KEEPING WARM 
stead to build a nest above ground. (In 
February of the same winter this female 
and her newborn cubs became the first 
radio-monitored black bear family to be 
killed by wolves.) 
For their dens, the black bears I ob- 
served generally chose burrows, caves, 
rock crevices, hollow trees, or excavated 
depressions under fallen trees or brush 
piles. A few bears simply built nests on 
the surface of the ground. Hollow trees 
were uncommon, but seemed to be pre- 
ferred for dens because they probably 
provided the best insulation and protec- 
tion from hunters and predators. En- 
trances to underground dens or tree cav- 
ities faced in all directions — here the 
bears had no discernible preferences. 
The entrances usually were just large 
enough for the occupant to squeeze 
through, opening into a chamber that 
was typically two and a half to five feet 
wide and two to three feet high. The 
distance from entrance to bear was usu- 
ally two to five feet, but in some cases as 
much as fifteen feet. 
Temperatures in the dens depended 
on snow cover — the bears, with their low 
metabolisms, did little to warm the 
space. In winters with several feet of 
snow, den entrances were often covered 
and temperatures inside were only 
slightly warmer than the soil, a few 
degrees below freezing. In winters with 
light snow, den entrances remained open 
and temperatures in the dens were 
about the same as that of the outside air, 
frequently 60° below freezing. Open en- 
trances were more common. Those dens 
acted as windbreaks, but provided little 
other insulation. 
The main insulation for a bear in 
winter is its fur, which more than dou- 
bles in insulative value during the fall. 
The fur is thickest on the back, neck, 
and sides and thinnest on the muzzle, 
legs, and underside. A hibernating bear 
sleeps in a curled-up position so that its 
crown is against the den floor and its 
nose is near its tail. This position mini- 
mizes a bear’s surface area and reduces 
heat loss from the thinly furred areas. 
For extra insulation, bears sleep on a 
nest of leaves, grass, and other material 
that they rake into the den. These nests 
also insulate cubs from the ground. 
Females usually give birth to two or 
three cubs every two years; cubs are 
born in January, weigh less than a 
pound, and have practically no hair at 
first. After giving birth, the female re- 
sumes her deep sleep, waking on occa- 
sion to tend to the cubs’ needs. She licks 
them to clean them and to stimulate 
defecation. She eats their feces and 
moves into position to facilitate nursing. 
The cubs do not hibernate. They suckle 
and sleep snuggled warmly against their 
mother’s sparsely furred underside and 
reach weights of four to eight pounds by 
the time they leave the den at three 
months of age. 
Ending her hibernation in mid-April, 
a black bear emerges from her den 
with two cubs that were born in 
January. She will suckle the cubs 
in the vicinity of the den for a few 
days, then move to snow-free ground. 
67 
