There are now around 30 bears in this one valley.
As they rub, each leaves an individual and recognisable scent.
So the tree soon carries a list of who's around,
which might help individuals to avoid a fight.
To best spread their scent, they really have to put their back into it.
But summer is short. Itches satisfactorily scratched, it's time to eat.
In a couple of months, they will have to return to their dens to hibernate,
so now they must put on as much weight as they can.
Winter in the mountains returns fast and hits hard.
Temperatures in the Rockies will drop to -54 degrees,
so cold that moisture in the air freezes into tiny crystals called diamond dust.