Do ball pythons need a heat lamp?

I have two albino ball pythons, one male and one female, I have them both in a good sized tank. The two do just fine together and have good accomidations for them ( a hollow rock for hiding, a water tank, climbing branch, log, and a whole lotta aspen bedding) I was wondering if they absolutely need a heat lamp, I was thinking since they hide or "sleep" together that they would produce some heat and also they sleep together in the rock or under the water dish so maybe that would conserve the heat. Any help?