The Bears Are Back in Town

Apr 15, 2019

Black bears are easily one of our most intriguing wildlife species up here.

A few weeks back an anonymous listener from the Rhinelander area submitted a question to our Curious North series: What time of year do bears come out of hibernation?

In this week's episode of Wildlife Matters, the Masked Biologist answers that question and more, as he discusses some of what is going on in the life of a bear as it emerges from winter.

Black bears are a continual source of questions for folks who contact me for information. The calls tend to taper off somewhat in the winter, naturally, as bear activity slows to a crawl. That is no longer the case, though, as we move to Mid-March to Mid-April, bears emerge from their dens, and the calls and concerns about bears pick up.

Black bears enter their dens in early winter. That time is different for every bear, but studies have shown that the less natural forage they have available, the sooner they hibernate. When their natural food is supplemented by unnatural food supplies, that can change. It used to be common for bears to den up as temps dropped in October; now it is more like November before the bear observations drop. Once they are in their selected winter den, they drop into a deep sleep, which slows their metabolism and allows them to live off their body fat stored through the summer and fall months.  True hibernation by mammals like chipmunks and groundhogs includes lowering body temperature to near freezing. Bears do not enter true hibernation, but more of a winter sleep, or torpor.  Torpor allows them to rouse more easily than true hibernation, so they reposition, groom themselves, give birth, and if necessary escape danger readily.  However, as researchers discovered that bears can lower their heart rate down to 8 beats per minute, and breathe once every 45 seconds, they have decided hibernation is still an accurate description of their overwinter condition. Their den may be completely underground, partially exposed until covered with snow, or out in the open (I heard one story about a bear denning in an eagle’s nest). If you are out in the woods, you can sometimes find a bear den by spotting a melted exhaust hole in the snow.

Bear cubs are born in February, at which time they weigh less than a pound and fit into the palm of your hand. They have a covering of fine stiff hairs, but for all practical purposes they are essentially naked and their eyes are closed. They nurse from their mother through the winter, growing rapidly. In less than a month, the eyes are open and they have a covering of short fuzzy fur. While the mother may snooze off and on during this time, they are very attentive to their cubs; if the den is too significantly disturbed she may even select a new den and move the cubs one at a time to finish waiting out the winter. In spring the cubs will emerge from their dens weighing about six pounds, still nursing and completely dependent on their mother. While it was once basically considered a fact that female bears have two cubs, we now know that three to five cubs are common. Success of the pregnancy depends heavily on how much fat the sow (female bear) puts on during her foraging before hibernating.

Spring emergence is a moving target much like the start of hibernation. Several studies have linked the shortening of their hibernation period to the observed change in climate over the last 50 years or so. As the winters have moderated in temperature and shortened in length, bears have started emerging earlier. Fifty years ago, mid-April was a reliable time for bears to emerge. Today, really any time after St. Patrick’s day would be fair game. Emerging science also seems to indicate that, the more “human food” a bear consumes before hibernation, the shorter the hibernation period. Early emergence is a real problem, as bears need to put on fat fast, and they are out before food sources like green plants and insect larvae are readily available. This bears further research especially here in Wisconsin where artificial food sources from hunting activities add countless tons of cookies, frosting and granola bars to the landscape.

Striving to make new things familiar and familiar things new, this is the Masked Biologist coming to you from the heart of Wisconsin’s great Northwoods.

Do you have questions about our local wildlife or the environment? Submit them to our Curious North series below and they could be featured in a future Wildlife Matters commentary with the Masked Biologist.

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