Despite previous annoyances from a reader in doing so, I am once again thinking out loud, and I pose a question which I, at least, find interesting. Why do squamates shed their skin? Well, the first conclusion one draws is that perhaps skin-shedding is primitive for Reptilia. But if that were the case, then one would expect turtles and crocs to shed, but they don't. Turtles will slough off old or damaged sections of their shell, but it's not what one would call molting. Modern lizards and snakes go through the same ritual that arthropods do: They crawl out of their old skin.
Perhaps skin-shedding is primitive to Diapsida, which would make it a derived trait. But then crocs remembered how to grow larger without shedding? And what about dinosaurs and their kin? Certainly Diplodocus wasn't shedding its skin all the time. Birds lose feathers on occassion, but not to the same extent that squamates do. Birds shedding feathers is not homologous to my leopard gecko losing his skin.
Perhaps molting is specific to Squamata. Do Tuataras shed their skin?
And if it's a derived trait, what makes it more advantageous than just growing? There are plenty of risks that come from skin-shedding. A predator could grab you by your flaking skin and pull you towards it. If the humidity isn't right, my geckos have a difficult time shedding. The skin will flake off, or be sticky in places. Sometimes they won't get their "gloves" or "socks" off, and the dead skin will harden to the end of a toe, resulting in the loss of a terminal phalange. Occassionally, in my older gecko, sand will build up on the inside rim of his upper jaw, and when he sheds, his "lip" skin will become stuck beneath that sand rim, and I have to get the tweezers and pop that hardened sand out of his mouth before I can get his shed mouth skin off.
It's easy to see how, in the wild, lizards might have a difficult time shedding properly!
I have considered, however, that shedding might help heal wounds. My frog-eyed gecko, Big Boss, had a scratch on his throat when I bought him. The top layer of scales had been taken off by some unknown means (perhaps a fight with his pet store roommate), and a red patch remained. It wasn't an open sore, but it was clearly an injury. Big Boss shed soon after we got him home, and after that initial molt, the red spot had faded. He just shed a second time last night, and the wound is barely noticeable. He has regrown the scales over the red spot.
Another interesting tidbit. I have three leopard geckos, and they have all gotten on the same "shed schedule," in that they all shed around the same time. Not simultaneously, but when one starts shedding, you can be sure that the others will soon follow. And it's not like the leopards are getting any larger--my oldest, Mr. Fat, is twelve years old and hasn't grown a centimeter half a decade. He is getting darker, though. But the leopards are never injured, so I wonder why they still shed at all (they are all adult size and quite fat).
Anyway, just thinking out loud again. Feel free to chime in.