Biology Reference
In-Depth Information
kin, a handful of European and African species, and Asian keelbacks, all in Natricid-
ae (Latin,
nato,
to swim;
natricis,
a watersnake). Tiger keelback bites occasionally kill
people, and when threatened this species displays a flattened, red nape, where pois-
onous skin gland secretions, sequestered from toads in its diet, are ejected into an at-
tacker's mouth. Tiger keelbacks also pass toad toxins across their placentas, so young
snakes are protected at birth.
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Viperidae, Elapidae, Atractaspididae, Colubridae, Dipsadidae, Natricidae, and
more—now let's contemplate thirty-five million years of venomous snake evolution in
the context of the last thirty-five years of snake research. My dissertation showed how
constriction allowed ancestral serpents to subdue heavy prey, favoring evolution of big
mouths and digestion of large items, whereas defensive displays are more recent, con-
vergent adaptations to particular lifestyles. Hundreds of people now study snakes, and
recent findings imply some additional, overarching patterns involving mobile-jawed, ad-
vanced species. The best-known new generality is that venoms, defined by natural his-
tory instead of human mortality, are ancient, widespread, and diverse. Moreover, the
rich variety of toxins and injection equipment facilitates feeding on many kinds of prey
under a wide range of circumstances, rather than representing linear stages toward a
single pinnacle of snake evolution.
Another, less widely recognized pattern is that front fangs radically changed prey-
predator dynamics in both directions. Boa and python teeth snare prey for constriction,
and rear-fanged species must hang on to inject toxins. Thus, if target animals are dan-
gerous, a snake's range of prey and feasibility of defensive biting are limited. Vipers,
elapids, and stiletto snakes, however, swiftly dispatch prey
and
punish attackers, mak-
ing possible not only heavy, infrequent meals, but also the effective use of threats as
predator deterrents. Among many examples from the field, several times the buzz of an
unseen terciopelo or golden lancehead, vibrating its tail on leaves, has stopped me cold
and likely prevented an accident. I'd be surprised if venoms aren't modified for defense
in spitting cobras, and I can't help wondering if some other snakes need all that toxicity
just to immobilize prey. I'm also impressed that so many Waorani survived bites, as do
with snakes. And the evolutionary significance of front fangs doesn't end with feeding
and defending their bearers.
To better appreciate the third pattern—that many snakes are mimics—note that
snakes similar in both appearance and ecological niche usually are on different contin-
ents. New Guinea green tree pythons and South American emerald tree boas, for ex-
ample, independently evolved to look like foliage on tree limbs and ambush prey with
dangling head postures. Conversely, when distantly related snakes look alike and live in
one place, they routinely have different niches—the coralsnake mimics I studied eat am-
phibians, not other snakes—and one of the lookalikes is front-fanged. Many mimics re-
semble dangerous models in one or two aspects, like head shape and general color pat-
tern, but some are so precisely similar that even herpetologists are cautious. On a field
trip to Vietnam, I almost lost an escaping spotted catsnake for fear of grabbing the more
commonly encountered and similarly patterned Chinese habu, and in Costa Rica we al-
ways treated toad-eating false terciopelos as the real deal until discerning their round,
unviperlike pupils. In fact, entire specialized lineages, totaling many dozens of species,
closely resemble vipers, cobras, or coralsnakes. Absent venomous models, there'd be no
