Sunday, February 10, 2013

Pelvic Spurs Revisited

Last week, one reader indicated that some herpetology textbooks refer to the pelvic spurs of some snakes are the remains of vestigial limbs. This is contrary to what I claimed in one of my previous posts ("How Pythons Get It On"), in which I said that pelvic spurs are actually modified scales and are not the remnants of legs. Since then, I have been doing some digging about this topic in an attempt to clear things up. What I found was that there is some debate upon the issue, and more research should be done to determine if pelvic spurs have any relation to legs.

I immediately brought the issue up to my herpetology professor here at my university. She had been taught that the pelvic spurs of snakes are related to vestigial legs, and she had never heard anything contrary to this idea. She managed to find a quick reference to pelvic spurs in my textbook, but the mention was brief and provided no analysis. Even though I would trust both of these sources, I also realize that they could be misinformed.

I began searching for reasons behind the conflict. It appears that there are two issues at hand here that make the connection between pelvic spurs and legs a bit fuzzy:

1.) Pelvic spurs are not connected to bones. Some use this argument to claim that pelvic spurs are not related to legs. However, I say this means nothing. Just because structures are not connected now, we cannot assume that they were not connected millions of years ago.

2.) Pelvic spurs have much greater mobility than other scales. If you have ever seen a snake move, you will recognize how the skin of the snake will stretch and bend as the snake winds, and the keratinous scales move along with it. The only thing that changes as the snake bends is the spacing between the scales. Pelvic spurs, on the other hand, are quite mobile, and the male will be able to make his pelvic spurs pivot, dig, and tap.

Legless lizard. What a cutie.
Found at http://lpfw.org/wp-content/uploads/2012/04/legless-lizard.jpg
In his 1985 article in Herptile, Raymond Hoser questions the assumption that pelvic spurs are related to legs and suggests that they may be "auxiliary sexual organs." Hoser points out that Australian legless lizards also have structures like pelvic spurs, and this is where I thought, "Aha! The lizards that have pelvic spurs are legless! We have our answer!" In fact, we do not. The legless lizards actually do have small legs that are separate structures from the spurs, according to Hoser. This is all really one big mess because I cannot find the original article in Herptile ( I found Hoser at this url: http://www.smuggled.com/pelspu1.htm), nor can I find another reference to both pelvic spurs and vestigial legs in the legless lizards. And, of course, I like to have multiple sources.

I realized quickly that this is becoming one large mess that is going to be very difficult to sort out. I would suggest exactly what Hoser recommended in 1985, and that's that we first examine the structure of the legless lizard in more depth. Perhaps legs and pelvic spurs have nothing in common. However, I would also recommend that we look into the exact musculature that is used to move the pelvic spurs because perhaps this could be related to the same muscles used in legs.

I believe the most defining evidence found will come from development. If we can look at which genes contribute to the development of the pelvic spurs (and in which patterns these genes are activated), we may be able to determine if pelvic spurs arose from legs. After all, many genes and developmental patterns are shared among species that look nothing alike. Using the same genes (which Sean Carroll refers to as "tool kit genes" in Endless Forms Most Beautiful), many animals follow the same patterns to develop very different structures. I suggest that if the pattern is the same, perhaps the structures are related.

In conclusion, it appears that pelvic spurs are generally accepted as vestigial leg remnants among the herpetologist community. Although I now say I do not have the answers, I think the topic should be investigated rather than assumed.

Through all of this mess, one thing has been cleared up. In my first post regarding pelvic spurs, I questioned whether female pythons had pelvic spurs. In fact, according to Slip and Shine (1988), females have pelvic spurs, though they are less prominent than the males'.

3 comments:

  1. Stacy,

    Have you yet found reference to the appearance of vestigial pelvis bones in such snakes as Morelia boeleni, or in some snakes in the families Typhlopidae and Leptotyphlopidae? In the case of M. boeleni, there is some evidence of connection of a vestigial femur to the pelvic spur.

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    1. Thanks for the comment, David. I have only heard reference to the pelvic bones of pythons and boas, so your comment about Typhlopidae and Leptotyphlopidae is interesting. I had to look them up because they were new to me. It is interesting that snakes that live sub-terrestrially would still retain some form of pelvic bones. I have heard the hypothesis that snakes evolved from burrowing lizards, which would make Typhlopidae and Leptotyphlopidae members possible stepping stones in the course of evolution.

      It is fascinating to heard that M. boeleni may yet retain a connection between its pelvic spurs and femurs. I wonder if there are muscles attached to the femur that may aid in pelvic spur movement. Aside from this, it is a gorgeous snake in general.

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  2. I'm extremely skeptical of this "modified scale" idea, and not entirely because Hoser's "work" is to science what Twilight is to great literature. Here's the Herpetile "paper" (http://www.smuggled.com/pelspu1.htm), which, as you can see, rises to to usual quality of his "scientific" contributions.

    First, there's the phylogenetic signal - most "basal" snakes have pelvic spurs or internal pelvic vestiges, with it being lost only in the Uropeltids and Caenophidians. However, the latter includes most living snakes, over 3000 species, and it seems odd that in none of those would some sort of "modified scale for mating" re-evolve.

    But mostly, it's the anatomy. I'm not sure who's said that the anal spur does not connect to bone, but that's not been my experience. Furthermore, the bones are clearly homologous to limb bones, with some species even having a distinct ilium, ischium and pubis that meet at the acetabulum, where the femur remnant joins them.

    If a snake were to evolve such a modified scale, why bother with the osteology? Snakes have copious muscular connections between their skin and skeleton, so there's no need for extra bones to make things mobile. And that the bones so closely match the pelvic osteology of tetrapods in general is the final nail in the coffin of this idea.

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