I've thought about this for a bit, but only recently did I see any proof.
Last weekend was relay for life here, so I brought Fatty as part of a reptile "room" fund raiser. It was about 50 (F) out in the morning, but I noticed that without a heat source Fatty was still very active. After a while the temps went back up then back down to 50, so I had my parents pick up the animals. My mother recalls picking Fatty out of her cold cage and saying that Fatty was incredibly warm. So, for this year's science fair I'm thinking about doing a project on whether or not garter snakes can generate their own body heat in suitable amounts. Has this been tested before? Is it outright true or false?