They sure don't. Conditions play a role in if they go into a fast. Mine is eating this year like a pig so much so I have to restrict his diet.
On a funny side note, this winter, Dante has taken up play hunting me. Just last night he wagged his tail like crazy and was stalking me while I was sitting in front of his enclosure. It was so hilarious watching him only move forward when I was not looking and shaking his booty like crazy.