Biology Reference
In-Depth Information
ic development and moved enlarged rear teeth forward, evolutionary transitions during
which grooves closed to form canals for high-pressure injection. Front fangs are either
short and fixed or long and folded, depending on whether the bones to which they're
attached move slightly or a lot; like hypodermic needles, they have beveled, usually el-
liptical tip openings. Cobras and other snakes with fixed front fangs typically hold prey
until ingestion starts, whereas vipers, with folding front fangs, seize frogs, lizards, and
birds but strike and quickly release mammals, then relocate them before swallowing.
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We know little of what venoms do in nature, despite countless studies of their bio-
chemistry, but tranquilizing and tenderizing prey surely are key roles. Rear-fanged
plains black-headed snakes kill centipedes within minutes, and desert nightsnakes sub-
due reptiles up to half their own weight; Puerto Rican racers more rapidly digest enven-
omed lizards, and separate glands of Brazilian snail-eaters tranquilize or tenderize mol-
lusks. Aside from toxic bites, hog-nosed snakes use huge rear fangs to puncture toads
that swell with air to thwart ingestion. Only front-fanged serpents, however, immobilize
truly heavy prey, illustrated by prey/predator weight ratios of 1.4 for an eastern cor-
alsnake that ate a glass lizard and 1.6 for a common lancehead containing a whiptail.
And as predicted if venomous digestion is important where warm seasons are short, out-
side the tropics, only vipers commonly eat heavy, bulky prey; the northernmost snakes
are Scandinavian adders, the southernmost are Patagonian lanceheads, and Himalayan
pitvipers and Mexican dusky rattlesnakes live in some of the world's highest mountains.
The biology of venomous serpents seemed straightforward when I studied them in
high school—coralsnakes, rattlers, and their kin kill prey and sometimes us; mildly
venomous rear-fanged snakes evolved from nonvenomous species, which in turn gave
rise to dangerous forms with fixed front fangs and thence, most recently, to folding front
fangs. Now, with better understanding of the entwined roles of development and evolu-
tion, we know the toxic serpents began flourishing tens of millions of years ago, as ad-
vanced snakes loosened those toothy tuning forks and modified their head glands, con-
verted enlarged hind teeth to grooved rear fangs, and repeatedly invented front fangs.
Researchers are increasingly focused on the diversification of venomous serpents, mim-
ics, and their adversaries—the players in Acts Two and Three—and I believe we'll soon
be asking if toxins have more to do with defense than heretofore realized.
Big-mouthed constrictors had our shifty-eyed, tree shrew-like ancestors to themselves
for the first several million years of serpent-primate conflict, but today descendants of
those earliest macrostomatans are prominent only where advanced snakes are rare or
lacking. Cuba has a nine-foot relative of the South American rainbow boa and fifteen
species of dwarf boas, whereas no other comparably sized landmass has more than five
total from those two groups; Australia is dominated by one highly specialized bunch of
venomous snakes and also boasts twenty species of pythons, whereas all of Africa, with
an otherwise much more diverse serpent fauna, has only four of the latter. Most contin-
ents are populated instead by hundreds of species of advanced snakes, many blatantly
venomous. Because Greek or Latin names denote their lineages, a little taxonomy will
help us appreciate them.
The 308 species of Viperidae (Latin,
vivus
and
parus,
to give live birth—as do many
vipers, rather than lay eggs) branched off soon after advanced snakes originated and
