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Hibernation

Black Bear Dens

2-25-15 bear dens1Black bears den in a range of locations, including under logs and stumps, under the branches of a fallen tree, and inside caves and hollow trees. Most adult black bears are not completely protected from the elements while they are hibernating and/or raising cubs, as there is usually a fairly large opening and the bear is exposed to the cold air. The amount of exposure can vary tremendously, from a relatively protected hollow under a log to complete exposure within a dense thicket or stand of conifers. Pictured are two black bear dens where cubs were raised; one is under a fallen tree and the other is in the middle of a stand of spruces.

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Black Bears Giving Birth

1-9-14  black bear cubIMG_0391Sometime between the end of January (the full moon in January is often called the ‘bear moon’) and the first part of February black bears give birth to between one and five (usually two) tiny, blind, almost hairless one-half pound cubs, each about the size of a chipmunk. The cubs are totally dependent on their mother for food and warmth.

Most dens are exposed to the cold air, as they are located under fallen logs and brush, or are dug into a bank. Occasionally they are on the ground with little or no cover; in all of these locations, the mother acts like a furnace, enveloping her young and breathing on them to keep them warm. The cubs do not hibernate, but nap frequently. Like human mothers, black bear mothers sleep when their young sleep, and are alert when their cubs cry and let them know that they are in need of attention. (Photo taken during NH Fish & Game/Ben Kilham spring research; cub in photo is two months old)


Aquatic Frogs Hibernating in Ponds

1-9-15  green frog IMG_0181Most aquatic frogs such as this Green Frog have been deep in hibernation for several months. A common misconception is that frogs spend the winter the way aquatic turtles do, dug into the mud at the bottom of a pond or stream. In fact, hibernating frogs would suffocate if they dug into the mud for an extended period of time. A hibernating turtle’s metabolism slows down so drastically that it can get by on the mud’s meager oxygen supply. Hibernating aquatic frogs, however, must be near oxygen-rich water and spend a good portion of the winter just lying on top of the mud or only partially buried. They may even slowly swim around from time to time.

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Black Bears Still Active

12-15-14  bear track in snow 049A7902While some, perhaps most, black bears have entered hibernation in northern New England by now, there are still some active individuals, probably males, who are finding enough food to delay denning. Typically a scarcity of food and cold weather trigger the reduction in the metabolism of a black bear, which signals the onset of hibernation. If the weather remains relatively warm and/or there is a large supply of beechnuts and/or acorns, signs of bears can occasionally be found even this late in the year. While there have been a few years when black bears have been sighted throughout the winter, most wildlife biologists say that it is safe to put up bird feeders in December, as sunflower seed-loving bears have usually retreated to their den sites by now. The pictured tracks and scat discovered in the past few days in central Vermont challenges that assumption (as does the stolen bird feeder). (Thanks to Erin Donahue and Charlie Berger for photo op – sensitive fern fertile frond for size comparison)

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Raccoons Fattening Up

11-13-14  raccoon tracks IMG_0045In the Northeast, raccoons spend the fall fattening up, for little, if any, food is consumed during the winter. Acorns, hickory nuts, beechnuts and hazelnuts are favorite foods. For as long as insects are available, they form a large part of a raccoon’s diet – delicacies include larvae of dug-up yellow jacket and bumblebee nests, and honeybees as well as the honey the bees have stored for the winter. Birds and rabbits injured by hunters, mice, bats, wild grapes and an occasional crayfish provide raccoons with enough sustenance so that fat makes up almost 50% of their body weight as they head into winter.

While signs of their presence, such as tracks, are still visible, they soon will be scarce. On the coldest winter days, raccoons will seek shelter in hollow trees, sometimes holing up for as long as a month at a time. Communal denning sometimes occurs, with up to 23 raccoons having been found in the same den. Considered “deep sleepers,” raccoons do not lower their metabolism significantly, and therefore are not considered true hibernators.

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Woodchucks Heading for Winter Burrows

11-10-14 woodchuck 122Woodchucks are one of the few species of mammals that enter into true hibernation. When the temperatures dip into the 40’s, usually in October or November in the Northeast, most woodchucks leave their summer burrows and head for the woods, where they dig a tunnel that ends in a chamber that is well below the frost line (and therefore above freezing). Here they curl up in a ball and live off of the 30% additional body weight they put on in the fall. In order to survive until March or April, a woodchuck’s metabolism slows way down. Its heartbeat goes from 100 beats a minute to five, and its body temperature goes from 96 degrees F. down to to 47 degrees F.

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Big Brown Bats Entering Hibernation

11-7-14  big brown bat IMG_7011Big Brown Bats, one of the most widespread mammals of North America, are one of the last species of bat to be seen flying in the fall. A relatively hardy species, the Big Brown Bat can tolerate conditions that other bats can’t. However, once cold weather arrives in the late fall and the nighttime temperatures dip down into the 30’s, they go into hibernation.

Both the Big Brown Bat and the endangered Little Brown Bat are considered “house bats,” because they are the most common bats found in houses in both summer and winter. During October, November and December, Big Brown Bats seek out caves, buildings and mines in which to hibernate. Some may migrate short distances to find an appropriate location for hibernating, but many find hibernacula close to their summer residence. Individuals often become active for brief periods during the winter months, sometimes even changing hibernation sites. Big Brown Bats can live up to 18-20 years in the wild but, unfortunately, most Big Brown Bats die during their first winter because they did not store enough fat to survive through their entire hibernation period.

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