Biology Reference
In-Depth Information
Act Three began at least thirty-five million years ago with diversification of vipers;
then, five to ten million years later, it continued with radiation of the cobra family,
stiletto snakes and their relatives, and—except in Australia, where cobra relatives pre-
dominate—roughly two thousand species of advanced snakes that mostly are not dan-
ratsnakes, racers, and kin; North American watersnakes and gartersnakes, along with
Old World counterparts; and a diverse group, including hog-nosed snakes and a few
more in the United States, that's primarily found in the New World tropics. Many spe-
cies are venomous to prey, though unless defined in terms of humans, it's impossible to
draw a crisp line between those that do and don't have toxins. In any case, dangerous
species and their mimics make up more than half of all serpents. Inventing venoms in
the early Cenozoic was clearly at the heart of serpentine success—among other things,
it facilitated again feeding on large prey—and one paleontologist facetiously renamed
that era the Age of Snakes.
As for the anatomy of venom delivery, imagine forward-directed hypodermic syr-
inges, in which the barrels—situated behind the eyes on a snake's head—serve as pro-
duction and storage sites for toxins that are forced by plungers through needles into
prey or adversaries. At a primitive level, it's all about modifying the glands, muscles,
and teeth typical of many vertebrates, and each of the components of this syringe ana-
logy varies among lineages, from vaguely obvious in some species to extremely soph-
isticated in vipers and cobras. We'll begin with glands because, although many spe-
cies with venom lack enlarged fangs, there are no instances of the reverse, suggesting
that toxins originated before the specialized teeth and muscles that are used to inject
them.
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To simplify a lot of fascinating research: all serpents have glands along the jaws that
secrete mucus and enzymes, compounds that, like those in our saliva, mainly lubric-
ate food and catalyze biochemical reactions. Toxic head glands use evolutionarily trans-
formed versions of those secretions mainly as prey tranquilizers—some work slowly,
others within seconds—and/or tenderizers, although some act as spreading factors,
pain inducers, and chemical signposts for finding bitten prey. Venom glands consist
of toxin-secreting cells packaged in a sheath of cellophanelike connective tissue. In
simplest and most widespread form, they lack a lumen (storage chamber) and com-
pressor muscles. Boomslangs and a few other dangerous rear-fanged species, however,
have the gland cells arranged in folded rows, such that space between them forms stor-
age tubules, as well as small muscle attachments that may move venom to the teeth.
Front-fanged snakes, in contrast, have a well-defined lumen, a duct to the fang, and
modified jaw muscles that pump toxins into other animals. Most impressively, although
stored venoms remain potent, they don't tranquilize or tenderize the snakes that make
them.
Isolated dry fangs are ivory white, but in live snakes they're translucent, reminiscent
of fine crystal. Rear fangs are situated behind two or more smaller teeth on the max-
illary bones; single, paired, or in triplets, with or without grooves, they are embedded
with side-to-side jaw cycles, penetrating tissue as each is pulled down and back, like a
fishing gaff. Venom flows under low pressure, from jaw compression and capillary action
along the rear fangs, and prey is held until dead or struggling weakly, then swallowed.
Front fangs arose at least three times when the maxillaries shortened during embryon-
