Herpetology. An Introductory Biology of Amphibians and Reptiles

514 Pages·1993·13.35 MB·English

by  George R. Zug (Auth.)

ΤΑΤΑΤΑΤΑΤΑΤΑΨΑΤΑΤΑΤΑΤΑΤΑΤΑΥΑΥΑΤΑ
HERPETOLOGY
AN INTRODUCTORY BIOLOGY OF
AMPHIBIANS AND REPTILES
George R. Zug
Department of Vertebrate Zoology
National Museum of Natural History
Smithsonian Institution
Washington, D.C.
Academic Press, Inc.
Harcourt Brace Jovanovich, Publishers
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This book is printed on acid-free paper. 0
Copyright © 1993 by ACADEMIC PRESS, INC.
All Rights Reserved.
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Library of Congress Cataloging-in-Publication Data
Zug, George R., date
Herpetology : An Introductory Biology of Amphibians and Reptiles /
George R. Zug.
p. cm.
Includes bibliographical references and index.
ISBN 0-12-782620-3
1. Herpetology. I. Title
QL641.Z84 1993
597.6-<lc20 92-30758
CIP
PRINTED IN THE UNITED STATES OF AMERICA
93 94 95 96 97 MM 9 8 7 6 5 4 3 2 1
Dedicated to my mentors:
A. F. and G. B. Zug,
A. Schwartz
y
W. J. Riemer,
and C. F. Walker
YAYAYAYAYAYAYAYAYAYAYAYAYAYAYAYA
PREFACE
It is an admirable feature of herpetologists that they are able to cross
the boundaries between different aspects of their subject, which
remains, perhaps more than other branches of zoology, a single
coherent discipline.
A. d'A. Bellairs and C. B. Cox, 1976
The classification followed in this book is a mixture of the old and the new.
Vertebrate zoologists will recognize that the division of the subphylum Verte -
brata into seven classes (Cyclostomata, Chondrichthyes, Osteichthyes, Am­
phibia, Reptilia, Aves, and Mammalia) does not recognize single evolutionary
lineages or groups of equivalent taxonomic rank. In a strictly phylogenetic sense,
the Tetrapoda (Amphibia through Mammalia) are a subgroup of bony fishes
specialized for terrestrial life. However, that classification level is not of immedi­
ate concern to us as herpetologists, our concern, rather, is with the classification
arrangement within the Tetrapoda and its accurate depiction of evolutionary
lineages. Breaking with traditional classifications, the following cladistic classifi­
cation more accurately reflects our knowledge of tetrapod evolution but dras­
tically alters the ranks of many long-used taxa and, in some cases, changes the
content of taxa.
xiii
xiv Preface
Tetrapoda
Amniota
Mammalia
Reptilia
Testudines
Sauria
Archosauria
Crocodylia
Aves
Lepidosauria
Sphenodontida
Squamata
Amphibia
Lissamphibia
Gymnophiona
Salientia
Caudata
Note particularly the difference in taxonomic rank of Amphibia and Reptilia and
the inclusion of birds in the Reptilia. These rankings and relationships do not
surprise the professional zoologist, but they are unsettling after having used
another classification for years. Due to this and to the lack of agreement on
category names for the new rankings, the formal names of the taxa are not
preceded with category titles for any category higher than family. Because herpe­
tology is defined by its history of study, I use "reptiles" in a vernacular sense to
include all the Reptilia, while generally excluding the Ornithosuchia (the lineage
that includes the dinosaurs and birds).
My major goals are to portray the diversity of living amphibians and reptiles
in most aspects of their biology and to introduce students (upper-level under­
graduates) to the breadth of herpetology today. Obviously, not all aspects of
herpetology can be covered in depth in an introductory text, so a bibliography is
included for each chapter. Each bibliography contains the references I consulted
in writing the chapter as well as summary and review articles that provide
information beyond that presented herein.
George R. Zug
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ACKNOWLEDGMENTS
The preparation of any book requires the assistance of many individuals. I
thank all of you—those who provided direct assistance and those whose ideas
and discoveries were extracted from research articles and seminars. A few individ­
uals deserve a special thanks: Pat Zug and Ron Heyer for constant support and
picking up the slack when I failed to do so, Ron Crombie for his amazingly
encyclopaedic recall of the herpetological literature, and Josh Seiff and Rob
Wilson for literature retrieval and map production. Seven reviewers—C. Gans,
L. M. Hardy, W. R. Heyer, G. Middendorf, J. Mitchell, W. Witt, and K.
Witt—graciously volunteered to read the text in its earliest and roughest stages;
their critical reviews and tolerances are much appreciated. I also thank the other
reviewers—M. Benabib, R. Crombie, C. Crumly, K. de Queiroz, C. Ernst, O.
Flores-V., J. Iverson, G. Mayer, R. W. McDiarmid, J. Slowinski, K. Troyer, V.
Wallach, P. Weldon, and A. Wynn—for their assistance on one or more chapters.
Their suggestions have made this a better text. Undoubtedly errors remain, since
I do not always follow good advice. For these, I am alone responsible.
I thank the following photographers for permitting me to reproduce their
materials in original or modified form: R. W. Barbour, A. Bauer, A. Cardoso,
R. E. Clark, Jr., C. K. Dodd, Jr., C. Gans, J. W. Lang, R. W. McDiarmid,
K. Miyata, E. O. Moll, K. Nemuras, R. A. Nussbaum, C. A. Ross, T. Schwaner,
R. G. Tuck, Jr., H. I. Uible, and R. W. Van Devender.
xv
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PART I
DIVERSITY
A ND
HISTORY
Lifi, life an endless march, un endless army < . ,
W. Whitman, 1891-1892
J^^^^iving amphibians and reptiles display an amazing variety of shapes and
• sizes. Some forms, such as the legless and wormlike caecilians and amphisbae-
nians, are easily mistaken for animais other than amphibians and reptiles,
whereas frogs and turtles are always recognized for what they are. The variety
of body forms is. matched by equally diverse behaviors, ecologies, and physiolo­
gies-. All of this variety reflects a complex and long evolutionary history
for both amphibians and reptiles and their adaptations to a multitude of
environments and life-styles.
The great variety of adaptations is matched by high Species richness or
diversity. There are approximately 4300 species of amphibians and over 6000
species of reptiles. Neither matches the diversity of the bony fishes, but both
ate comparable to the diversity of mammals (ca. 5000 species). This great
diversity demonstrates that amphibians and reptiles are not degenerate groups
hanging on at the edge of survival in this "Age of Mammals/ Their life-style
3
is different but not inferior to that of mammals. In fact, they have been able
to adapt and radiate widely in several environments that have proved more
hostile for mammals. "Cold-bloodedness, a supposed maladapted characteris­
55
tic of amphibians and reptiles, is less energy demanding and allows a lower
metabolism and discontinuous growth thereby permitting survival in many
hostile environments. However, "cold-blooded' is a poor descriptor for the
7
ectothermic amphibians and reptiles. Ectothermy denotes the physiological
condition in which an animales body temperature depends on heat.derived
from an external (ecto-) source. Ectothermy is not an adaptation; rather it
is the original (ancestral) state of lite. Its importance lies at two extremes:
( 1 ) what it permits an animal to do for immediate survival; and (2) evolution­
ary, what adaptive modes hâve beenpossible within the boundaries imposed
by this physical state. The latter aspect is emphasized in the subsequent four
chapters through a general introduction to the anatomy of amphibians and
reptiles and their present and past diversity. .
CHAPTER 1
Amphibians
any living amphibians have a two-phase life-style.
Indeed, they are the only vertebrates with a free-living
aquatic developmental stage and a terrestrial juvenile/
adult stage. The larval stage highlights the ancestral ori­
gin from fish, and the adult stage demonstrates the mas­
tery of the land. Their name derives from the Greek
amphibios for "double life," recognizing their duplex life­
style.
Living amphibians share a number of other unique
traits. These traits distinguish them from fish and the
other limbed vertebrates (tetrapods) and indicate that
the three dissimilar groups of modern amphibians pos­
sessed a common ancestor. All share a reliance on cutane­
ous respiration, a pair of sensory papillae in the inner
3
4 1 • Amphibians
ear, doubled transmission channels in the middle ear, specialized visual cells in
the retina, pedicellate teeth, two types of skin glands, as well as several other
unique traits.
Three respiratory surfaces are used by amphibians, usually two of them
operating simultaneously. Aquatic amphibians, particularly the larvae, use gills;
terrestrial ones use lungs. However, in both air and water, the skin serves as a
major surface for the transfer of oxygen and carbon dioxide. The skin has proved
so effective that some terrestrial amphibians have lost or reduced their lungs,
and some aquatic ones have eliminated their gills, depending entirely on the skin
for the exchange of gases. All lunged species use a force-pump mechanism for
moving air in and out of the lungs.
The double-channeled auditory system of amphibians has one channel that
is common to all tetrapods, the columella-basilar papilla channel. The other
channel, opercular-amphibian papilla, allows the reception of low-frequency
sounds (<1000 Hz). The possession of double receptors may not seem peculiar
for the frogs since they are vocal animals. However, for the largely mute salaman­
ders, a dual hearing system seems peculiar and redundant.
Salamanders and frogs have green rods in the retina, presumably absent in
the degenerate-eyed caecilians. This type of visual cell is found in no other
animals, and its particular function is as yet unknown.
The teeth of modern amphibians are two-part structures: an elongate base
(pedicel) anchored in the jaw bone and a crown protruding above the gum. Each
tooth is usually constricted where the crown attaches to the pedicel. As the
crowns wear down, they break free at the constriction and are replaced by a new
crown emerging from within the pedicel. Only a few living amphibians lack
pedicellate teeth.
Two types of skin glands are present in all living amphibians: mucous and
granular (poison) glands. The mucous glands secrete mucus, which keeps the
skin surface moist for cutaneous respiration. Although the structure of the poison
glands is identical in all species, the toxicity of the secretions is highly variable,
ranging from barely irritating to lethal.
Living amphibians possess other unique traits. All have fat bodies that
develop from the germinal ridge of the embryo and retain an association with
the gonads in adults. Frogs and salamanders are the only vertebrates able to raise
and lower their eyes. The bony orbit of all amphibians opens into the roof of
the mouth with a special muscle stretched across this opening; this muscle
elevates the eye. The ribs of amphibians do not encircle the body.
MODERN GROUPS OF AMPHIBIANS
Three major lineages of amphibians exist today: frogs, salamanders, and
caecilians (Fig. 1.1). Although these three groups share a number of unique
traits for tetrapods, the question remains whether they evolved from the same