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best exemplify the hypodermic syringe analogy. Their short maxillary bones are hinged
with the front of the skull, so as the upper jaws push forward, both fangs—an inch or
more long in gaboon adders—rotate to almost 180 degrees for high-pressure venom in-
jection. Striking, biting, and releasing take less than a second, during which the snake
adjusts fang placement to avoid hitting the victim's bones and dispenses venom accord-
ing to victim size. Today vipers, having arisen in Asia, are diverse in tropical and semiar-
id regions; sometimes they are among the largest local predators and remarkably com-
mon. Most are terrestrial, but prehensile-tailed arboreal species have evolved independ-
ently in Asia, Africa, and the New World, as have side-winding desert forms. African
night adders with upturned snouts seem to be the only burrowing viperids, North Amer-
ica's cottonmouth the sole aquatic one. 16
Vipers are generally stout-bodied ambush hunters, sometimes luring frogs and birds
by wriggling a caterpillarlike tail. The young often eat frogs and lizards, whereas adults
mostly take mammals. I observed one little western rattlesnake eat a western fence liz-
ard that slightly outweighed it, and an adult had swallowed a California ground squirrel
equal to 50 percent of the rattler's premeal weight; the rodent's whiskers were still in-
tact but its abdomen was open, evidently digested from inside out by deeply injected
venom. Peringuey's adder and several other small species eat lizards throughout life,
while the large neonates of eastern diamond-backed rattlers and urutús take adult ro-
dents. Emphasis on tranquilizing versus tenderizing venoms parallels predation on lizar-
ds or mammals, exemplified by their relative importance for juvenile and adult western
rattlers and by geographic diet variation in Malayan pitvipers and massasaugas. Some
saw-scaled adders are unusual for having evolved a predilection for scorpions and tox-
ins specialized for killing them.
Vipers show fangs, hiss, rub scales, or vibrate tails prior to striking at enemies—so,
despite signature feeding adaptations, defensive novelties also have played key roles in
their evolution. Moreover, infrared-imaging pits, whose neural input is integrated with
vision, characterize 213 species (69 percent of all vipers), and surprisingly, although
capable of prey detection, a pitviper's namesake organs provide no known hunting ad-
vantage compared to pitless Old World relatives. These snakes do, however, emphasize
risk avoidance and a “good offense is the best defense” strategy: superbly camouflaged,
they never rely on striped patterns with which some other snakes facilitate locomotor
escape, and their pits evolved right along with protection of offspring and defensive tail
vibration. Later, the Mexican ancestor of thirty-seven species of rattlesnakes (17 per-
cent of pitvipers) morphed its tail spine into a string of castanets, perhaps to warn of
the probing snouts and paws of carnivores in talus habitats—and today their defensive
displays can be heard from Canadian bogs to Argentine thorn-scrub. 17
The 351 species in Elapidae (Greek, elop, a kind of serpent) have fixed front fangs
(half an inch long in king cobras) and usually neurotoxic venoms. Typically slender, elap-
ids are more diverse in size, shape, and ecology than vipers; they include secretive ser-
pents no larger than ordinary earthworms, fifteen-foot king cobras that construct nests
out of vegetation and guard their clutches, and a seasnake so specialized on fish eggs
that its venom apparatus is vestigial. Nine-foot black mambas are the epitome of terri-
fying, but small, banded, burrowing snake-eaters are prevalent across most elapid lin-
eages, so the group's overall common ancestor likely resembled the one hundred spe-
cies of Asian and New World coralsnakes in appearance and natural history, including
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