As winter turns into more winter, the Brown Bears of the Northern Panhandle turn in to den and lay in torpor until the spring. KHNS’s Alain d’Epremesnil caught up with Alaska Fish And Game Research Biologist Anthoni Crupi to hear what has been learned about the process.
25 bears are currently tagged in his study of the northern Lynn Canal brown bear population. As of last week two of them were still fishing on the Chilkat river. The transmitter signal from all the others indicates they are at higher altitude. This means they have settled into their dens or are about to. On average in the upper Lynn Canal, half of the bears will have gone to den by Thanksgivings. Every year around this time, each bear has to make a calculation familiar to many human fishmongers.
Here is Crupi
“They are weighing this balance of ‘Am I taking in more energy than it is to stay warm in this cold weather, or is it smarter for me to kind of shut everything down and go into a winter hibernacula?’ and be able to conserve that energy that they’ve stored.”
Their decision will depend on the weather, how good the fishing is, and how much fat they have already accumulated. Once they are ready, bears leave the river and head for the mountains. The average altitude of dens in this region is just under 2500 feet. The bears will look for a site that will accumulate enough snow for maximum thermal insulation, on a sufficient slope so as to drain well. A den can become quite humid, with mushrooms finding a proper growth environment. If the moisture becomes too much, the den will collapse, forcing the bear to dig another one to finish the winter. The most common type of den in the upper Lynn Canal is an excavation in the soil on the side of the mountain. Very often the bears will start to dig under vegetation, the roots serving to reinforce the structure of the den’s roof. Sometimes they will dig under a boulder, which will then act like the header over a door.
Some dens are rock caves, those will be used year after year, generation after generation, and the inside appears polished by the repeated rubbing of the fur against the walls. The bears will build a nest in those rock caves, using blueberry branches, duff from decayed trees, willow boughs, or sand when near shore. Some bears in more coastal climates have been known to simply build a nest on the ground, and let falling snow cover them with its insulative protection.
One thing seems to be consistent, according to Crupi’s observations.
“And one thing that is really neat with all the dens that we’ve gone and visited so far, is it’s almost always in a place where they have a spectacular view of the watershed below them.”
Once comfortably installed, the bears settle into a torpor that will see their body temperature drop 10 to 12 Degrees Fahrenheit, their heart rates drop 20 to 40 percent, and their metabolism slow to less than half its active rate. They will also stop defecating or urinating. They will spend the next 5 to 6 months in this state. Crupi says he once tracked a three year old female who went in in mid october, and emerged mid June, spending a full eight months underground.
And in the middle of it all…
“In January is when females will have their cubs in the den, and then come late April would come out of the den, the cubs are going to be very immobile, so not being able to move that much, so one thing that’s common in females that have cubs in the springtime is they spend a lot of time just on the outside of the den before the things green up where they can start to eat vegetation, they are very limited by how far they can take those smaller cubs.”
Over the winter a bear will lose a quarter to a half of its body mass, but because their accumulated fat gets recycled into their muscles, bears are able to emerge from the dens with enough strength to begin the next seasonal feeding cycle. Most of the excavated dens will be abandoned and will collapse from excessive moisture. There is no indication that other animals move in and use the cavity.
This thorough knowledge of denning behavior allowed Crupi to draw a map of critical denning areas that is surprisingly detailed and useful in making predictions. This can be used to make management recommendations to regulate the timber, heliskiing and commercial flight industries. Suggestions include prescribed flight paths and predictable routes, and minimum approach distances and altitudes. Disrupting a bear’s torpor imposes a high energetic cost. A lot of work goes into that sleep.
Here is a link to Anthoni Crupi’s hour long presentation of the research on our website.
