Biology Reference
In-Depth Information
resonated during my time there, and terciopelos remain the most impressively danger-
ous snakes with which I have ever worked.
Serpents evolved venoms not to kill pursuers like us but to subdue and digest prey.
Their defensive threat displays protect harmless species as well, as in the case of the
coralsnake mimics I studied in graduate school. Nonetheless, something sinister lurks
beneath all that rationality. During decades of work with venomous species I've suffered
only the mild copperhead bite as a teenager, but in 2003 Kelly heard explosive rattling
underfoot, saw the blacktail and two puncture marks on her shin, and endured a tortur-
ous recovery. The very day I learned of young Sweeten's tombstone, my herpetologist-
rancher friend David Hillis reached under some hay, expecting tiger salamanders, and
jerked back from a western diamondback—then within thirty minutes leapt over anoth-
er one by his porch! Snakes, it turns out, remind even those of us who love and study
them of how, by evolving new ways to make a living, they've shaped our own, distinct-
ively primate biology. I'll argue shortly that we've influenced theirs too.
At least half of all serpents possess venoms, defined as injected toxins—themselves
organism-produced poisons, which are chemicals deleterious to life—and browsing any
of herpetology's continental-scale treatises will provide a sense of the dangerous-to-
humans fraction of overall snake diversity. In Campbell and Lamar's epic
Venomous
Reptiles of the Western Hemisphere,
for example, we can savor images of the yellow-
bellied seasnake, 72 species of coralsnakes, and 117 species of pitvipers; lingering
over intricate color patterns and individual species' natural histories, we can learn
that coralsnakes and lanceheads occupied South America long before wild dogs and
semblages populate Old World landmasses. Because venoms are mostly about food (stay
tuned for exceptions), we'll discuss three general aspects of getting a meal, before ex-
ploring global snake diversity.
First, to maximize their bottom line, predators should pass up prey only if, during
the time required to eat it, something better could be taken. “Better” has to account for
effort spent searching, catching, and swallowing, as well as predation risks and oppor-
tunity costs (such as not mating right then), all relative to food value and, ultimately, re-
productive success. Foraging optimally thus amounts to making cost-benefit trade-offs
in gaining an adequate diet. Indigo snakes and other wide-foraging searchers may daily
travel hundreds of yards, for example, whereas bushmasters and other sit-and-wait am-
bushers can occupy one site for weeks, during which taking small prey might not be
worth the risk of attracting enemies. Generalists often are jacks-of-all-trades but mas-
ters of none, like coachwhips that eat grasshoppers, lizards, baby birds, mice, and even
small rattlesnakes, but not large centipedes. The plains black-headed snake, a dietary
specialist, takes only those formidable myriapods.
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Second, because snakes swallow food intact, shape matters. Prey can be elongate
and cylindrical, like worms or eels; noncircular in cross section, like laterally com-
pressed fish and animals with awkward protuberances, such as birds; or bulky, like ro-
dents or eggs. For snakes of equal gape, elongate cylindrical prey yield more food than
other shapes (hotdogs weigh more than meatballs of equal caliber); a heavier prey item,
however, can fight back and, if also bulky, might rot before stomach juices seep into
