Heat from Above or Below?

The following text is part of a discussion on the IguanaDen List about heat from above versus heat from below. Text in italics is from Christina, responses are by Veronica.

I was thinking, in their natural habitat, do iggy's not warm themselves on rocks and absorb heat from them as well as the sun above? Would that not be similiar to what I'm proposing?

**No...in their original, natural habitat, they sun themselves from the canopies of the trees. Coming down to the forest/jungle floor is asking to be eaten so they limit this action for laying eggs mainly. Absorbing heat from above is how they've adapted over the milennia. They have adapted to life as a heliotherm - which means to bask in warm air rather than "plastering themselves to hot surfaces" (Melissa Kaplan's words)...or being thigmothermic.

Other than the risks of injuries, etc. can someone tell me healthwise what does having heat come from above as opposed to below do? If it affects their digestion, etc, can you tell me how? I'm one of those people, I really like knowing the why's of everything.

**Igs cannot feel bottom heat the same way we can...or other reptiles that DO rely on ground heat like many other lizards, snakes and turtles do. The nerves have devolved into a minimal number of sensory collectors. Their circulatory system depends so much on the air around them they have lost the ability to feel how hot something gets on their bellies...like a malfunctioniung heat rock.

All they know is that they feel the heat, but by the time the warm blood circulates to the top of the ig it is cold again making them stay on the bottom heat source in a futile attempt to get warm. The igs internal, or core, temperature never gets to the proper temps for ideal bodily functions which include, but not limiting to, proper food digestion (ranging from the proper mineral and vitamin dispersion/absorption to eliminating wastes), mood changes (mostly irritable and depressed), and the belly heat actually cooking the foods inside the igs' stomach creating a whole set of problems there.

Melissa Kaplan has a good piece in her book (Iguanas for Dummies) that explains a bit about the different presentations the igs use with their body when basking, or when it's time to cool down. This is where she gets into the smaller piece on being a heliotherm (pg. 66).Hatfield (Green Iguana Ultimate Owner's Manual) also goes into the different presentaions the ig will use in order to get the maximum efficiency from the heat emitted from overhead. This heat would come from whatever overhead would be available, sun, CHE or basking bulb.