up fat reserves to feed off of throughout this ordeal. Those that can’t hold enough fat will store nonperishable food with them or nearby so they can access it during the winter. While hibernating the bodies of these animals halt nearly all activity, including excreting waste. Bears for instance will recycle nitrogen and other chemicals from fecal and urine waste back into their system.

Bears, along with several other species, don’t actually hibernate by the traditional understanding of the word. They undergo a type of hibernation, sometimes referred to as brumation, which actually allows them to wake up periodically. Their bodies undergo the same physiological changes as other hibernators, but they are still somewhat aware of their surroundings. They will only wake up when disturbed, and this is most likely due to the fact that they only lower body temperature marginally. In general, they require a lot more energy. To lower their body temperatures anymore than just a few degrees (from approx. 100EF to about 88EF) and then return to normal would require a great deal of time and energy. Reptiles will drop 40EF to 70EF or more from their normal body temperature! Depending on temperature black bears will enter their winter dens in October and November and undergo hibernation for about three to five months.

Similarly, bats also undergo this type of hibernation. Bats eat mostly insects, at least bats in our area, and during the winter insects also hibernate. Thus, it’s easiest for them to hibernate during this difficult season. While bats undergo brumation, and will only awaken if disturbed, they face a much different challenge than other hibernators.

Currently there is an invasive fungus, geomyces destructans, creating a fatal condition in bats known as White-Nose Syndrome (WNS). This white fungus is introduced into cave systems on the soles of shoes from other infected caves and spreads rapidly once introduced. The fungus generally damages wing tissue, causes erratic behavior, and infected bats generally become very active during their usual hibernation period causing them to starve. The Little Brown Bat, currently the most common bat in MD, is expected to become extinct within 16 years. Because of the large number of bats lost it is estimated that 700 metric tons (that’s 14 million lbs!) of insects will go uneaten, potentially causing millions of dollars in economic damage to agribusiness.

While there is still much to learn about this disease research is underway. Caving activities have been banned in several states in order to curb the spread. If you do happen to venture into or out of any caves in the future make sure to disinfect clothes before and after to help stop WNS. You can also construct bat boxes to provide habitat for bats during the summer months. Planting native species of night blooming flowers will also attract insects for bats to feast on.

Eastern Box Turtles, and other reptiles, will also hunker down for the winter. They’re ectothermic, meaning they obtain their body heat from outside sources, not through internal production and regulation like mammals. When the air temperature around turtles begins to drop and sunlight gets shorter body temperature drops and it is difficult to maintain normal metabolic rates. When this begins box turtles will find a suitable location, generally with good leaf pack or dense roots, and bury down (sometimes as much as 2 feet). This protects them from the elements, and would be predators. Some cold-blooded creatures can survive much closer to the surface because their blood contains a sort of "anti-freeze", made up of mostly glucose (which is just a fancy name for sugar). Species like the wood frog and painted turtle can essentially freeze solid during the winter, and then thaw out in the spring!

During the summer months forests are bustling with activity. The winter can be tough times for those of us still up to endure it. Maybe I should take a cue from geese and fly south, or follow suit with the black bear and check out until the spring. Either way there is way more than I could cover in about 1000 words so if you’re interested in more on hibernation, or what happens to the forest in the winter I highly recommend checking out this website put together by the BBC:

Read other articles by Ranger Tim Iverson