what makes bears to hibernate in winters?

Answer 1

From Ohio State University:

"Bears don't hibernate through winter. They just go into a really deep sleep.

It's easy to understand why you might think bears hibernate. In fact, wildlife specialists are finding that the different kinds of winter sleep have a lot of similarities. "

Click the link for the rest of the information.

Answer 2

http://en.wikipedia.org/wiki/Hibernation

Its a hormone that's released in response to the environment, so bears can survive at colder temperatures because their metabolism is greatly slowed down...its very amazing..you should do a search in scientific journals..theres a lot of research on this topic..