Biology Reference
In-Depth Information
its interior—a serious problem at high latitudes and elevations because an ectotherm's
digestion slows down at lower temperatures. Eels are easily swallowed and especially
nutritious for their diameter, whereas fish and fowl require more gape for the same cal-
ories. High-payoff mammals require large gape, safe dispatch, and rapid digestion. The
insects eaten by blindsnakes and most lizards don't require much gape or special hand-
ling, but they provide little value per item. For other snakes, feeding infrequently on
heavy prey means less exposure to predators—assuming the prey can be efficiently sub-
dued, swallowed, and digested.
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The third general point concerns a hunter's own prospects of being dispatched by a
predator. As potential prey, a snake might be so cryptic as to impose intolerably high
search costs, and/or appear so well defended as not to warrant the energy costs and
risks of capture. To avoid enemies, venomous serpents are usually well camouflaged
and back up threats with toxic bites, whereas mimics capitalize on the presence of
those dangerous models and lie about their own defensive abilities. This all amounts to
a high-stakes game, because hungrier predators will pay higher costs, and availability
of alternative prey varies. The natural history literature confirms that even extremely
dangerous animals aren't invincible—honey badgers and snake eagles have killed black
mambas and puff adders, an ocelot mortally wounded a five-and-a-half-foot Totonacan
rattlesnake—yet most predators, when they can, take the easy way out, as in red-tailed
hawks that will attack western rattlesnakes but more often take nonvenomous gopher
snakes.
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Given that background, serpent evolution amounts to a three-act play, with plot
twists imposed by climate and habitat change, foraging economics, and the origin of
novel traits. If Act One was the Mesozoic rise of stout, big-mouthed constrictors in the
Age of Reptiles, described in the last chapter, Act Two began with early-Cenozoic expan-
sion of cooler, drier grasslands during the Age of Mammals, when horses and rodents
were on the make and slender, supple ancestors of advanced snakes posed an alternat-
ive to earlier macrostomatan lineages. By analogy with modern boas on the one hand
and racers on the other, those first ambushers survived on a few large meals per year,
whereas the stripped-down innovators were out poking, peering, and tongue-flicking in-
to smaller creatures' hideouts. Items that fought back much wouldn't be worth the risk
of detection by bigger predators, and depending on prey abundance, diet specialization
might have been difficult. On the plus side, advanced snakes could rapidly slither away
from their foes, which was not an option for more heavy-bodied species.
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Act Two featured another subplot, in which the slimmer, faster models freed up their
jaws, perhaps to more quickly ingest struggling prey that otherwise might attract snake
predators. Using my analogy of “toothy tuning forks” for the upper jaws of macrosto-
matans, recall that right and left arches alternate in forward-backward cycles when
muscles from the palate tug on them. Most importantly, as individual bones, analog-
ous to the fork prongs, evolved mobility, their inner tooth rows proved adequate for
jaw-walking over prey, thereby freeing the outer tooth rows for specialized new roles.
Consistent with that shift, dental adaptations are modest at best among boas, pythons,
and other remnants of early serpent evolution—emerald tree boas, suspended by their
tails from branches, secure prey with long front teeth—whereas advanced snakes in-
clude folding-toothed skink-eaters, needle-toothed slug-eaters, club-toothed crab-crush-
ers, and species with diverse styles of venom injection.
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