KEEPING WARM 
Jim Brandenburg 
The bears under study usually spent 
considerable time in the fall gathering 
nest material and arranging it in their 
dens. Most of them arrived at their den 
locations well before snow covered the 
nesting materials, usually in late Sep- 
tember or early October. (One arrived 
on September 2.) After digging a bur- 
row or depression or renovating a rock 
cave that another bear had used in years 
past, the bears spent most of their time 
resting in and around the den or gather- 
ing nest material until they entered their 
dens to stay, usually in late October. 
Whenever cedar trees were present, 
bears stripped the fibrous bark from 
them to use in making nests. The narrow 
strips of bark, up to several feet long, 
make a durable bed that does not par- 
tially disintegrate and lose insulation 
value by spring in the way that beds 
made of finer materials such as leaves, 
grass, and clubmoss often do. These less 
sturdy materials were generally the only 
ones available, however, and so most 
nests were built with them. A few bears 
waited until snow covered the ground to 
start the work of creating a nest, but 
these were mainly bears that had access 
to an unusually long-lasting food supply, 
such as acorns, mountain ash berries, or 
garbage. They either did not make nests 
or made them of conifer boughs that 
they bit from trees. 
From September 29 through October 
18, 1972, Steve Wilson, Leon Konz, and 
I worked in shifts around the clock, 
watching a family of bears settle in for 
the winter. During the previous week, 
By the time they emerge from their 
natal dens, black bear cubs weigh 
from four to eight pounds. As adult 
females give birth at intervals of 
two or more years, these cubs, above 
and right, will remain with their 
highly protective mothers through 
the following summer. 
the animals had completed a burrow 
under the roots of a fallen pine and with 
all three cubs (of the previous winter) 
helping their mother, had begun to rake 
up bedding from a fifty-foot radius 
around it. The cubs raked leaves, grass, 
and forest litter, backing toward the den 
as they pulled piles of material with 
their front paws. The five-year-old 
mother did the same, but in addition, 
she bit branch tips from willow bushes 
and pulled up supple Labrador tea 
plants with her teeth, dropping them on 
her own piles. She also took charge of 
arranging all the nest material in the 
den. The cubs could have done a good 
job of this, too, as orphaned cubs have 
demonstrated, but the mother had her 
own ideas about the nest. Twice she 
scooped all the material out of the den, 
sending it flying backward between her 
hind legs and then putting it back her 
own way. Except for these vigorous re- 
modeling jobs and instances when the 
cubs quickly grouped behind their 
mother after hearing a strange noise, all 
the actions we saw during this period of 
den preparation were languid and list- 
less. There was no play. The bears were 
active only during the day, and during 
the three weeks of observation the activ- 
ity decreased from a few hours per day 
to a few minutes before ceasing alto- 
gether. 
The activities of these bears followed 
about the same daily schedule as those 
of a captive black bear in Alaska whose 
heart rate Ed Folk of the University of 
Iowa monitored by telemetry. That 
bear, too, was active mainly by day as it 
neared the hibernation period. For most 
of each day in early fall the bear’s heart 
beat between fifty and ninety times per 
minute, then for most of each night, 
while the bear rested, its heart rate 
slowed to between forty and fifty beats 
per minute. As fall progressed, the bear 
began sleeping for more and more of 
each day. By December, when monitor- 
ing ended, its sleeping heart rate had 
gradually declined to as few as eight 
beats per minute. This heart rate was as 
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