Bears Fattening Up for Winter’s Slumber

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Illustration by Adelaide Tyrol

Conventional wisdom says that if you put up a bird feeder on Nov. 1 and take it down on April 1, you won’t have a problem with marauding bears, because they hibernate between those dates. After a bear’s visit in mid-November two years ago, I assumed put-up date should be rescheduled to late November. A month later, almost on New Year’s Day, I was astonished to hear that a large bear had just been seen on a porch a few miles from my home in Thetford Center, VT.

What gives? Why aren’t bears in bed by then?

Pregnant females are first to hibernate, reports Ben Kilham, of Lyme, NH, a wildlife researcher, who has studied black bears extensively in the Northeast. He says they don’t begin slumbering until around the second week of December. Next, he says, are the unbred females, who start hibernating around Christmas. The hibernation schedule of adult male bears is harder to predict, because they generally stay active as long as food is available, or until forced to their dens by heavy snow or plunging temperatures.

Kilham says young males often are the last to retire because they have the greatest incentive to keep feeding—so they can grow to compete with mature males. In our area young males can still be active until mid-January.

When they do call it a winter, where do they go? Many people probably envision bears hibernating in rocky caves. Bears certainly use caves or rock crevices if they exist within their home territories. But many bears here must look to other places for winter shelter.  They excavate dens under tree roots, or make their own protected cavities beneath tangles of fallen boughs. They spend time collecting leaves, branches and grasses to help insulate their winter quarters. Sometimes the female and her cubs from the previous winter work on the den as a team.

For some reason a bear builds a new den each year even if the old one remains in fine condition. They will build anew even if the den offers less protection. Such was the case in Michigan not long ago, when a female bear spurned her cave of the previous winter, built a new den above ground, but then fell prey, with cubs, to wolves.

Contrary to another popular belief, a hibernating bear is not comatose; it can be roused from sleep in mid-winter in several minutes. Nor does its body temperature drop substantially as is the case with smaller mammals, which must awaken at times during winter to eat and move about. A thick coat of hair and body fat allows a hibernating black bear to maintain a temperature of about 88 degrees F., about eight degrees lower than its summer temperature.

Female bears are likely to become at least somewhat active in winter. They give birth in late January or early February, and they clean and nurse their cubs. Their 1 to 3 cubs are hairless and weigh less than a pound when born, but they weigh 3-to-4 pounds by the time they leave the den, which seems impressive given the fact the mother doesn’t eat or drink over winter. (Nor does she defecate or urinate.)  Females may weigh more than 200 pounds before winter, but by spring they are likely to have lost 60 or 70 of those pounds.

Black bears in our region mate in June, and the females must put on as much fat as possible over the following six months to assure survival of both her and offspring. However, fertilized eggs do not implant in the female’s uterus and become embryos until November or December. This process of implantation apparently is disrupted if she hasn’t put on enough fat—which would help assure her survival, without cubs, over winter. This would explain why bear birth rates drop dramatically after lean summers.

To build fat, bears might travel miles in a single day to forage for berries, nuts, apples and other woodland plants.  They eat much and sleep little.

Black bears in our region face a special challenge: the growing fragmentation of forests by housing, commercial developments and highways, which can cut off their access to natural feeding areas and tempt them to try other food, such as sunflower seed. Not all homeowners with birdfeeders are as tolerant of bears as those in the Thetford neighborhood where I live. The last thing a homeowner wants is a bear coming to his or her house for food.

“A fed bear is a dead bear,” is an old saying that too often becomes a sad reality.

Li Shen is an adjunct professor at Dartmouth Medical School and a member of the Thetford conservation commission.

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Discussion
  1. Carol LaBree → in Bradford NH
    Nov 20, 2009

    My significant other and I have hiked the old logging roads and woodlands of Antrim NH, and over to Windsor via woods roads, and bushwhacking..overland; we have seen many claw scratched trees…but have not found a makings of a den yet. I had recalled finding a den in a bush pile in the woodlands I had cleared that was covered with snow, but had steam coming out the top, and what looked like a dug out spot on the side…needless to say I was never brave enough to look inside. My question to you is would a bear make its winter home in a old foundation covered with old logs next to a logging road, that is traversed by man?...Any Comments welcome.

    Carol

  2. Dave Chase → in East Saint Johnsbury, VT
    Nov 21, 2009

    I always enjoy reading your magazine and online stories, and I found the article about bears fattening up for the winter very informative.  I found the delay of embryo development amazing. However, remembering back to my Wildlife Ecology class, I learned that bears do not actually hibernate, they enter into a state of torpor.  We learned that only smaller animals are capable of true hibernation.  As an example, frogs burrow down into the mud for winter and actually do hibernate, their bodies produce a natural “antifreeze” that keeps their bodies from freezing solid and therefore preventing cellular damage. Perhaps someone more knowledgeable than myself could write an article on the differences between hibernation and torpor. 

    Thanks, Dave.

  3. Carolyn Haley → in East Wallingford, VT
    Nov 23, 2009

    Thank you for this article. Having lost several feeders this year to hungry bears, we’ve been wondering how/when they operate so we can manage our bird feeding appropriately. The information provided answers our questions.

  4. Dave
    Nov 23, 2009

    Foundation near logging road doesn’t sound like optimal bear den site, Carol, but you never know. It could have been. Check out this video:

    http://news.bbc.co.uk/earth/hi/earth_news/newsid_8320000/8320414.stm

  5. jayla gabriel → in louisianna
    Feb 03, 2010

    Why do the un-bred females have there slumber around Christmas?

  6. Bonnie Ovitt → in Hanover, NH
    Nov 18, 2013

    We seem to be on a path that bears use to move from one area to another! It is not unusual to see more than one bear a day here, in fact on one day in the summer of 2013 we saw a female with two very young cubs come down one side of our house and through the back yard, then about 5 mins later two larger bears,still smaller than the female came down the other side of our house. We thought perhaps they were the previous cubs of the same female as they appeared to be following her and I guess cubs stay with Mom for about two years, until the next young is born. Needless to say, we have many pictures and video’s of bears. We thought perhaps our geographical closeness to Ben Kilham may have something to do with the population we observe. Thanks for your article, will wait a couple more weeks to put our birdfeeders down lower…on upper side of barn now.

  7. Moose → in Vermont
    Nov 29, 2013

    If bears “can still be active until mid-January”, when is the earliest they can be expected to resume activity? That is to say, the reasonably bear-safe bird-feeding window in a “normal” Winter is from mid-January to ... when?

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