Monday, March 17, 2008

Drawings of little-known lizards or parts thereof. IV. Getting small with Brachymeles, and then what happens.

In the forests of the Philippine Islands there exists a genus of small, cocoa-brown, long-bodied, scincid lizards (skinks) you might never see unless you have the urge to flip over a log, or scratch through the duff beneath the vegetation. These are Brachymeles, which means “short- something.” I’m guessing “meles” has do with limbs, because these lizards have either very small limbs, some with reduced numbers of digits, or no external limbs at all. They are burrowing invertivores, and their general body forms are shared by unrelated, burrowing lizards -- some skinks, some from other families-- in other parts of the world.

Brachymeles gracilis, a "large species." From Polillo Biodiversity.

Within the genus there is an inverse relationship between size and degree of elongation. That is, the smallest species, which are chopstick-thin, have the highest numbers of vertebrae. The larger species, what I’ll call, cleverly, "large species," which have functional limbs (First photo), have 32 or 33 vertebrae in front of the sacrum (pre-sacral vertebrae). The others, henceforth referred to as "small species," with very reduced or absent limbs, have approximately 50 pre-sacral vertebrae (Second photo). The totally limbless species B. vermis has 60. Incidentally, the species can easily be divided into two distinct groups; there isn’t a gradual grading of body size and form.

Brachymeles bonitae, a "small species." From Polillo Biodiversity, in case you missed it.

Being small, long-bodied, with more vertebral articulations, therefore presumably more axial flexibility, may work well for a functionally limbless lizard living within the forest litter. Lizard spines bend side-to-side, so these little skinks should be able to undulate, snake-like, through loose soil and other friable materials. However, there are functional issues related to smallness, to becoming smaller. One is reproduction. A female lizard has to contain eggs within her abdomen. The abdomen can be defined as the body cavity posterior to the ribs, a space that also contains the digestive system and other organs. So how to fit everything in?

The large species have a trick, or at least a trait. The relative size of the female abdomen grows faster than in males. The individual vertebrae toward the back of the body, and therefore the abdomen, increase in relative size, creating an elongated, more spacious cavity in which eggs are contained.

Diagram of gravid female Brachymeles drawn to scale showing position, size and shape of eggs, and position of the last parasternal rib (PSR).

The small species have more abdominal vertebrae, which would suggest relatively more room to carry eggs, especially if the body segments also became relatively longer in females. But this doesn’t happen. As in a juvenile lizard of either large or small species, the pre-sacral vertebrae remain approximately the same size along the length of the body. (Perhaps functionally related to a reliance on undulatory locomotion?)

Developing eggs have their own requirements and properties, based on physiological needs of the embryo and the evolutionary history of the group to which the lizards belong; one thing: they can only be so large, or so small.

A large species female, possessing what is probably closer to the body form of the common ancestor of the genus, can produce and carry up to six eggs, which are almost spherical and packed within the abdomen as far forward as beyond the last parasternal rib. The slender, small species can carry one or two cylindrical eggs, entirely posterior to the ribs.

So what?

So being a pregnant lizard is probably no fun, no matter what shape you are. Beyond that, the evolution of elongation (and this is nowhere near the degree of elongation seen in snakes, which have at least a hundred, in some, several hundred vertebrae -- and not to mention significant rearrangements of internal organs that affect available abdominal volume) is tied to changes in reproductive output, and, possibly other life history traits such as age at reproductive maturity. The small, elongate, limbless species may reproduce at an earlier age, essentially as morphological juveniles, an evolutionary change that may be a form of progenesis. There are no data to test this, unfortunately. Mostly there are pickled Brachymeles in museum jars to x-ray, measure, and muse about.
Is there a take-home message to this post, which seems to have zig-zagged as much as a panicked skink? Perhaps this: things evolve from other things, and not by changing just one thing. Things that change (size, shape, life history) are functionally and developmentally connected and historically limited.

Another elongate, burrowing skink, Plestiodon egregius. This one from Florida.
Again, much smaller, more vertebrae, fewer eggs than large Plestiodons. There are rules to building these things, not yet figured out.

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