That’s a great question! Scientists are
still trying to work out exactly what makes bears
enter and leave hibernation. There’s probably
a combination of external factors (like the
temperature of the environment) and internal
factors (like hormone or nutrition levels).
First, it’s important to understand what
hibernation is about. It’s an adaptation to living
in a climate that has big seasonal
variation. Usually, it means a cold winter
with low food availability. Hibernation is
a sort of super-sleep. A hibernating animal slows
down all of its processes (its metabolism). The
slower the metabolism, the less energy they burn.
Heating ourselves (as birds and mammals do)
takes a LOT of energy, so hibernators drop
their body temperature. Some animals actually
have body temperatures below freezing in winter,
but bears don’t drop their temperature as
much. If the timing is right, they enter
hibernation after they have put on a lot of extra
fat to fuel themselves during the winter, and
before food gets scarce. They should leave when
there’s enough food around to support them.
When you think about it, external factors would be
important for helping bears survive in different
environments. Let’s just look at black
bears. They can be found from Mexico to
Alaska. Obviously, the conditions are very
different in the northern and southern parts of
their range. If their hibernation clock were
completely determined by genetics, southern bears
would hibernate too much and northern bears would
go into hibernation too late and leave too early.
That means that they must be born with a flexible
“hibernation clock” that can be adjusted by
things like temperature of the environment.
There seems to be more to it than just
temperature, though. Availability of berries
explains some of the differences in when bears
Even in the same location, bears may enter or
leave hibernation at different times. For example,
female bears give birth during the hibernation
season. The female keeps hibernating, but the
newborn cubs just snuggle up, suckle, sleep, and
grow. If they hibernated, their metabolism
would be low and they couldn’t do the work of
growing. Females with new cubs stay in hibernation
the longest. This probably allows the cubs to be
bigger. Adult male bears come out of hibernation
first. They may need to do this to get an
advantage over other males. A bigger, stronger
bear will be able to defend a bigger territory.
Then females with no cubs or cubs from earlier
years emerge after the males.
There’s some great information at this site:
nature. Did you know that bears may move
a ton of dirt while digging their dens?
What else do you think is important in causing
bears to enter or leave hibernation? You may
want to study physiology if this kind of
question interests you.