AMER. ZOOL., 23:339-342 (1983)
Feeding Mechanisms, Body Size, and the
Ecology and Evolution of Snakes1
Introduction to the Symposium
F. Harvey Pough
Section of Ecology and Systematics, Corson Hall,
Cornell University, Ithaca. Xew York 14853
The popular view of the Age of Reptiles
as a Mesozoic phenomenon that closed with
the extinction of dinosaurs does not consider the remarkable evolutionary success
of one group of living reptiles, the snakes.
This suborder of the Squamata is first
known from Cretaceous fossils of extinct
groups and boa-like forms (Hoffstetter,
1962). Ancestors of modern snakes, the
Caenophidia, appeared in the Paleocene
and their radiation in the second half of
the Cenozoic has been so rapid and extensive that the Neogene could appropriately
be known as "The Age of Snakes" (Stanley, 1979). What is it about snakes that has
made them so successful?
The elongate, legless serpentine body
form is a distinctive characteristic of snakes
and it influences all areas of their biology.
Snakes differ markedly from their closest
relatives, the lizards, in body size. Most lizards are small animals weighing less than
20 g as adults and feed on insects. Nearly
80% of living lizard species fit that description and morphological adaptations that
appear to be related to insectivorism as a
dietary specialty have characterized lizards
from their initial appearance in the fossil
record (Carroll, 1977: Pough, 1980). In
contrast, nearly 75% of snake species have
adult body masses greater than 20 g.
Because most small mammals also have
body masses of 20 g or more, the disparity
of body sizes of snakes and lizards places
these two suborders of squamate reptiles
in quite different relationships with mammals. Lizards appear to avoid competition
1
From the Symposium on Adaptive Radiation Within
a Highly Specialized System: The Diversity of Feeding Mechanisms of Snakes presented at the Annual Meeting of
the American Society of Zoologists, 27-30 December
1981, at Dallas, Texas.
with mammals by being small and using
different resources from those exploited
by mammals. Snakes, which are the same
body size as many predatory mammals,
appear to occupy a distinctive adaptive zone
that is the consequence of their specialized
morphology. Because of their ability to
pursue prey down narrow burrows or into
treetops, snakes may be able to exploit kinds
of prey and methods of hunting that are
not feasible for mammals.
Most of the predatory specializations of
snakes are directly related to their attenuate body form, and that shape is probably
energetically viable only for an ectotherm
because of the large heat loss that would
be experienced by an endotherm with the
surface/mass ratio of a snake. Possessing
a specialized body form that mammals are
unable to emulate, snakes appear to have
succeeded in achieving an overlap of body
size with mammals that has not been possible for lizards.
There are, of course, complications in
locomotion and predation that are associated with the extreme morphological specialization of snakes. The evolution of leglessness initially required a body form that
permitted an alternative method of locomotion (Gans, 1975). The diverse array of
legless tetrapods that have existed among
amphibians and reptiles since the Carboniferous indicates that the problems are
readily solved and that there are advantages to be gained from a serpentine body
plan.
A second mechanical difficulty that
snakes must have encountered early in their
evolution was the problem of supplying
energy to an elongate body via a relatively
small mouth (Gans, 1961). The success of
snakes in solving that problem appears to
lie at the heart of their successful radiation.
A feeding mode that is based on consuming
339
340
F. HARVEY POUCH
Fie. 1. Carved stone effigy vessel from the Santa Cruz phase of the Hohokam culture of southern Arizona
(700-900 A.D.). A rattlesnake encircling a vessel with its head in close proximity to a large toad is a common
motif of this period. (Arizona State Museum, A-26,753, photo by Helga Teiwes.)
relatively few large meals rather than many
small ones is quite different from that typifying most lizards and mammals and has
been a subject of fascination for centuries
(Fig. 1). Such a mode is feasible for snakes
because of the modifications of the skull
that had their genesis at the initial stages
of the evolution of the suborder. In turn,
that feeding specialization is associated with
distinctive methods of detecting, capturing, subduing, swallowing, and digesting
prey. Many features of ecology, behavior,
and physiology are related to those secondary specializations.
An enormous array of further modifications and specializations has evolved
within the already complex morphology of
the ophidian feeding apparatus. These
modifications allow snakes to exploit a
broad diversity of prey including hardbodied vertebrates and invertebrates, softand hard-shelled eggs, and prey that is most
effectively dispatched or digested with the
aid of venom. Parallelism and convergence
associated with specific dietary modes have
made interpretation of the taxonomic relationships of even major groups of snakes
problematic. At the opposite extreme, the
capacity that snakes have shown to evolve
new specializations that were not part of
the initial radiation of the group is probably unique among tetrapods. Some of
these specializations, for example the ability to crush the calcified exoskeletons of
crabs, seem to be the very antitheses of
what could be expected from the structures most snakes use for capturing and
ingesting prey.
The only common vertebrate dietary
mode that is not represented among snakes
is herbivory. A diverse assemblage of lizards subsists on a diet of plant material
(Pough, 1973). Considering the variation
in ecology, morphology, and behavior of
341
INTRODUCTION
herbivorous lizards, neither energetic constraints nor masticatory inefficiency seems
adequate to explain the absence of herbivorous snakes. Perhaps the critical limitation for a snake is the volume of its gut.
Small mammals that depend upon fermentation of plants encounter that problem
(Van Soest, 1982), and a serpentiform body
may be incompatible with an herbivorous
diet. Even the stoutest snakes are half the
relative girth of lizards (Pough and Groves,
1983). The fatsnake (Dixon, 1981, p. 98)
may illustrate the morphology that would
be necessary for a snake to be an herbivore.
A view of snakes as adaptive systems that
require simultaneous consideration of a
broad range of biological characteristics
focuses attention on function in a broader
and more integrative context than has been
traditional. This approach suggests that
features of snake morphology that have
been considered separately in systematic
interpretations are linked in functional
complexes. For example, coordinated
modifications of cranial morphology and
kinesis, cephalic musculature, venom gland
histology, and protein synthesis have
appeared in at least three and perhaps as
many as five lineages of snakes. The products of these lineages are the front-fanged
snakes popularly considered "the venomous snakes." Further parallelisms within
these groups can be associated with characteristics of particular habitats, with prey
texture, and perhaps with predation (Savitzky, 1978). Other taxa, such as the highly
specialized vipers of the genus Bids, diverge
from this assemblage by carrying familial
trends in fang length, venom characteristics, behavior, and body form to such an
extreme that a quantitatively new adaptive
regime emerges, ecologically distinct from
less specialized genera in the same family
(Pough, 1977). Soon after the demonstration that certain boid and colubrid snakes
were characterized by suites of physiological, myological, and behavioral traits
(Ruben, 1977«, b), the same suites of characters were found to bear on the cranial
anatomy and fossil record of these higher
taxa (Savitzky, 1980).
The study of adaptation succeeds best in
situations in which it has proceeded to an
extreme, and the feeding mechanisms of
snakes represent a series of these extremes.
How has this diversity of feeding specializations been achieved? Are functional and
structural aspects of prey capture separated from those features associated with
subduing and swallowing prey? Can one set
of functions be conservative while others
are progressive? How is the widespread
occurrence of shared, derived character
states among snakes to be interpreted?
These questions, and related ones, are
addressed in this symposium.
ACKNOWLEDGMENTS
In retrospect I appreciate the invitation
from the Division of Vertebrate Morphology to organize this symposium, and I am
most grateful to Sharon B. Emerson and
George E. Goslow, Jr., for their help and
advice. Mary Wiley, as always, made working with the ASZ a pleasure. My thanks to
the Systematic Biology Program of the
National Science Foundation for its support of the symposium (DEB-8107917).
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