Biology Reference
In-Depth Information
limbless birds or frogs, arms and legs being the essence of flying and jumping, but
the amphibian relatives of frogs include superficially wormlike caecilians as well as
sirens and several other near-limbless salamanders. Most impressively, limbs have un-
dergone independent reduction and loss more than a hundred times just among lizar-
ds, repeatedly within skinks and certain other groups—and one of those evolutionary
experiments led to snakes. Extant species with reduced limbs invariably have elongate
bodies, implying that over and over tetrapods first got skinny, then gave up appendages
as other travel modes evolved. 10 Over geological time, the giant python that might eat
you today came from a small, insectivorous lizard with tiny legs.
Locomotion entails support, propulsion, and steering, accomplished without limbs in
serpents and best appreciated while watching an undisturbed snake moving in nature.
At a bare bones level, limbless reptiles are stretched-out versions of more convention-
al tetrapods. Drop girdles, arms, and legs, multiply vertebrae and ribs about twenty-
fold, and you've got a snaky human. Whereas our skeleton totals 206 bones, an average
snake boasts about that many vertebrae, and then there's the ribs and skull elements.
(Pythons, boas, and a few others also retain a tiny pelvis and vestigial hind legs, called
spurs.) Serpentine muscles resemble in anatomy and function those tissues that move
our ribs and backbone, but some span up to thirty or more vertebrae, in overlapping,
weblike fashion. As many as twenty or more repeated sets of muscles on each side tug
on vertebrae, ribs, and skin, thereby effecting different, often continuously changing
movements along the body.
What are the reasons for this radical shift by which limbs were repeatedly lost?
Skinny organisms move in ways so distinctive they're given special names: “laterally un-
dulating” in symmetrical waves, “sidewinding” by lifting themselves forward, crawling
“rectilinearly” like caterpillars, and pulling and pushing through tunnels and up trees
in “concertina” fashion. Reduced diameter also permits entering crevices, requires less
force for burrowing (shove a tent stake into soil, then try a piece of fence post of the
same length), and allows passage over frail substrates (backpackers fall forward in
quicksand, spreading out their weight while crawling to safety, whereas serpents skim
across foliage that couldn't support a running squirrel of the same mass). 11 And as ob-
served in our black-tailed rattler field studies, snakes, unlike stoutly built lizards, can
leave a body loop in the sun and warm an abdomen swollen with food or embryos even
while hiding from predators under a rock or log. 12
Costs are the other side of the skinny-animal coin, two of which involve energy. First,
just as our arms and legs cool rapidly due to high surface area, a snake-shaped mam-
mal couldn't sustain high body temperatures. A weasel, the closest approximation to
an endothermic serpent, barely hangs on by eating nonstop, whereas ectotherms warm
up by basking, have lower metabolic rates, and simply hide out when it's cold. Second,
slender amphibians and reptiles have narrower heads than stout ones do, yet have to
gain equivalent nutrition. They've solved this little-mouth dilemma by eating lots of little
things, eating chunks of big things, or reorganizing their heads and eating big things
intact. Accordingly, blindsnake stomachs contain hundreds of tiny insect larvae, caecili-
ans shear off earthworm pieces, and some snakes consume prey exceeding their own
weight, a stupendous feat I'll explain shortly.
Avoiding being eaten poses a third problem for skinny creatures because, all else
equal, greater length makes detection more likely, and disabling blows can fall any-
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