Tuesday, February 19, 2008

Why do squamates molt?

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.


Amanda said...

Wow...I somehow missed that annoying thing with the comments on the pterosaur post. That sucks.

Some of the stuff I've learned by posting questions on my blog (things I could have researched myself if I had the time) are the most lasting. Usually, someone with MUCH MORE expertise than I answers me and they put things into words I can understand. Like Jerry's comment on pnuematicity. I could have looked up all of that, but I would have had to look at many different sources, some of which may have been hard to find and then I would have had to undestand them as written. Thinking out loud is awesome, as is answering questions. Just because something is easy to look up, doesn't mean it's not worth asking about...

About the molting? I don't know. But I want your gecko...little fat, cute thing.

Neil said...

Periodically replacing portions of the integument is seen in most (all?) vertebrates. Although humans tend to shed skin and hair piecemeal, many mammals do molt seasonally as do all birds.

Squamates do take the process to extremes however, and it's interesting to note that certain squamates can replace body parts, so I think your observation about healing is salient here. Lissamphibians also molt their entire skin (remember those crazy caecilians that eat their mother's skin), and some can replace entire limbs.

Many protosomes (annelids, arthropods) molt as well. Perhaps in addition to the regenerative aspect you note, molting is related to allometric growth of a segmented body?

Your observation about synchronized molting is very interesting...I wonder if it's mediated by pheromones? Dig into the literature and see if anyone has done any work on this

lantaro said...

Big Boss is the shit. Liquid and Solid rock house, too. And Fatty is still plotting to kill your wife.

Maybe it's just a jacked up part of their evolution. Like humans and our appendix. It doesn't do anything but burst and poison us with bile. In the past, maybe it was useful, but now it just serves to inconvenience them.

I have no idea what I'm talking about.