Biology Reference
In-Depth Information
where along a slender body. As for solutions, like most other animals, reptiles generally
avoid discovery, flee if found, and confront enemies only as a last-ditch gambit. Judging
by their closest living relatives, early snakes were secretive burrowers with drab
colors for camouflage when they crawled on the surface, and if attacked their writhing
struggles exposed brightly barred bellies that mimicked unpalatable millipedes and
centipedes. If harder pressed by a predator they expelled noxious secretions from tail
glands uniquely characteristic of serpents, whereas venomous bites and more elaborate
defensive tactics came later in their evolution. 13
One other attribute deserves emphasis. Although a forked tongue evolved repeatedly
among lizards, most obviously in monitors and tegus, only snakes have a throat skeleton
so simplified that it no longer assists swallowing and functions solely for lingual move-
ment; only the lightly built and exquisitely controlled snake tongue waves about re-
peatedly during protrusion, its tines together or widely splayed, tips curved upward or
down; and only a snake's tongue delivers airborne molecules to corresponding right and
left parts of a supersensitive vomeronasal organ in the roof of its mouth, thereby provid-
ing directional cues. 14 Thanks to rapid volleys and sustained flicking of this remarkable
structure, that favorite blacktail of mine mentioned earlier routinely tracked her bitten
prey dozens of yards, all the while ignoring the abundant spoor of other mammals. And
thanks to the same extraordinary capacity for processing complex chemical informa-
tion, male 26 followed superfemale 21's every turn over far longer distances—but only
in years when her trail advertised sexual receptivity.
The first serpents perhaps used their fancy tongues and palatal chemosensors to
search for insects, as do many modern lizards and blindsnakes, but their descendants
soon shifted to heavier items. And just as modern pipesnakes eat eels nearly equaling
themselves in weight, ancestral snakes could have radically changed foraging econom-
ics without increasing gape. Whereas lizards consume many arthropods daily, each
hunt-and-capture bout exposing them to danger, proto-pipesnakes could have cut that
to a few risky forays per year by pulling their heads over long slender prey. And dur-
ing that early lifestyle shift, regardless of details, snakes likely subdued their adversar-
ies with a limbless version of the bear hug—a small boa tightened around one's arm
elicits more discomfort than a blood pressure cuff, and can dispatch a rat in about a
minute. Constriction in turn facilitated the evolution of macrostomatans, able to swal-
low items shaped more like hamburgers than hotdogs. One six-foot-long, fifty-million-
year-old fossil “big-mouth” I studied contained a two-foot crocodile weighing 45 percent
of its own mass. 15
How, then, does an anaconda eat a caiman, or a python an antelope? After all, even
their average meals are tantamount to me swallowing a ninety-pound object without be-
nefit of hands or utensils.
Eating big prey obviously involves anatomy, including highly elastic skin and liga-
ments, but one oft-recited characterization is simply wrong. Rather than unhinging or
disjointing their jaws, early serpents invented a method for ingesting bulky prey by
dropping one hinge and adding others. In most vertebrates, lower jaw halves fuse dur-
ing development at a joint—felt as a groove between bumps on our chins—but that su-
ture never forms in snakes. Their ancestors also freed up some skull bones and stuck
in extra joints, so the lower jaws hang from struts rather than articulating directly with
the skull. And while vertebrate upper jaws are usually firmly attached and support a
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