Crocodiles - Biology, Husbandry and Diseases

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Plate 1. Adult captive gharials at the Madras Crocodile Bank. Note the ghara, the nasal excrescence, of the male gharial. Plate 2. The right gonad, macroscopically undifferentiated, of a juvenile Nile crocodile can be seen between the spleen and the right kidney. The left gonad is hidden by the mesentery. The yellowish adrenals are almost completely obscured by the paler gonads. Plate 3. The dark-brown right thyroid situated laterally of the right bronchus. The pale right parathyroid is visible slightly caudally of the thyroid on the right aortic arch, medially of the precaval vein. Plate 4. Fighting male Nile crocodiles on a crocodile farm in South Africa. Plate 5. Oral cavity, gular valve and pharynx exposed after the ventral skin has been removed and the tongue has been cut loose from the mandibles. Plate 6. Cutting lines for a belly skin.

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Plate 7. Spectacled caiman hatchling with the greyish-white crusty lesions of caiman pox on the dorsal and lateral surfaces of head, body, tail and limbs. Plate 8. Nile crocodile hatchling with ventral, dark-brown crocodile pox lesions in patterns suggesting bite marks. Plate 9. Reddening of the ventral skin of the hind legs and around the cloaca of a juvenile Nile crocodile with septicaemia. Plate 10. Right elbow joint of a juvenile Nile crocodile with exudative arthritis. Plate 11. Heart of an adult Nile crocodile with exudative epicarditis. Plate 12. Hepatozoon sp. gametocyte in a red blood cell of a Nile crocodile.

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13. Ascaridoids in the stomach of an adult wild-caught Nile crocodile. 14. Juvenile Nile crocodile with fat necrosis involving the thoracic and abdominal fat deposits. 15. Fat necrosis: hardened yellow fat between the tail muscles of a Nile crocodile. 16. Renal gout in a juvenile Nile crocodile with deposits of uric acid in the pelvic portions of the renal folds. 17. Close-up of winter sores on the ventral surface of the tail of a juvenile Nile crocodile. 18. Advanced case of stress dermatitis with lesions affecting all parts of the body.

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Plate Plate Plate Plate Plate Plate

19. Large fungal granuloma on the right hind foot of an adult captive Indo-Pacific crocodile. 20. Exudative enteritis causing the intestine to be grossly distended by the fibrinous exudate. 21. Haemorrhagic enteritis in a juvenile Nile crocodile. 22. Tonsillitis in a juvenile Nile crocodile. 23. Laryngitis in a juvenile Nile crocodile. 24. Lacrimal cyst under the eye of a captive Nile crocodile (photo Marc Gansuana).

Crocodiles

Biology, Husbandry and Diseases

Crocodiles Biology, Husbandry and Diseases

F.W. Huchzermeyer Onderstepoort Veterinary Institute, South Africa

CABI Publishing

CABI Publishing is a division of CAB International CABI Publishing CAB International Wallingford Oxon OX10 8DE UK Tel: +44 (0)1491 832111 Fax: +44 (0)1491 833508 E-mail: [email protected] Website: www.cabi-publishing.org

CABI Publishing 44 Brattle Street 4th Floor Cambridge, MA 02138 USA Tel: +1 617 395 4056 Fax: +1 617 354 6875 E-mail: [email protected]

© CAB International 2003. All rights reserved. No part of this publication may be reproduced in any form or by any means, electronically, mechanically, by photocopying, recording or otherwise, without the prior permission of the copyright owners. A catalogue record for this book is available from the British Library, London, UK. Library of Congress Cataloging-in-Publication Data Huchzermeyer, F. W. (Fritz W.) Crocodiles : biology, husbandry and diseases / by F. W. Huchzermeyer. p. cm. Includes bibliographical references (p. ). ISBN 0-85199-656-6 1. Crocodile farming. 2. Crocodiles. 3. Captive reptiles. I. Title. SF515.5.C75 H83 2003 639.3982--dc21 2002013734 ISBN 0 85199 656 6 Typeset in Palatino by Columns Design Ltd, Reading. Printed and bound in the UK by Biddles Ltd, Guildford and King’s Lynn.

Contents

Foreword Disclaimer

vii viii

Introduction

ix

1

Crocodiles and Alligators The Species of Crocodilians Crocodilian Anatomy Crocodilian Physiology Crocodilian Biochemistry Crocodilian Behaviour

1 1 6 32 47 52

2

Examination of Crocodiles and Clinical Procedures Clinical Examination Post-mortem Examination Medication Surgical Interventions

57 57 75 86 91

3

Important Aspects of Crocodile Farming Nutrition Incubation of Crocodile Eggs Rearing Breeding Slaughter Crocodiles in Zoos and Private Collections Animal Welfare

98 98 102 107 118 123 133 136

4

Diseases of Eggs and Hatchlings Diseases of the Egg Diseases of the Yolk-sac Hatchling Diseases Congenital Malformations

139 139 142 145 148

5

Transmissible Diseases Viral Infections Bacterial Infections Fungal Infections Parasitic Protozoa Metazoan Endoparasites Ectoparasites

157 157 163 176 182 192 203 v

vi

Contents

6

Non-transmissible Diseases Nutritional Diseases Poisoning Multifactorial Diseases

211 211 221 226

7

Organ Diseases and Miscellaneous Conditions Skin Diseases Eye Diseases Diseases of the Digestive System Diseases of the Urogenital System Diseases of the Nervous System Diseases of the Circulatory System Diseases of the Respiratory System Diseases of the Skeletal–Muscular System Diseases of the Endocrine System Miscellaneous Pathological Conditions

240 240 245 247 263 266 268 270 272 274 277

References

292

Index

323

Foreword

Crocodilians have been the subject of international study for hundreds of years. Research on their biology began appearing in the literature during the 1800s, increased considerably in the 1920s and then really took off after the mid-1950s, when funding for academic research became more readily available. These early studies concentrated on various aspects of population dynamics of the species in the wild, and of crocodilians kept in display centres and zoos. Husbandry and diseases of crocodiles received early attention by zoo veterinarians and keepers, but interest in these matters took a dramatic turn upwards once ranching and farming of the species became a serious business, worth hundreds of millions of dollars. I well remember some 25 years ago how at meetings of the Crocodile Specialist Group (CSG), someone would mention seeing blemishes on crocodile skins and how this would drastically decrease the value of the skins. Soon dozens of husbandry problems and diseases came to the fore and it became imperative that the issues be looked at in a systematic and deliberate fashion. More and more researchers became interested in the field, and by the year 2000 the CSG had decided to establish, with the encouragement and leadership by Dr Fritz Huchzermeyer, a veterinary group within the CSG. I am delighted to see that Dr Huchzermeyer has put pen to paper and produced this most important and valuable book on husbandry and diseases of crocodilians. It will have many avid readers. Professor Harry Messel Chairman, CSG

vii

Disclaimer

Although every effort has been made in the collection and presentation of facts the author cannot accept any responsibility for damages arising from actions based on information contained in this book.

viii

Introduction

This book has been written for veterinarians, scientists, wildlife officials, students and crocodile farmers. The knowledge was gained mainly by my work over many years with farmed Nile crocodiles, some work with wild and wild-caught African dwarf crocodiles in the Congo Republic and, in addition, by the study of the available literature, which embraces most of the other crocodilian species as well. I believe this to be the first comprehensive book on crocodile diseases. Being a pathologist, poultry pathologist, and not a clinician, I placed the emphasis in this book on the diagnosis and treatment, or rather prevention, of diseases as they occur on crocodile farms. However, an effort has been made to cover all clinical aspects as well. I believe that my avian background has helped me to grasp the peculiarities of crocodilian physiology and pathology, while my poultry background has guided me towards a herd health approach. Diseases cannot be understood without a background knowledge of normal body functions, nor without knowledge of farming conditions. These are therefore treated in a somewhat introductory fashion in the first and third part, without any claim of completeness. Wherever possible, common names have been used as well as scientific names, in an effort to make the book more accessible. Unless emphasized specifically the term ‘crocodiles’ is used to denote all crocodilians (see Chapter 1). Basically we know very little about crocodiles and their diseases. Research into their biology is carried out and funded in the course of normal biological studies and conservation efforts. However, most veterinary research is centred on domestic animals and may at best involve some of the major wildlife species, possibly stimulated by a need to protect the inhabitants of national parks and zoological gardens. The crocodile farming and ranching industries in the various countries are in competition with each other and are most unlikely to be able to provide funding for a concerted and specialized veterinary research effort. There are no catastrophic crocodile diseases, and consequently veterinary research will always be regarded as not so important. I had the good fortune that the Poultry Section of the Onderstepoort Veterinary Institute (OVI) was closed a few years before my retirement and that I was allowed then to devote all my time to crocodile work. For some years I had some nearby crocodile farms submit practically all their mortalities. The many post-mortems carried out during that time and the generous permission to make full use of the Institute’s library, even up to the present, have laid a foundation of knowledge, on which I became confident to tackle writing this book. The past and present directors of the OVI, Dr D.W. Verwoerd, Dr G.R. Thomson, Dr S.T. Cornelius and Dr F.T. Potgieter, are thanked for the provision of an office for my use and continued access to the Institute’s infrastructure since my retirement, as well as for their continued interest in my work. The secretary of the now also defunct Pathology Section of the OVI, Mrs Mara Stoltz, has always been at hand to solve computer and program problems ix

x

Introduction

most speedily and efficiently, and Mr D. Swanepoel of the Institute’s library and his staff have been most helpful in trying to procure even the most obscure items of literature. Over the years I have been most fortunate in having been able to draw on the knowledge of many colleagues, for which I want to express my gratitude. Many crocodile farmers in southern Africa have welcomed me on their farms and have allowed me to study their animals in their environment. Dr Jenny Turton and Dr Jane Walker reviewed parts of the manuscript and helped me to overcome some of my language problems. To all go my heartfelt thanks. And, last but not least, I have to thank my wife Hildegard, who not only put up with my periods of withdrawal while writing, but always showed a keen interest in my work and encouraged me to carry on. F.W. Huchzermeyer Pretoria June 2002

Chapter 1 Crocodiles and Alligators

The Species of Crocodilians The crocodilians are classified as reptiles, together with lizards, snakes, tuataras and chelonians (tortoises, terrapins and turtles – note that the Americans use the term ‘turtles’ for all chelonians), because of their exothermia and their skin architecture. However, many features, particularly behaviour (vocalizations and parental care), heart morphology and fat body, clearly separate them from the other reptiles. All living crocodilians are grouped in the family Crocodylidae. They occur in a broad band around the globe in the tropics and subtropics of the Old and New World. At present the distinctions between subfamilies, genera and species are based mainly on anatomical features, particularly of the skull, and on scale patterns of the skin. DNA analyses may, in the near future, add new information and cause some revisions (Densmore and Owen, 1989; Ray et al., 2001; White and Densmore, 2001). The following details were taken mainly from Ross and Magnusson (1989). Please note that several common names can be in use for any one species. An effort has been made in this book to use only one common name per species, as listed below. Many synonyms of the scientific names can be found in the older literature. Where this

literature is cited, these synonyms have been replaced in most cases by the current names.

Crocodiles The subfamily Crocodylinae contains three genera: Crocodylus (the true crocodiles, with 13 species), Osteolaemus and Tomistoma (each with one species). The genus Crocodylus: C. rhombifer C. moreletii C. acutus C. cataphractus

C. niloticus C. intermedius C. porosus C. johnsoni C. palustris

Cuban crocodile Morelet’s crocodile American crocodile African slendersnouted crocodile (Fig. 1.1) Nile crocodile (see Fig. 1.7) Orinoco crocodile Indo-Pacific crocodile Johnston’s crocodile Mugger (Fig. 1.2)

C. siamensis Siamese crocodile C. mindorensis Philippine crocodile C. novaeguineae New Guinea crocodile C. raninus Bornean crocodile

© CAB International 2003. Crocodiles: Biology, Husbandry and Diseases (F.W. Huchzermeyer)

Cuba Central America Central America Africa

Africa and Madagascar South America Asia and Australia Australia Indian subcontinent Asia Philippines New Guinea Borneo (see Ross, 1990; Ross et al., 1998) 1

2

Chapter 1

Due to a consistent spelling error in its original description, the scientific name of Johnston’s crocodile is C. johnsoni. As the rules of nomenclature do not allow a subsequent correction, the original spelling of the scientific name must be retained. The wide distribution of C. porosus in the

Indo-Pacific area, C. niloticus throughout Africa and Madagascar, and C. acutus in Central America is probably due to their ability to tolerate varying degrees of salinity. This has allowed them to spread to different river systems and even islands, unlike more localized species that do not have any salt

Fig. 1.1. Captive Crocodylus cataphractus at the St Lucia Crocodile Centre.

Fig. 1.2. Captive Crocodylus palustris at the Madras Crocodile Bank.

Crocodiles and Alligators

tolerance. It therefore appears to be incorrect to use the names saltwater and freshwater crocodiles for C. porosus and C. johnsoni, respectively, outside Australia.

Dwarf crocodile

The genus Tomistoma: T. schlegelii

False gharial (Fig. 1.5)

Asia

Alligators

The genus Osteolaemus: O. tetraspis

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Africa

This has two subspecies, as follows: O. t. tetraspis from coastal West Africa (Fig. 1.3); and O. t. osborni from the Congo basin (Fig. 1.4).

The subfamily Alligatorinae contains four genera: Alligator (the true alligators, with two species), Caiman (the caimans, with two species), Palaeosuchus (the dwarf caimans,

Fig. 1.3. Captive juvenile Osteolaemus tetraspis tetraspis at the St Lucia Crocodile Centre. Their colouring is yellow and black, while that of O. t. osborni hatchlings is green and black.

Fig. 1.4. Young adult wild-caught Osteolaemus tetraspis osborni trussed up for transport to the market.

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Chapter 1

Fig. 1.5. Captive Tomistoma schlegelii on a farm in Kuching, East Malaysia.

with two species) and Melanosuchus (the black caiman, with only one species).

The genus Caiman:

The genus Alligator:

C. crocodilus

A. mississippiensis A. sinensis

American alligator USA Chinese alligator China

C. latirostris

Broad-snouted caiman Common caiman (Fig. 1.6)

Fig. 1.6. Juvenile Caiman crocodilus on a farm in São Paulo State, Brazil.

South America South America

Crocodiles and Alligators

The genus Palaeosuchus: P. palpebrosus P. trigonatus

Cuvier’s dwarf caiman Schneider’s dwarf caiman

South America South America

The genus Melanosuchus: M. niger

Black caiman

South America

Gharials The subfamily Gavialinae only has one genus, Gavialis, with a single species.

5

live further north than caimans and crocodiles in both North America and in China. 2. In alligators and caimans the teeth of the lower jaw fit into pits in the upper jaw, consequently when the mouth is closed no mandibular teeth are visible. In crocodiles the fourth mandibular tooth fits into a notch in the upper jaw and thus remains visible when the mouth is closed (Fig. 1.7). 3. Crocodiles and gharials have sensory pits in the ventral scales (Fig. 1.8). These are absent in alligators and caimans. This is one of the important features used in the species identification of goods made from crocodilian leather.

The genus Gavialis: G. gangeticus

Gharial (Plate 1)

Indian subcontinent

Differences between crocodiles and alligators This question is asked quite regularly. There are many anatomical and physiological differences, but for the purposes of this book it will suffice to name only three reasonably obvious ones: 1. Alligators are more cold resistant than caimans and crocodiles. They can therefore

Wild or captive? This refers to the description of the different ways in which the crocodiles are living or kept. Wild Crocodiles in the wild may be either left entirely to their own devices or subjected to a certain degree of management. They are hardly ever seen to be suffering from disease

Fig. 1.7. Adult Nile crocodile on a farm in South Africa. Note the visible fourth mandibular tooth in its maxillar notch.

6

Chapter 1

Fig. 1.8. Sensory pits in the ventral skin of Crocodylus palustris.

or dying, and often they live in such remote areas that suitable specimens rarely reach the laboratory (see also p. 239). Captive Crocodiles kept in zoos and other collections without a productive goal are referred to as captive. They may be bred or exhibited only, but they may also be subjected to scientific studies. Wild-caught Crocodiles caught in the wild and kept for a short period restrained for the purpose of sample collection or transported alive to a market, where they are slaughtered. They are under very severe stress which may affect many of their physiological and biochemical parameters. Such animals should be referred to as wild-caught. Ranched Crocodiles kept on farms for commercial (productive) purposes, but either hatched from eggs collected in the wild or having been collected as hatchlings, are referred to as ranched. Their diseases are substantially

the same as those of farmed crocodiles, except for their closer contact with wild populations, which may constitute a natural reservoir of crocodile-specific infectious agents. Farmed Crocodiles hatched from eggs laid by breeding stock kept on a farm for commercial purposes are called farmed crocodiles. The on-farm breeding of these crocodiles allows the genetic selection for certain productive parameters. These animals no longer have a direct link to the wild. Their only contribution to the conservation of wild crocodiles may be to keep commodity prices low, thereby lowering the incentive for poaching. However, they may also provide a substantial additional gene pool. Where such crocodiles are farmed far away from wild crocodile populations the incidence of crocodile-specific infectious diseases is usually very low.

Crocodilian Anatomy The aim of this section is to provide sufficient information for the normal functions of

Crocodiles and Alligators

the body to be understood and for the recognition of the organs during post-mortem examinations. This information is based largely on my own experience with Nile crocodiles. For a reasonably detailed and accurate study of the anatomy of the American alligator see Chiasson (1962). We are still waiting for a standard textbook on crocodilian anatomy. A dissection guide for post-mortem examinations is given in Chapter 2 (p. 75).

The skeleton Skull The pitted appearance of the dorsal skull surface (Fig. 1.9) is due to its fusion with the skin. There are three pairs of foramina dorsally on the skull: the external nares opening into one nasal orifice, the orbits and the supertemporal fossae (Fig. 1.9). On the ventral aspect, almost at the same level, are the anterior palatine foramina (foramen), the posterior palatine foramina and, partially hidden, the internal nares (Fig. 1.10). The cranium, which houses the brain, lies

7

roughly between the orbits and the supertemporal fossae. The articulation of the jaw is caudal to the atlanto-occipital joint, allowing the jaws to open extremely widely (Fig. 1.11). Vertebrae The cervical and thoracic vertebrae have ribs. The cervical ribs lie alongside the vertebral column pointing caudally, but only the thoracic ribs connect with the sternum. A cartilaginous portion in the midrib allows flexibility for collapsing the thorax during deep diving. The lumbar vertebrae do not have ribs, but the sacral ones do. Dorsally all the vertebrae bear neural spines; and ventrally, chevron bones, which point in an obliquely caudal direction, are attached to the caudal vertebrae. A fibrous membrane bearing abdominal ribs (gastralia) connects the sternum with the os pubis and supports the abdominal viscera. Legs The pectoral girdle, consisting of the scapula, coracoid and sternum, together with the first

Fig. 1.9. Pitted appearance of the skull bones of a mature Nile crocodile, dorsal aspect.

8

Chapter 1

Fig. 1.10. Ventral aspect of the skull of an adult Nile crocodile.

Fig. 1.11. Lateral aspect of the skull of a juvenile Nile crocodile.

thoracic ribs, surrounds the wide cranial aperture of the thorax. This allows large masses to be swallowed. The bones of the forelimb (humerus, radius and ulna) are shorter than their counterparts in the hind limb. The front feet have five digits, the first three carrying claws. The pelvic girdle consists of an os ileum, an os ischium directed caudoventrally and an os pubis pointing cranially. The hind limbs are twice as long as the forelimbs, allowing

for a galloping action. Femur, tibia and fibula are well developed. The foot has four digits, the first three carrying claws (Fig. 1.12). The skin Scales and osteoderms Crocodile skin, like that of all reptiles, is covered with scales or scutes and is devoid of sweat glands. On the head the skin is fused

Crocodiles and Alligators

9

Fig. 1.12. Claws on the left hind foot of an adult Tomistoma schlegelii at Singapore Zoological Gardens (photo P. Martelli).

to the bones of the skull. The large scales on the back, and in some species some of the ventral scales also, contain bony plates, the osteoderms. Muscles connect the ossified dorsal scales with the vertebral column, and when the muscles contract this results in a dorso-ventrally rigid, beam-like structure that allows the crocodile to keep its back and tail straight when walking or running (Frey, 1988a,b). In this context it is interesting to note that recent mitochondrial DNA analyses, as well as studies of nuclear genes, suggested a close relationship between crocodilians and chelonians (tortoises and turtles). The latter also have osteoderms and both dorsal and ventral armour (Hedges and Poling, 1999).

parenchymal cells contain lipid droplets (Weldon and Sampson, 1987). For the analysis of the aromatic secreta of these glands, see p. 52. In some species there are also rudimentary dorsal glands – in the Chinese alligator beneath the second row of scales from the dorsal midline, but in various positions from the 2nd to the 15th transverse row (Chen et al., 1991).

Skin glands

Pigmentation

Crocodilians have a few holocrine skin glands. The cloacal (paracloacal) glands are situated laterally within the lips of the cloaca. The mandibular (gular) glands are in the skin under the tongue, between the mandibles (Fig. 1.13). The septa of the gular glands are lined with melanocytes, giving the gland tissue its black appearance (Weldon and Sampson, 1988). The paracloacal gland is a single secretory sac with a single duct and a single lumen. The

Hatchlings of many species have light and dark transverse striations, which in some species are maintained almost into adulthood. These striations mimic rippling shadows in shallow water (see Fig. 1.3). The chromatophores in the skin can contract and expand following nervous impulses from the eyes via the brain. Blind crocodiles and those kept in complete darkness usually display lighter colours than those exposed to bright daylight.

Identification The patterns of scales, both dorsal and ventral, are species specific, although some slight individual variations may occur. A key for the identification of tanned whole crocodilian skins can be found in Brazaitis (1987).

10

Chapter 1

Fig. 1.13. Mandibular (gular) glands of a juvenile Nile crocodile.

The muscles There are no external muscles on the head because the skin adheres to the skull. The powerful jaw muscles are all on the median aspect of the mandible, thus broadening the posterior skull. Sphincters close the external

nares and depressors close the auricular flap over the tympanum for diving. The long dorsal muscles of the trunk extend into the tail. These muscles, plus the ventral tail muscles, musculus (m.) caudofemoralis medially and m. ilioischiocaudalis externally (Frey, 1988a), provide the power for swimming (Fig. 1.14).

Fig. 1.14. Schematic drawing of a cross-section of the tail of a Nile crocodile: 1, musculus (m.) longissimus dorsi; 2, m. caudofemoralis; 3, m. ilioischiocaudalis.

Crocodiles and Alligators

The respiratory system Respiratory tract The external nares are slightly raised above the level of the upper jaw, allowing the crocodile to surface and breathe when most of its body is submerged. Adult male gharials develop a large nasal excrescence, the ghara (see Plate 1), which is thought to function as a vocal resonator (Whitaker and Basu, 1983).

11

In the long nasal passage the olfactory nerve endings are exposed to the air. Except when swallowing, bellowing or yawning, the posterior part of the mouth is closed by the gular valve, consisting of the dorsal flap of the tongue and the palatal flap (velum palati) extending from the soft palate (Putterill and Soley, 1998a) (Fig. 1.15). The Eustachian tubes enter the pharynx in a joined opening just caudally of the internal nares (Colbert, 1946) (Fig. 1.16). Their func-

Fig. 1.15. Schematic drawing of the oral and pharyngeal cavities of the crocodile: 1, gular valve; 2, tongue; 3, larynx and trachea; 4, oesophagus; 5, internal nares; 6, tonsils; 7, Eustachian tubes; 8, nasal passages.

Fig. 1.16. Tonsils of the crocodile caudally of the internal nares.

12

Chapter 1

tion is to equalize the pressure on the two sides of the tympanum (the ear membrane). Close to the opening of the Eustachian tubes into the pharynx there are two mucosal folds, one on either side and extending caudally, which contain tonsillar tissue (Putterill and Soley, 2001) (Fig. 1.16). The glottis has two soft lips (Fig. 1.17) which close when the crocodile swallows. In crocodiles (but not in alligators) the trachea bends to the left inside the thorax before its bifurcation, a substantial distance before entering the lungs (Fig. 1.18). This allows large chunks of prey to be swallowed without exerting any pressure on the trachea or bronchi. Lungs The lungs are multi-cameral sac-like structures, highly vascularized but with thicker walls than a mammalian lung. These thick walls may be necessary to counteract the outside pressure during diving. The lungs lie in pleural chambers which are separated by

a complete mediastinum. The posterior part of the lungs is connected tightly to the anterior transverse membrane (postpulmonary membrane). In crocodiles the remainder of the lungs lies loosely in the thoracic cavity, not as described by Duncker (1989), while in the caiman the lungs are fused to the ventral wall of the thorax. For a detailed study of lung morphology of the Nile crocodile, see Perry (1988). Respiratory muscles The thorax is divided from the abdomen by two transverse membranes. The postpulmonary membrane separates the lungs from the liver, and its ventral third is muscular. The posthepatic (posterior transverse) membrane is attached to a sheet of muscle (m. diaphragmaticus) which extends to the os pubis (Van der Merwe and Kotzé, 1993). Together, the two membranes, with their muscular components, act like a diaphragm, pulling the liver in a caudal direction for inspiration.

Fig. 1.17. Tongue and ventral aspect of the pharyngeal cavity with protruding glottis; juvenile Nile crocodile.

Crocodiles and Alligators

13

Fig. 1.18. Tracheal loop in the thorax of Crocodylus palustris.

Voice organ? Crocodiles can produce a range of sounds, but have neither vocal cords (like mammals) nor a syrinx with tympaniform membranes (like birds). It is believed that sounds are produced by forcing the air through the compressed lips of the glottis (Fig. 1.17), much as sounds are produced by human lips in the mouthpiece of a trumpet.

The digestive system Teeth Crocodilian teeth are pointed, very sharp and are constantly replaced throughout life. The replacement rate varies with the growth rate and slows down as the animal becomes older. In small American alligators (