When do bears emerge from hibernation?

Some trivia books still try to pose this as a trick question, stating that bears don’t emerge because they never hibernated in the first place. The bear’s body temperature doesn’t fall as much as that of other hibernating animals, and this was once thought to disqualify them as true hibernators. But now we know that their higher body temperature is simply a function of their larger size.

Like other hibernators, bears don’t sleep through the winter, but rather wake up periodically and eat what they’ve stored in their cave. They emerge for good when average temperatures are above freezing.

How does the bear determine the average temperature? Damned if I know. I’m more interested in how we knew about the bear’s hibernating temperature in the first place. Would you enter the cave of a sleeping bear and stick a thermometer… well, wherever?

Source: The World Book Encyclopedia by World Book Inc.