OK, the thing is if you've seen enhanced views of the Blast Door Map from "Lockdown", you might have noticed the latin "Ursus Maritimus" in the bottom right corner near where it locates "Zoological Research" (ie. where Kate and Sawyer and Jack are). It also says "stated goal accelerated deterritorialization of Ursus Maritimus".

The polar bears may look normal on the outside, but have been engineering/conditioned to behave differently than they would in their natural environment. This may well include hibernating, seeking shelter, and perhaps more aggressive behaviour.

It is also possible, imho, that the Others we've seen so far have undergone some similar process to make them stealthier, more aggressive, more intelligent...and who knows what else.