Two years ago, immediately after the Portal Road opened, I was getting ready for a dayhike, and stowed some items in a steel locker. As I closed the door, I commented to a fellow nearby that it was so early in the season I probably didn't need to worry about the bears being out. He replied, to the effect "I work in the backcountry, and we see bears all winter long. Assume that they're out".
So, I guess not all of them hibernate, or if so, it's not a long hibernation? Personally, I've never seen bear tracks on snow in mid-winter in the Eastern Sierra, but if this fellow was believable, then some remain active all year.