Garters (all reptiles) are by definition ectothermic... they regulate their body temperature via outside sources (sunlight, shade, heat-mat, water etc). They are not 'cold blooded', 'cold blooded' is a misnomer, they simple do not create their own heat internally, but rather get it elsewhere.
It is this ability that makes them so successful in colonising some of the most inhospitible places on Earth. Being ectothermic allows them to brumate (or aestivate) for such extended periods, it allows them to go without food for such extended periods. We're they endothermic, like mammals, they would be.... well, mammals!
