They don't have to be, but some seem to be doing it on their own. Some of my leos (and my AFT pair as well) are hanging out more and more on the cool side and are not hungry. Since the leos in question are in the neighborhood of 100 grams each, for the most part, I'm not worried about weight. I have never cooled them but since I"m leaving the country for about 10 days at the end of the year, I'm thinking of cooling the ones that are cooling themselves to make life easier for my gecko feeder.