The Species of CrocodiliansThe crocodilians are classified as reptiles,together with lizards, snakes, tuataras andchelonians (tortoises, terrapins and turtles –note that the Americans use the term ‘turtles’for all chelonians), because of their exothermiaand their skin architecture. However,many features, particularly behaviour(vocalizations and parental care), heart morphologyand fat body, clearly separate themfrom the other reptiles.All living crocodilians are grouped in thefamily Crocodylidae. They occur in a broadband around the globe in the tropics andsubtropics of the Old and New World. Atpresent the distinctions between subfamilies,genera and species are based mainly onanatomical features, particularly of the skull,and on scale patterns of the skin. DNAanalyses may, in the near future, add newinformation and cause some revisions(Densmore and Owen, 1989; Ray et al., 2001;White and Densmore, 2001). The followingdetails were taken mainly from Ross andMagnusson (1989).Please note that several common namescan be in use for any one species. An efforthas been made in this book to use only onecommon name per species, as listed below.Many synonyms of the scientific names canbe found in the older literature. Where thisliterature is cited, these synonyms have beenreplaced in most cases by the current names.CrocodilesThe subfamily Crocodylinae contains threegenera: Crocodylus (the true crocodiles, with13 species), Osteolaemus and Tomistoma (eachwith one species).

Due to a consistent spelling error in itsoriginal description, the scientific name ofJohnston’s crocodile is C. johnsoni. As therules of nomenclature do not allow a subsequentcorrection, the original spelling of thescientific name must be retained.The wide distribution of C. porosus in theIndo-Pacific area, C. niloticus throughoutAfrica and Madagascar, and C. acutus inCentral America is probably due to theirability to tolerate varying degrees of salinity.This has allowed them to spread to differentriver systems and even islands, unlike morelocalized species that do not have any salt tolerance. It therefore appears to be incorrectto use the names saltwater and freshwatercrocodiles for C. porosus and C. johnsoni,respectively, outside Australia.The genus Osteolaemus:O. tetraspis Dwarf crocodile AfricaThis has two subspecies, as follows:O. t. tetraspis from coastal West Africa (Fig. 1.3); andO. t. osborni from the Congo basin (Fig. 1.4).The genus Tomistoma:T. schlegelii False gharial Asia(Fig. 1.5)AlligatorsThe subfamily Alligatorinae contains fourgenera: Alligator (the true alligators, with

two species), Caiman (the caimans, with twospecies), Palaeosuchus (the dwarf caimans, with two species) and Melanosuchus (theblack caiman, with only one species).

GharialsThe subfamily Gavialinae only has onegenus, Gavialis, with a single species.

Differences between crocodiles andalligatorsThis question is asked quite regularly. Thereare many anatomical and physiological differences,but for the purposes of this book itwill suffice to name only three reasonablyobvious ones:1. Alligators are more cold resistant thancaimans and crocodiles. They can thereforelive further north than caimans and crocodilesin both North America and in China.2. In alligators and caimans the teeth of thelower jaw fit into pits in the upper jaw, consequentlywhen the mouth is closed nomandibular teeth are visible. In crocodilesthe fourth mandibular tooth fits into a notchin the upper jaw and thus remains visiblewhen the mouth is closed (Fig. 1.7).3. Crocodiles and gharials have sensory pitsin the ventral scales (Fig. 1.8). These areabsent in alligators and caimans. This is oneof the important features used in the speciesidentification of goods made from crocodilianleather.Wild or captive?This refers to the description of the differentways in which the crocodiles are living orkept.WildCrocodiles in the wild may be either leftentirely to their own devices or subjected toa certain degree of management. They arehardly ever seen to be suffering from disease or dying, and often they live in such remoteareas that suitable specimens rarely reach thelaboratory (see also p. 239).CaptiveCrocodiles kept in zoos and other collectionswithout a productive goal are referred to ascaptive. They may be bred or exhibited only,but they may also be subjected to scientificstudies.Wild-caughtCrocodiles caught in the wild and kept for ashort period restrained for the purpose ofsample collection or transported alive to amarket, where they are slaughtered. They areunder very severe stress which may affectmany of their physiological and biochemicalparameters. Such animals should be referredto as wild-caught.RanchedCrocodiles kept on farms for commercial(productive) purposes, but either hatchedfrom eggs collected in the wild or havingbeen collected as hatchlings, are referred toas ranched. Their diseases are substantiallythe same as those of farmed crocodiles,except for their closer contact with wild populations,

which may constitute a naturalreservoir of crocodile-specific infectiousagents.FarmedCrocodiles hatched from eggs laid by breedingstock kept on a farm for commercial purposesare called farmed crocodiles. Theon-farm breeding of these crocodiles allowsthe genetic selection for certain productiveparameters. These animals no longer have adirect link to the wild. Their only contributionto the conservation of wild crocodilesmay be to keep commodity prices low,thereby lowering the incentive for poaching.However, they may also provide a substantialadditional gene pool.Where such crocodiles are farmed faraway from wild crocodile populations theincidence of crocodile-specific infectious diseasesis usually very low.Crocodilian AnatomyThe aim of this section is to provide sufficientinformation for the normal functions of the body to be understood and for the recognitionof the organs during post-mortemexaminations. This information is basedlargely on my own experience with Nilecrocodiles. For a reasonably detailed andaccurate study of the anatomy of theAmerican alligator see Chiasson (1962). Weare still waiting for a standard textbook oncrocodilian anatomy. A dissection guide forpost-mortem examinations is given inChapter 2 (p. 75).The skeletonSkullThe pitted appearance of the dorsal skullsurface (Fig. 1.9) is due to its fusion with theskin. There are three pairs of foramina dorsallyon the skull: the external nares openinginto one nasal orifice, the orbits and thesupertemporal fossae (Fig. 1.9). On the ventralaspect, almost at the same level, are theanterior palatine foramina (foramen), theposterior palatine foramina and, partiallyhidden, the internal nares (Fig. 1.10). Thecranium, which houses the brain, liesroughly between the orbits and thesupertemporal fossae. The articulation of thejaw is caudal to the atlanto-occipital joint,allowing the jaws to open extremely widely(Fig. 1.11).VertebraeThe cervical and thoracic vertebrae haveribs. The cervical ribs lie alongside the vertebralcolumn pointing caudally, but only thethoracic ribs connect with the sternum. Acartilaginous portion in the midrib allowsflexibility for collapsing the thorax duringdeep diving. The lumbar vertebrae do nothave ribs, but the sacral ones do. Dorsally allthe vertebrae bear neural spines; and ventrally,chevron bones, which point in anobliquely caudal direction, are attached tothe caudal vertebrae. A fibrous membranebearing abdominal ribs (gastralia) connectsthe sternum with the os pubis and supports

the abdominal viscera.LegsThe pectoral girdle, consisting of the scapula,coracoid and sternum, together with the first thoracic ribs, surrounds the wide cranialaperture of the thorax. This allows largemasses to be swallowed. The bones of theforelimb (humerus, radius and ulna) areshorter than their counterparts in the hindlimb. The front feet have five digits, the firstthree carrying claws.The pelvic girdle consists of an os ileum,an os ischium directed caudoventrally and anos pubis pointing cranially. The hind limbsare twice as long as the forelimbs, allowingfor a galloping action. Femur, tibia and fibulaare well developed. The foot has four digits,the first three carrying claws (Fig. 1.12).The skinScales and osteodermsCrocodile skin, like that of all reptiles, is coveredwith scales or scutes and is devoid ofsweat glands. On the head the skin is fused to the bones of the skull. The large scales onthe back, and in some species some of theventral scales also, contain bony plates, theosteoderms. Muscles connect the ossifieddorsal scales with the vertebral column, andwhen the muscles contract this results in adorso-ventrally rigid, beam-like structurethat allows the crocodile to keep its back andtail straight when walking or running (Frey,1988a,b). In this context it is interesting tonote that recent mitochondrial DNA analyses,as well as studies of nuclear genes, suggesteda close relationship betweencrocodilians and chelonians (tortoises andturtles). The latter also have osteoderms andboth dorsal and ventral armour (Hedges andPoling, 1999).Skin glandsCrocodilians have a few holocrine skinglands. The cloacal (paracloacal) glands aresituated laterally within the lips of thecloaca. The mandibular (gular) glands are inthe skin under the tongue, between themandibles (Fig. 1.13). The septa of the gularglands are lined with melanocytes, givingthe gland tissue its black appearance(Weldon and Sampson, 1988). The paracloacalgland is a single secretory sac with a singleduct and a single lumen. Theparenchymal cells contain lipid droplets(Weldon and Sampson, 1987). For the analysisof the aromatic secreta of these glands,see p. 52. In some species there are also rudimentarydorsal glands – in the Chinese alligatorbeneath the second row of scales fromthe dorsal midline, but in various positionsfrom the 2nd to the 15th transverse row(Chen et al., 1991).IdentificationThe patterns of scales, both dorsal and ventral,are species specific, although someslight individual variations may occur. A keyfor the identification of tanned whole crocodilianskins can be found in Brazaitis (1987).Pigmentation

Hatchlings of many species have light anddark transverse striations, which in somespecies are maintained almost into adulthood.These striations mimic rippling shadowsin shallow water (see Fig. 1.3). Thechromatophores in the skin can contract andexpand following nervous impulses from theeyes via the brain. Blind crocodiles and thosekept in complete darkness usually displaylighter colours than those exposed to brightdaylight. The musclesThere are no external muscles on the headbecause the skin adheres to the skull. Thepowerful jaw muscles are all on the medianaspect of the mandible, thus broadening theposterior skull. Sphincters close the externalnares and depressors close the auricular flapover the tympanum for diving. The long dorsalmuscles of the trunk extend into the tail.These muscles, plus the ventral tail muscles,musculus (m.) caudofemoralis medially andm. ilioischiocaudalis externally (Frey, 1988a),provide the power for swimming (Fig. 1.14).

The respiratory systemRespiratory tractThe external nares are slightly raised abovethe level of the upper jaw, allowing the crocodileto surface and breathe when most of itsbody is submerged. Adult male gharialsdevelop a large nasal excrescence, the ghara(see Plate 1), which is thought to function asa vocal resonator (Whitaker and Basu, 1983).In the long nasal passage the olfactorynerve endings are exposed to the air. Exceptwhen swallowing, bellowing or yawning,the posterior part of the mouth is closed bythe gular valve, consisting of the dorsal flapof the tongue and the palatal flap (velumpalati) extending from the soft palate(Putterill and Soley, 1998a) (Fig. 1.15). TheEustachian tubes enter the pharynx in ajoined opening just caudally of the internalnares (Colbert, 1946) (Fig. 1.16). Their function is to equalize the pressure on the twosides of the tympanum (the ear membrane).Close to the opening of the Eustachian tubesinto the pharynx there are two mucosalfolds, one on either side and extending caudally,which contain tonsillar tissue (Putterilland Soley, 2001) (Fig. 1.16).The glottis has two soft lips (Fig. 1.17)which close when the crocodile swallows. Incrocodiles (but not in alligators) the tracheabends to the left inside the thorax before itsbifurcation, a substantial distance beforeentering the lungs (Fig. 1.18). This allowslarge chunks of prey to be swallowed withoutexerting any pressure on the trachea orbronchi.LungsThe lungs are multi-cameral sac-like structures,highly vascularized but with thickerwalls than a mammalian lung. These thickwalls may be necessary to counteract theoutside pressure during diving. The lungs lie

in pleural chambers which are separated bya complete mediastinum. The posterior partof the lungs is connected tightly to the anteriortransverse membrane (postpulmonarymembrane). In crocodiles the remainder ofthe lungs lies loosely in the thoracic cavity,not as described by Duncker (1989), while inthe caiman the lungs are fused to the ventralwall of the thorax. For a detailed study oflung morphology of the Nile crocodile, seePerry (1988).Respiratory musclesThe thorax is divided from the abdomen bytwo transverse membranes. The postpulmonarymembrane separates the lungs fromthe liver, and its ventral third is muscular.The posthepatic (posterior transverse) membraneis attached to a sheet of muscle (m.diaphragmaticus) which extends to the ospubis (Van der Merwe and Kotzé, 1993).Together, the two membranes, with theirmuscular components, act like a diaphragm,pulling the liver in a caudal direction forinspiration.

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 areproduced by forcing the air through the compressedlips of the glottis (Fig. 1.17), much assounds are produced by human lips in themouthpiece of a trumpet.The digestive systemTeethCrocodilian teeth are pointed, very sharp andare constantly replaced throughout life. Thereplacement rate varies with the growth rateand slows down as the animal becomes older.In small American alligators (<1.5 m) the estimatedreplacement rate varied from 3 to 4months (Erickson, 1996a). Early in life, toothreplacement occurs in waves, passing alongalternately numbered tooth series from backto front, while later in life the direction isreversed (Edmund, 1962). Although very old,toothless individuals are sometimes found,this may be due to accumulated damage tothe alveoli (Erickson, 1996b) (see p. 247).While crocodilian teeth are homomorphic,they may be categorized by their position inthe maxilla and mandible. Kieser et al. (1993)group the teeth of the Nile crocodile as follows(Fig. 1.19):● maxilla: 5 incisors – gap – 5 canines – gap– 5 molars;● mandible: 3 incisors – gap – 5 canines –gap – 7 molars.The first mandibular canine is the longestand often extends above the dorsal plane ofthe snout (Fig. 1.20). This can cause damageto belly skins when frightened young crocodilespile in the corner of their pen (see alsopp. 114 and 241). In some individuals thetwo mandibular incisors 1 on each side

sometimes penetrate the maxilla behind themaxillary incisors and produce openings inthe upper lips in front of the nostrils, whichcould be called ‘false nostrils’ (Figs 1.9 and1.21; see also p. 153).TongueThe tongue occupies the floor of the mouthcavity. It is not free but is held in place laterallyby a folded membrane. The dorsal surfaceof the tongue contains mucus glandswhich are associated with lymphoid tissue forming ‘lingual tonsils’, while sensoryorgans are found along the sides of thetongue (Putterill and Soley, 1998b). In crocodilesthe dorsal surface also contains saltglands (Taplin and Grigg, 1981; Franklin andGrigg, 1993).OesophagusThe oesophagus extends from the epihyalcartilage of the larynx to the clearly definedgastro-oesophageal junction. It has manylongitudinal folds, allowing distension whenthe crocodile swallows large chunks. Theentire epithelial surface contains manygoblet cells that function as an intra-epithelialgland (Putterill et al., 1991).StomachThe stomach lies to the left, immediatelybehind the left lobe of the liver and the posteriortransverse membrane. Its junction withthe oesophagus (cardia) is defined by a welldevelopedsphincter muscle. The pyloric exitalso lies in the cranial aspect, slightly to theright of the cardia, and is defined by a smallbulbus, the pyloric antrum, which in turnopens into the duodenum (Figs 1.22–1.24).The entire interior surface of the stomach is lined uniformly by mucosal glands. Gastrinand somatostatin cells are found only in theglands of the pyloric antrum (Rawdon et al.,1980; Dimaline et al., 1982). The pyloric openingto the duodenum is very small, thus preventingthe escape of accidentally swallowedforeign bodies (see p. 254).The gastric wall is strongly muscularizedover the fundus, which gives the crocodilianstomach a somewhat gizzard-like appearance.However, the internal glandular liningwould not be able to protect the mucosa froma strong chewing action, as the koilin layerdoes in the avian gizzard. The function of thegastroliths that are often found in crocodilianstomachs, i.e. whether they are ballast, have achewing function as in birds or have beentaken in accidentally, is the subject of anongoing debate (Steel, 1989) (see also pp. 36and 290). The fact that stomachs of crocodiles in the Okawango swamps in Botswana containincreasing amounts of plant material,such as papyrus roots and palm tree seeds,with increasing body length (Blomberg, 1976)tends to indicate accidental ingestion.IntestineThe looped duodenum starts from thepyloric antrum and extends to the end of theloop. In many crocodile species the duodenumfolds over again, forming a doubleloop, although this is apparently not the casein alligators. Both forms occur in different

populations of Osteolaemus tetraspis(Huchzermeyer et al., 1995; Huchzermeyer,1996b). Part of the pancreas is embeddedbetween the limbs of this loop (see Fig. 1.26).From the end of the loop the jejunum runsinitially straight along the dorsal aspect ofthe abdominal cavity, then becomes suspendedin loose coils by the mesentery to the point at which the cranial mesenteric arterymeets the intestine (van der Merwe andKotze, 1993). From this point the ileumextends in similar coils to the very short rectum,which in turn enters into the cloaca(Hunter, 1861) (Fig. 1.22). The rectum is suspendedby a short mesentery and liesbetween, and ventral to, the two kidneys.The internal surface of the intestine doesnot have villi, but a system of complexzigzagging, ridge-like folds, which alternatewith each other and are oriented longitudinally(Kotzé et al., 1992; Kotzé and Soley,1995) (Fig. 1.25).PancreasThe proximal (ventral) pancreas lies betweenthe limbs of the duodenal loop, while thedistal (dorsal) part surrounds the cranialaspect of the spleen (Miller and Lagios, 1970;Huchzermeyer, 1995) (Fig. 1.26).LiverThe liver lies between the two transversemembranes in the hepatic coelom and hastwo lobes of almost equal size – the rightlobe being slightly larger than the left. The heart separates these two lobes. In somespecies the lobes are completely separate,while in others they are joined by a dorsalbridge of liver tissue.Substantial collagenous trabeculae havebeen found in the liver of American alligatorsand to a lesser extent in Caiman crocodilus(Beresford, 1992). Storch et al. (1989)found abundant Kupffer cells, as well asfat-storing cells, in the sinusoidal lining ofthe liver of O. tetraspis. Numerous Kupffercells are also present in Nile crocodilelivers.The gall bladder lies between the twoliver lobes within the hepatic coelom, andreceives bile from both. The bile duct entersthe intestine in the proximal duodenum (Vander Merwe and Kotzé, 1993). In most of theAmerican alligators examined by Xu et al.(1997) the right and left hepatic ducts wereinterconnected, the right duct entering thegall bladder while the left duct continuedthrough the pancreas directly into the duodenum.The urinary systemThe two kidneys are firmly attached to thedorsal abdominal wall in the most posteriorpart of the abdomen. As in birds, they arenot embedded in perirenal fat and lack acapsule. The renal tissue, consisting of corticaland pelvic layers, is folded over, in a singlefold in the African dwarf crocodile and inmultiple folds in other crocodile species.These folds continue to grow as the crocodilegrows. These multiple folds give the kidneyof the Nile crocodile a triangular shape on

transverse section, while the kidney of theAfrican dwarf crocodile appears flattened.The folding patterns appear to be speciesspecific (Figs 1.27–1.29).Crocodilians do not have a urinary bladder.The two ureters open into the cloaca.However, urine may be stored in the rectum(Fig. 1.30).The reproductive organsFemaleTwo ovaries are attached to the dorsal bodywall cranioventrally to the kidneys, and arepartially attached to the cranial part of thekidneys. The ovaries are elongate, and in veryyoung animals they are difficult to differentiatemacroscopically from testes. In largerjuveniles the follicular structure becomes evident.In adult crocodiles all the folliclesmature at the same time (Fig. 1.31). The ovarianhistology of the American alligator wasstudied by Uribe and Guillette (2000). In adultfemale American alligators the corpora luteaform after ovulation. Their morphology issimilar to that in birds and their size can beused to judge whether a female had laid eggsduring the preceding season, recent corpora lutea having a minimum diameter of 0.4 cm(Guillette et al., 1995b).The ostium of the oviduct lies close to thecranial apex of each ovary. The oviducts areconvoluted and increase in size with maturityand sexual activity. They enter the uteri(the glandular part), followed by the vaginae,where the eggs are stored before laying(Fig. 1.32). The vaginae join the cloaca, caudallyto the ureters. A small clitoralappendage, which resembles the male penisin shape, is situated ventrally in the cloaca.MaleThe slightly flattened testes are situated inthe same position as the ovaries in thefemale (Plate 2). A convoluted deferent ductruns along the caudolateral border of eachtestis and enters the cloaca close to the baseof the copulatory organ. This crocodilianpenis is folded around a ventral seminalgroove (Fig. 1.33). Note that crocodiles andtortoises have only one penis, while lizardsand snakes have paired hemipenises.

The endocrine organsPituitaryThe pituitary gland lies on the ventral aspectof the brain, at the level of the optic lobes(Fig. 2.26) (Chiasson, 1962).ThymusThe thymus gland consists of a series of lobulesof varying sizes on both sides of the tracheaalong the neck and in the thorax to thebase of the heart. In well-nourished crocodilesthese glands are embedded in fatty tissue, which is almost the same colour. Thismakes it difficult to differentiate the individuallobules. Note that Huchzermeyer’s (1995)description of the thymus glands of the Nilecrocodile was based on emaciated individualsfor improved visibility. Consequenly the lobuleshad vanished from the necks of these animals.

However, lobules were subsequentlyseen in the neck of healthier Nile crocodiles asdescribed previously from other crocodilespecies (Gegenbauer, 1901; Bockman, 1970).It is believed that in crocodilians thethymus remains active throughout life. For adiscussion of the striated muscle cells occasionallyfound in the reptilian thymus, seeRaviola and Raviola (1967).ThyroidsWhile some crocodilians have a single thyroidgland with two well-defined lobes oneither side of the trachea, connected by anarrow isthmus (Lynn, 1970), other specieshave two separate lobes. They are recognizedby their dark-brown colour. In the Nilecrocodile these are situated not on either sideof the trachea, but on the lateral side of eachof the two bronchi and medially of the commoncarotid artery, the right one closer to theentrance of the right bronchus into the lungand the left one closer to the bifurcation ofthe trachea (Plate 3) (Huchzermeyer, 1995).ParathyroidsThe two parathyroid glands are normallyhidden by thymus tissue and difficult to see.In the Nile crocodile they are situated caudolaterallyof the thyroid glands on eachside, between the precaval vein and the commoncarotid artery, immediately cranially ofthe dorsal bend of the aortic arch (Plate 3and Fig. 1.35) (Huchzermeyer, 1995). The situationappears to be similar in C. crocodilus,apart from the fact that additional (accessory)parathyroid glands occasionally occur(Oguro and Sasayama, 1976).AdrenalsThe two adrenal glands are found in theabdominal cavity adhering to the dorsalbody wall. Ventrally they partially overlapthe proximal part of the kidneys. Theyextend cranially beyond the two kidneys andsomewhat laterally of the midline (Plate 2)(Huchzermeyer, 1995).Pancreas and intestinal tractThe topography of the pancreas has beendescribed above. In the Nile crocodile theislets of Langerhans appear to be presentin the distal (dorsal) pancreas only(Huchzermeyer, 1995). A similar distributionwas found in the American alligator, inwhich smaller groups were also found in theventral (proximal) portion (Jackintell andLance, 1994).Endocrine cells have also been found inthe pyloric part of the stomach and in theintestine of crocodiles (Rawdon et al., 1980;Dimaline et al., 1982; van Aswegen et al.,1992).The circulatory system and blood cellsHeartIn the Nile crocodile the heart is situatedbetween the 4th and 8th thoracic ribs (Vander Merwe and Kotzé, 1993) and betweenthe two lobes of the liver (Fig. 1.34). The situationis similar in the other crocodilians. A

ligament at its apex, the gubernaculumcordis (Webb, 1979), connects it to the pericardialsac and beyond this to the postpulmonarytransverse membrane. There is nofat in the coronary groove. The two auriclesstretch caudally on either side halfway alongthe ventricles, the larger right auricle sometimesfurther. The four chambers are completelyseparated.CirculationThe major blood vessels leaving the heart ofthe Nile crocodile are identified in Fig. 1.35.Crocodiles have two aortic arches like otherreptiles. The left aortic arch leaves the rightventricle alongside the pulmonary arteryand becomes the coeliac artery; this suppliesthe digestive organs of the abdomen. The right aortic arch emerges from the leftventricle and runs posteriorly as the dorsalaorta.The left and right aortic arches communicatein two places: the foramen of Panizzaand the anastomosis (Axelsson and Franklin,1997). The foramen of Panizza is located atthe base of the heart within the aortic archvalves (Webb, 1979), and the anastomosis is ashort vessel connecting the two aortic arches.In American alligators of length 1–2 m, theforamen of Panizza had a diameter of 1–2 mm (Greenfield and Morrow, 1961).During systole it is completely covered bythe aortic valves, thus allowing an exchangeof blood during diastole only (Axelsson andFranklin, 1997).As in all reptiles and birds, part of thevenous blood of the caudal half of the bodyis drained through the renal portal system.According to Chiasson (1962) this appears tobe bypassed partially by the ventral abdominalveins which, together with the mesentericvein, enter the hepatic portal system.Superficial veinsI have been unable to identify any large, easilyaccessible superficial veins for intravenousinjections. Blood can be drawn fromthe dorsal and ventral vertebral veins of theneck and tail (see p. 64). The statement aboutdrawing blood from the temporal vein,which lies just below the temporal muscle onthe dorsal aspect of the head (Lance, quotedby Samour et al., 1984), is in error. Such atechnique has never been used or describedby Lance (personal communication, V.A.Lance, San Diego, 1999).TonsilsIn the Nile crocodile the tonsils are situatedin the roof of the pharynx (Putterill andSoley, 2001) (see above, Fig. 1.16).SpleenThe pear-shaped spleen lies dorsally in themesentery, close to the base of the duodenalloop (Plate 2). Its broad cranial end is embeddedin the caudal limb of the pancreas. Thespleen is covered by a strong capsule. Thehistology and vascular architecture of thespleen of the American alligator were studiedby Tanaka and Elsey (1997).Lymphatics

The lymphatic system was studied byMcCauley (1956). There are no subcutaneouslymph vessels and no lymph nodes. Lymphvessels from the head and the anterior limbs,thorax and abdomen anastomose with theexternal jugular vein just proximal to thejuncture with the subclavian vein. Thelymph from the caudal and pelvic regions ispumped by the posterior lymph hearts intosmall vessels which empty into the pelvicvenous plexus. These lymph hearts are single-chambered muscular structures, measuring3 _ 5–7 mm in alligators 50–75 cm long.They are situated superficially at the junctionof the hind limb with the pelvis, below thesuperficial layer of the deep fascia in thetriangle formed by the m. longissimuscaudae, the crest of the ileum and the m.flexor caudae.In the absence of lymph nodes, the thymus(see above), tonsils, spleen and numerouslymphoid masses in the walls of thedigestive tract act as reservoirs of lymphocytes.Blood volumeThe total blood volume of one juvenileAmerican alligator was 4.2% of body mass(Coulson et al., 1950), of another two alligators5.1% and 5.5% (Andersen, 1961), andthat of 3.5-year-old Cuban crocodiles (n = 19)was 4.0 ± 0.3% for males and 3.6 ± 0.2% forfemales (Carmena-Suero et al., 1979).Blood cellsAll crocodilian blood cells are nucleated. Theerythrocytes are oval in shape with round oroval nuclei. The dimensions of the red bloodcells of some crocodilian species are given inTable 1.1.The following descriptions of the variousblood cells were taken from Mateo et al.(1984b) and refer to the American alligator.Detailed descriptions of the blood cells ofCrocodylus porosus and Crocodylus johnsoni aregiven by Canfield (1985). See also Hawkeyand Dennett (1989). However, there is someconfusion in the literature, with differentauthors using different definitions for thedifferent leucocytes.THROMBOCYTES. Length 14.3 _m, oval or ellipticalwith smooth cell borders, smooth paleblue or almost colourless cytoplasm thatoften contains numerous clear confluent vacuoles with poorly demarcated borders.The uniform oval nuclei are located centrallyand oriented longitudinally, stainingintensely dark purple, with coarsely condensedchromatin.A phagocytic function of avian thrombocyteswas discovered recently (Wigley et al.,1999) and the same function may be postulatedfor reptilian thrombocytes. The functionsof the other blood cells are presumed tobe the same as in mammals and birds.HETEROPHILS. Heterophils, or type I granulocytes(Canfield, 1985), are round to oval cellswith mean diameters of 17.3 _m and distinctsmooth cytoplasmic borders. The nuclei arelenticular, oval or, rarely, bilobed, with

indistinctly clumped purple chromatin,usually eccentrically located at one pole ofthe cell. The cytoplasm contains abundant,3–4 _m long, fusiform refractile granules,occasionally arranged in perinuclear radiallysymmetrical star-like configurations(Fig. 1.36).EOSINOPHILS. Eosinophils, or type II granulocytes(Canfield, 1985), are oval or occasionallyround, with a mean diameter of 14.9 _mand smooth cytoplasmic outlines. The lenticularor oval nuclei are purple with prominentcoarsely clumped chromatin and verysharply demarcated borders, usually locatedat one pole of the cell, often causing a slightoutward bulge of the cell outline. Somenuclei are located more centrally. The paleblue,smooth cytoplasm is visible only as athin rim surrounding the many bright pinkplump granules measuring 2–3 _m (Fig.1.37). Often a few granules are present on theface of the nucleus.BASOPHILS. Basophils, or type III granulocytes(Canfield, 1985), are round cells with irregularexternal ‘cobblestone’ contours and adiameter of 12.8 _m. Abundant, dark-purpleto purplish-red, round granules measuring0.1–0.5 _m pack the cell to the point wherethey frequently obscure the nucleus.Sometimes the granules are arranged in aperipheral rim with a central cluster over thenucleus.LYMPHOCYTES. Lymphocytes are generallyround or oval, with a diameter of 10.7 _m,but irregular, polygonal forms are also seen.A large nucleus, with smooth outlines andfollowing the cell contours, almost fills thecell. The nucleus is pale violet with finelyclumped chromatin. The cytoplasm is visibleonly as a thin, slate-grey or pale-blue rim bordering the nucleus. Occasionally dustlikered granules and/or a few clear vacuoles,1 _m in diameter, are scatteredthrough the cytoplasm. External cell bordersrange from smooth to ragged, frequentlywith bleb-like protrusions of cytoplasm.MONOCYTES. Monocytes are oval or roundwith a diameter of 14.3 _m, with somewhatindistinct external cell borders and numerousdelicate cytoplasmic projections. Theabundant grey-blue cytoplasm sometimescontains a few clear, refractile vacuoles, measuring1.2 _m. Many cells have fine dust-likegranules, usually arranged in crescentic perinuclearaggregates. The plump, ovalnucleus, measuring 7.1 _m, is usuallylocated centrally, but is sometimes eccentrically situated adjacent to one pole of the cell.The nucleus is homogeneous light purplewith finely stippled chromatin (Fig. 1.38). Inaddition to these typical monocytes, largecells, up to 20 _m in diameter, with undulatingborders, pale-blue cytoplasm with fewinclusions, and prominent, indented or evenhorseshoe-shaped nuclei are seen occasionally.Details of crocodilian haematology are givenin Chapter 2.Chromosomes

The chromosomes of 21 species of crocodilianswere studied by Cohen and Gans (1970)and of five species and several crossings byChavananikul et al. (1994). The number ofchromosomes ranges from 30 to 42 and thefundamental number from 56 to 62, for detailssee Table 1.2. There are no sex chromosomes.The nervous system and sensory organsBrainThe most striking features of the crocodilianbrain are the two olfactory bulbs, whichextend anteriorly far beyond the two cerebralhemispheres. The optic lobes are exposedbetween the hemispheres and the relativelysmall cerebellum. The base is formed by arelatively broad medulla oblongata.Spinal cordThe spinal cord extends almost to the tip ofthe tail. There is no cauda equina, as eachpair of caudal nerves leaves the cord at thesite of exit from the vertebral column(Chiasson, 1962).Peripheral nervesThe peripheral nerves exit the spinal cord inpairs. At the level of the pectoral and pelvicgirdles they are organized into a brachialand lumbo-sacral plexus, respectively. For adetailed description see Chiasson (1962).Autonomic nervous systemLike higher vertebrates, crocodiles also havean autonomous nervous system, consistingof two components. The vagus nerve startsas the tenth cranial nerve and runs along thejugular to the thoracic and abdominal viscera.The sympathetic trunk runs parallel tothe spinal cord and communicates with eachspinal nerve, thickening at each site of communicationin the form of a sympathetic ganglion(Chiasson, 1962).

EarThe ear has two sensory functions, hearingand spatial orientation.HEARING. The tympanic membrane of the earis protected by a fibrous flap which closeswhen diving. In the middle ear the columellais attached to the tympanic membrane and atthe other end it has a large basal plate set inthe fenestra ovalis of the inner ear.SPATIAL ORIENTATION. The membranouslabyrinth consists of three semicircularcanals, each with an ampulla, the utriculusand its ventral extension, the lagena, allenclosed in bone.Both functions are served by the acousticnerve (eighth cranial nerve) (Chiasson, 1962).EyeThe eye is protected by three eyelids. Thethird eyelid is the nictitating membrane,which is optically clear and protects thecornea during diving. A special muscle canretract the eye into the orbital fossa. A reflectinglayer behind the retina improves nightvision and causes crocodile eyes to light upat night in the beam of a torch (Fig. 1.39).There is no night vision in complete darkness

without a minimum of residual light.Fat storageFat bodyBeing exothermic, crocodiles do not need fatfor insulation. In fact, subcutaneous fatdeposits would impede thermoregulation (seep. 44). Also, crocodiles do not store fat in thecoronary groove of the heart. In mammals,there is growing evidence that the heart usesmainly fat as a source of energy (Medeirosand Wildman, 1997). It is believed that this isalso the case in crocodiles, and that the fatsupply for the heart is stored in the abdominalfat body, for which I propose the anatomicalname the steatotheca (Greek stear = fat; th\k\ = container, store). This organ is locatedin a mesenteric fold close to the right abdominalwall, immediately posterior to the liver(Fig. 1.40) (Vorstman, 1939; Mushonga andHorowitz, 1996). Its volume varies with thestate of nutrition, while its shape varies fromspecies to species (Fig. 1.41). The fat cells havelarge nuclei, demonstrating their ability toactivate the stored fat rapidly (Fig. 1.42).Somatic fatAdditional fat may be stored in somaticfat cells with small nuclei: in the mediastinumof the thorax, under the peritoneumand between muscles, particularlyventrally in the tail between the inner(caudofemoralis) and external (ilioischiocaudalis)muscles.

The eggCrocodilian eggs are elongate elliptical andhave a hard shell. The size of the egg varieswith the species, with the age of the femalethat lays the egg – young females layingsmaller eggs than mature females – and individuallybetween females. Larger eggsproduce stronger and more viable hatchlings,which rapidly outgrow hatchlingsfrom smaller eggs. Parameters of Americanalligator eggs were determined byCardeilhac et al. (1999b) and are summarizedin Table 1.3.

EggshellThe calcareous shell consists of an outer,densely calcified layer, in which the calcitecrystals are stacked vertically; a honeycomblayer of horizontally stacked crystals; anorganic layer, which contains a higher percentageof organic matrix; and a mammillarylayer. Pores penetrate the shell surface andend between the mammillae. These pores aremost frequent in the opaque zone (Ferguson,1982).A thin, organic, probably mucinous, layerwas found to cover the outer surface of somenewly laid eggs and was believed to consistof the remnants of oviductal secretions. Thislayer was no longer present after 2 weeks ofincubation. It is therefore not an equivalentof the wax cuticle present on most avianeggs (Ferguson, 1982).Under the calcareous shell lies theeggshell membrane, consisting of two layers,

a fibrous membrane facing the shell and alimiting membrane facing the embryo. Thelimiting membrane contains a large numberof tiny pores and fewer large pores. Most ofthese pores are closed at the onset of incubationand others open up as incubation proceeds.Consequently, the shell membrane isless permeable to oxygen than the calcareousshell (Kern and Ferguson, 1997).The opaque band around the lesser circumferenceof the egg develops during incubationin parallel with the expansion of thechorioallantoic membrane and the mobilizationof calcium out of the shell for use by theembryo (Fig. 1.43). At the same time, anextrinsic acidic degradation of the outer shelloccurs due to microbial action in the nest.This produces erosion craters around thepores and increases the permeability of theshell (Ferguson, 1982).Unlike the avian egg, the crocodile eggdoes not have an air chamber between theshell and the shell membrane (Ferguson,1982).Internal componentsThe yolk, with the embryonic disc floatingon top, is surrounded by a large quantity ofthin albumen, which in turn is contained in a layer of thick albumen separating it from theshell (Magnusson and Taylor, 1980). If theegg is turned during laying, gravity causesthe yolk with the embryo to rotate. Within24 h of laying, the developing vitelline membraneand the embryo adhere to the shellmembrane, displacing the albumen towardsthe poles of the egg (Webb et al., 1987).The embryoFrom the start of embryonic development inthe oviduct, water is drawn from the albumenand secreted beneath the embryo on theinside of the vitelline membrane, where itforms the subembryonic fluid. After laying,the volume of subembryonic fluid increasesrapidly, causing the volume within thevitelline membrane (containing embryo,subembryonic fluid and yolk) to expand(Webb et al., 1987).Albumen dehydration and the productionof subembryonic fluid peak at the time of theexpansion of the allantois. Along the shellthe allantois fuses with the chorion andforms the chorioallantois (Webb et al., 1987),which becomes highly vascularized andtakes on the gas-exchange function untilhatching, when the lungs are able to fill withair. The different embryonic membranes andspaces are shown schematically in Fig. 1.44.Crocodilian PhysiologyYolk-sac resorptionJust before hatching, the yolk-sac is drawninto the abdominal cavity and the body wallcloses around the navel. At this point gasexchange can no longer take place via themembranes and the young pre-hatchling hasto start using its lungs.The yolk-sac has already provided nutritionduring embryonic and fetal development,

and (in American alligators) has lost25% of its mass (Fischer et al., 1991), but stillcontains sufficient nutrients (75% of its contentsin American alligators) for the first fewweeks, until the hatchling is strong enoughto find its own food (Fischer et al., 1991). Thecontents of the yolk-sac are resorbed in twodistinct ways: (i) direct resorption into thebloodstream via a capillary bed which hasdeveloped in the wall of the yolk-sac; and (ii)voiding via the vitello-intestinal duct intothe intestine, digestion there and finallyresorption through the intestinal mucosa.The open vitello-intestinal duct is also amajor pathway for infection of the yolk-sacwith intestinal bacteria, depending onintestinal colonization and peristaltic movements(see p. 142). The vitelline duct connectingthe yolk-sac to the intestine is shown in Fig. 1.45. Unlike the situation in birds, thecrocodilian yolk-sac does not appear to beanchored to the navel.There do not appear to be any reportsabout the time it takes for the yolk-sac to becompletely resorbed under normal circumstances.It is probably 3–4 weeks. This wouldbe temperature dependent, with a slower rateof resorption at lower (suboptimal) temperatures.Infection of the vitello-intestinal ductcan lead to its closure and, in this case, theyolk-sac will remain unresorbed (see p. 143).Sex differentiationCrocodiles do not have sex chromosomes(see above). Instead, the sex of the embryo isdetermined by the incubation temperature.Recently, Crews and Ross (1998) reviewedcurrent knowledge about the mechanismsinvolved, as follows.At the temperature-sensitive stage earlyin embryonic development, temperatureinfluences the expression of stereogenic factor1, which in turn upregulates the expressionof the gene for aromatase, the criticalenzyme in the synthesis of oestrogen.Oestrogen then binds to the oestrogen receptor,the expression of which is also modulatedby the incubation temperature. Via thiscascade of events low incubation temperaturesfavour the development of ovaries,while at high temperatures testes are produced.However, this cascade can easily beinfluenced, or even disrupted, by the actionof external steroids (see p. 223).GrowthFactors influencing growthThe growth of juvenile crocodiles dependsmainly on the environmental temperatureconditions and on nutrition, althoughgenetic and clutch-related factors probablyalso play a role (Garnett and Murray, 1986).The most important clutch-related factor isegg size and consequently hatchling size, assmall hatchlings are generally poor growers.Stress caused by high stocking density candepress the growth rate (Elsey et al., 1990a)(see also pp. 116 and 280).

Metabolic rateThe metabolic rate of crocodiles depends ontheir size and activity, and the temperature(Baldwin et al., 1995; Munns et al., 1998). At28°C a 70 kg American alligator producesabout 72 kcal day_1, i.e. about 4% of that of aperson of equal mass. At 32°C the rate doubles.However, a hatchling at 28°C has half thehuman metabolic rate (Coulson et al., 1989).Stress-related reduction of the growth rate– runting – of some individuals is a commonoccurrence on crocodile farms (see p. 234). Apositive influence of sunlight on the growthrate was found by Zilber et al. (1991), but thesmall number of individuals involved, thepoor overall growth rates achieved and thehigh mortality in the experimental groupsseverely limit the usefulness and credibilityof their results. Generally, growth rates, particularlyweights, achieved on farms exceedthose in the wild. One-year-old wild Indo-Pacific crocodiles attained 0.73 m and0.87 kg, while farmed ones of the same ageaveraged 0.75 m and 1.36 kg (Webb et al.,1991).Crocodiles may continue growingthroughout their life, males faster thanfemales. The growth of adult females is furtherreduced by reproductive demands. Withincreasing age the growth in length slowsdown and is replaced by growth in width,leading to a maximum length, at least inAmerican alligators, which might not beexceeded (Woodward et al., 1995). Youngfemales lay smaller eggs and smallerclutches than older ones. There is also someindication that, in individual females, eggsize and clutch size are inversely related.AllometryAllometric studies have shown that the bodyand tail grow faster than the head and legs,although at some stage the snout lengthgrows faster than any other part measured.These changes in the proportions of the differentparts of the body allow the growingcrocodiles to adjust to the different demandsmade by the environment on crocodiles ofdifferent sizes (Kramer and Medem, 1955;Junprasert and Youngprapakorn, 1994).The correlation between myocardial mass,i.e. the mass of the two ventricles of theheart, and body length of Nile crocodileswas examined by Huchzermeyer (1994). Theventricular mass can be used as a standardfor the evaluation of other more variableorgans, particularly the fat body and spleen(see p. 85).Bone ringsIn most crocodilian species growth is seasonaland this is reflected by bone deposition.Such growth rings can be detectedhistologically and are used to estimate theage of the crocodile in question (de Buffrénil,1980a,b; de Buffrénil and Buffetaud, 1981;Wagner et al., 1990). Experimentally, thismethod can be enhanced by feeding tetracycline

which is deposited in the bone in theform of visible, stained rings (Roberts et al.,1988).The growth rate of crocodilians is limitedby the slow deposition of lamellar bone. Thiswas the case even in the giant crocodileDeinosuchus of the Late Cretaceous period ofNorth America (up to 10 m in length), whichis estimated to have taken 35 years to reachadult size (Erickson and Brochu, 1999).Age–length–weight relationsThe age–length relation depends on thegrowth rate, while the length–weight relationdepends on the actual state of nutrition.Consequently these relations differ betweenwild and farmed crocodiles, the latter growingfaster and being fatter. There are alsoindividual differences. Some examples ofsuch relations in American alligators, Nilecrocodiles and African dwarf crocodiles aregiven in Tables 1.4 to 1.6. Furtherlength–weight relations for Nile crocodilescan be found in Table 2.10. Mathematicalapproaches to length–mass relationships ofcrocodilians were explored by Wilkinson etal. (1997).LongevityWhile captive American alligators may livefor up to 70 years, they do not appear to reach more than 50 years in the wild(Woodward et al., 1995). Similar ages may beattained by individuals of other crocodilianspecies. However, estimates may be far out.An American crocodile with an estimatedage of 100 years was mentioned by Jasminand Baucom (1967).LocomotionSwimmingThe crocodilian body is designed primarilyfor swimming. During this action the frontlegs are held parallel to the thorax, while thehind legs are partially spread out to act asrudders. Sideways movements of the tailprovide the propelling force for both slowand rapid swimming. Rapid swimming canbe extremely fast and can catapult the crocodileout of the water at a very high speedwhen it attacks a prey on land close to thewater.At lower temperatures the swimmingspeed is reduced. In juvenile American alligatorsthe swimming speed increased at temperatures from 15°C to 20°C, butnot between 20°C and 30°C (Gatten et al.,1991).SlidingSliding occurs when the body is not lifted offthe ground. This kind of motion is used overshort distances on land and always whengoing into water. Sometimes referred to as‘sprawling’, it is also seen in the transitionfrom stationary to ‘high walk’ (Elias andReilley, 1996). On farms sliding can damagethe chin, the belly skin and the soles of thefeet if the floor of the pen consists of concretethat is not absolutely smooth or covered witha protective paint.Gharials cannot walk. On land they slide,

moving their body forward with all four legsacting simultaneously.WalkingWhen walking the crocodile lifts its wholebody off the ground. In this way it can moveover rough terrain without getting scratchedor torn. It is a stately motion, similar to thatof a tortoise when walking. It is also referredto as ‘high walk’.RunningA faster way of moving on land is running,which is a kind of galloping motion. This canbe quite fast, but can only be sustained overshort distances.JumpingHatchlings and yearlings of the Africandwarf crocodile have an additional mode oflocomotion. They use their relatively stronghind legs to jump in a frog-like fashionwhen frightened while on land. Each jumppropels the hatchling forward by up to1 m and it may jump several times insuccession.Crocodiles can also jump out of deepwater to catch prey high above the water orout on land. To achieve this they gatherspeed under water before surfacing.DigestionIngestionSmall prey is swallowed whole, though it isat least punctured during the act of catchingand killing. Larger prey is masticated for awhile before deglutination (Diefenbach,1975a). However, crocodiles do not reducethe size of the morsels by prolonged chewing.Excessively large prey is reduced byworrying and ripping off bits or limbs byrapid rotation around the longitudinal axisof the crocodile. Ripping is facilitated whenseveral crocodiles feed from the same carcass.Small bits are taken off the ground byholding the head sideways (see Fig. 3.20).ReductionIn the stomach the swallowed food isexposed to the action of hydrochloric acid(HCl) and peptic proteolysis. Their secretionis stimulated by the presence of the food,while penetration into the food is facilitatedby the puncturing and chewing that hastaken place before swallowing. Gastric pHdrops as low as 1.2 and in fasting animalseven stays below 2.5 (Diefenbach, 1975a).Gastric contractions mixing the stomach contentstake place 2–3 times per minute whenthe stomach is full (Diefenbach, 1975b). At30°C complete emptying of the stomach took99 h on average and at 15°C 315 h(Diefenbach, 1975b). However, Kanui et al.(1991) recorded gastrointestinal passagetimes in 12-week-old Nile crocodiles as 35 hat 30°C and 44 h at 25°C.LithophagyStones (gastroliths) are often found amongcrocodilian stomach contents. The questionremains whether these stones are needed togrind the ingested food, similar to the situation

in an avian gizzard, whether they areneeded as ballast, or whether they are swallowedaccidentally (Sokol, 1971) (see alsop. 15). Here it should be noted that predatory(carnivorous) birds do not use stones in theirgizzards. Fitch-Snyder and Lance (1993)

observed captive juvenile American alligatorsactively seeking out and swallowinggravel. However, this could have been due toa behavioural disturbance similar to the frequentlyseen ingestion of foreign objects bystressed farmed or captive ostriches(Huchzermeyer, 1996a) (see also pp. 281 and290).RegurgitationWhen American alligators eat hairy prey, theindigestible hair forms hair balls, which arethen regurgitated. Smaller foreign bodiesmay also become incorporated in these hairballs and regurgitated as well. Even radiocollars attached to released juvenile alligatorshave been found regurgitated after thebearers had been cannibalized (Chabreck,1996; Chabreck et al., 1996).DigestionThe combined action of pepsin and HCl inthe stomach digests most of the protein inthe food and dissolves the bones of the prey.Further protein, glycogen and fats aredigested in the upper small intestine underthe action of bile and pancreatic secretions.There is some evidence that frequent fillingof the stomach reduces the digestive efficiencyof the system (Webb et al., 1991).There is a suspicion that excess fat in the dietmight interfere with proteolytic activity andtherefore Webb et al. (1991) recommend amaximum of 9% fat in crocodile rations.AssimilationAssimilation is the uptake of the digestedfood from the intestine either into the venouscirculation and hence into the liver, or via thelymph directly into the general circulation.This takes place throughout the length of thesmall intestine (duodenum, jejunum andileum).Seasonal suppression of appetiteCoulson et al. (1950) observed that captiveAmerican alligators practically stopped feedingduring autumn and winter, althoughthey were kept at a constant temperature. Itis unclear whether this response was triggeredby diminishing daylength or whetherit might be governed by a built-in bodyclock. This phenomenon has also beenobserved in captive Nile crocodiles (personalcommunication, L. Fougeirol, Pierrelatte,2002).Normal oral floraIdentification of the oral flora of crocodiliansis important for the treatment of bitewounds. The work done on American alligatorscan be taken as representative for allcrocodilian species. Doering et al. (1971)isolated Clostridium spp., Citrobacter,

Enterococcus spp. ‘and others’ from twoAmerican alligators. Flandry et al. (1989)examined ten alligators from three differentlocations and found both aerobic and anaerobicbacteria in all of them, but isolated fungifrom only seven individuals (Tables 1.7–1.9).The bacterial oral flora of 19 farmed spectacledcaimans in Brazil comprised the generaCitrobacter, Providencia, Escherichia,Proteus, Morganella, Serratia, Edwardsiella,Aeromonas, Acinetobacter, Staphylococcus,Streptococcus and Bacillus (Matushima andRamos, 1993).

Crocodiles, particularly in captive or farmsituations, tend to contaminate their aquaticenvironment with faecal bacteria and fungi.Thus it is not surprising that the oral florashould be similar to that of the intestine.Flora of the gular and paracloacal glandsWilliams et al. (1990) isolated the followingaquatic and intestinal bacteria from either orboth pairs of the exocrine skin glands of 23adult American alligators: Acinetobacter anitratus,A. wolffi, Aeromonas hydrophila, Bacillussp., Citrobacter amalonaticus, C. freundii,Corynebacterium sp., Enterobacter agglomerans,E. cloacae, Edwardsiella tarda, Escherichia coli,E. hermanii, Flavobacterium indoltheticum, F.gleum, F. multivorum, Hafnia alvei, Klebsiellapneumoniae, Proteus mirabilis, Pseudomonascepacia, P. maltophila, Serratia marcescens andYersinia enterocolitica.Normal intestinal floraThe intestinal flora plays an important protectiverole by occupying the availableattachment sites and thereby displacingpathogenic intruders, a phenomenon referredto as competitive exclusion. Intensivelyreared crocodiles often have a single-speciesflora, an abnormal situation that makes themprone to intestinal infection. Despite itsimportance, this appears to be a neglectedsubject, probably partly due to the difficultyof obtaining specimens from animals in thewild, since they are usually in remote places.Most of the published results are from captivecrocodiles and it is doubtful that theyare representative of a normal intestinalflora.Campylobacter fetus subspecies jejuniserotype 23 was isolated from a captiveAfrican dwarf crocodile (Luechtefeld et al.,1981). Misra et al. (1993) examined cloacalswabs of 23 captive gharials and the resultsare shown in Table 1.10.

Roggendorf and Müller (1976) isolatedCitrobacter sp., Escherichia coli, Proteusmirabilis, P. vulgaris and Aeromonas hydrophilafrom the faeces of one captive Nile crocodileand Citrobacter sp., Providentia rettgeri andAeromonas hydrophila from the faeces of a C.crocodilus.Huchzermeyer and Agnagna (1994)reported the isolation of aerobic bacteriaand fungi from 21 wild-caught and severely

stressed African dwarf crocodiles whichwere sampled when they were slaughteredat markets in Brazzaville, Congo Republic.These and subsequently published isolationsfrom samples collected during asecond expedition in 1995 (Huchzermeyeret al., 1999) are shown in Tables 1.11 and1.12.RespirationVentilationThere are two types of ventilatory movements,pharyngeal and thoraco-abdominal.Pharyngeal ventilation does not contribute tothe air flow to the lung. It only serves to moveair through the nasal passages for olfaction.Thoraco-abdominal movements involve thediaphragmatic muscles for inhalation and theintercostal and abdominal muscles for exhalation(Gans and Clark, 1976).

Respiratory rateRespiration takes place in cycles of two tothree rapid movements followed by a longerpause (Gans and Clark, 1976). The respiratoryrate depends on the size of the animal,decreasing with increasing body mass (Gansand Clark, 1976). It is also influenced by temperature,increasing with increasing bodytemperature (Campos, 1964; Smith, 1976),with lower rates during warming than duringcooling (Smith, 1976). There appeared to be alow correlation between the metabolic rateand the respiratory rate (Huggins et al., 1971).Respiratory rates for different sized crocodilesare given in Table 1.13. The respiratory rate of123 crocodiles of 27.4 min_1 reported bySigler (1991) falls entirely outside the range ofall the other observations and may possiblyinclude pharyngeal (gular) movements.DivingThe following is based on work with Indo-Pacific crocodiles by Wright (1987). Most voluntarydives are short, only lasting 5 min.During these dives the metabolism stays aerobic.Forced dives occur when the crocodile isdisturbed and can last for up to 1 h. Duringthese dives the metabolism slows down andbecomes anaerobic as an oxygen debt develops.A crocodile disturbed during a voluntarydive immediately changes its metabolism.The diversion of the arterial blood flow awayfrom muscles during forced diving conservesoxygen reserves for the functioning of thebrain. Lactic acid accumulated in the musclesenters the circulation only after the crocodileemerges (Andersen, 1961).Oxygen consumptionIn both American alligators and Nile crocodiles,the oxygen consumption of inactiveanimals was found to increase as the temperaturerose. However, in Nile crocodiles itwas found to decrease between 25 and 30°Cand then rise again steeply to 35°C (Brownand Loveridge, 1981; Lewis and Gatten,1985). The decrease is seen as an adaptationto nocturnal activity, which is usually at

lower temperatures (Brown and Loveridge,1981). In the American alligator values rangingfrom 0.08 to 0.2 ml g_1 h_1 correspond with those cited from a number of reports(Lewis and Gatten, 1985). Similar valueswere established for the Nile crocodile(Brown and Loveridge, 1981).Non-respiratory CO2 excretionA relatively low respiratory quotient in crocodiliansis explained by the excretion of largeamounts of ammonium bicarbonate in theurine (Coulson and Hernandez, 1964; Grigg,1978), while Davies (1978) suggested cutaneousCO2 loss as an explanation (see alsobelow).Acid–base balanceIn American alligators and in Indo-Pacificcrocodiles, arterial pH decreased with risingbody temperature, while arterial PCO2increased (Davies, 1978; Davies et al., 1982;Seymour et al., 1985; Douse and Mitchell,1991).Respiratory regulationIn progressively anaesthetized American alligatorsit was shown that central chemoreceptorsplay a significant role in ventilatoryregulation (Branco and Wood, 1993).ExcretionFasting crocodiles and alligators produceapproximately equal quantities of ammoniaand uric acid in their urine, but when theyare fed maximally the excretion of ammoniaincreases while the proportion of uric acid inthe urine decreases. This decrease in uricacid clearance leads to increased plasma uricacid levels, predisposing the animals to gout(see p. 230). Only negligible amounts of ureaare produced (Khalil and Hagagg, 1958;Herbert, 1981). The white deposits in crocodileurine consist mainly of uric acid crystals(Khalil and Hagagg, 1958).The glomerular filtration rate remainsfairly constant under different conditions,and the tubules have little capacity to regulatethe osmolality of the urine. However,cloacal absorption varies with the salt load(Schmidt-Nielsen and Skadhauge, 1967).Salt lost into the freshwater environment isreplaced constantly by the salt contained inthe prey. Excess salt is excreted by specializedsalt glands, as is the case in othermarine reptiles (Schmidt-Nielsen andFange, 1958) (see also p. 14). Ammonia isthought to be excreted in the form ofNH4HCO3 which may be responsible for asubstantial deficit in respiratory CO2(Schmidt-Nielsen and Skadhauge, 1967;Grigg, 1978) (see above).Responses to high salinityAll alligatorines and most crocodiles arefreshwater species with poor salt tolerance.However, four crocodile species (C. porosus,C. johnsoni, C. niloticus and C. acutus) have estuarine populations. Large specimens of C.acutus lose weight more slowly in sea waterthan small ones, and NaCl loading causes areduction in cloacal flow rate, thus conservingbody water (Ellis, 1981).

Alligators osmoregulate by keeping a lowbody sodium turnover, by the low permeabilityof the skin to sodium and even bykeeping a relatively low water turnover.Estuarine crocodiles add to that effect by theexcreting of excess salt through the lingualsalt glands (p. 14). Freshwater species ofcrocodiles also have these salt glands andmay use them in aestivation during droughtperiods (Mazzotti and Dunson, 1989).Water loss through the skin when it isexposed to dry air may be considerable andthe lost water can only be replaced by drinking,not absorbed through the skin(Cloudesley-Thompson, 1968).ReproductionLaying cycleCrocodiles reproduce by laying eggs, as thetemperature control of sex determinationdoes not allow internal incubation (ovovivipary)as occurs in some snakes andlizards. Most species lay only one clutch ofeggs per year, the mugger being the exception,with two cycles per year occurring regularly(Whitaker and Whitaker, 1984).However, many females in the wild do notreproduce every year, probably dependingon their nutritional state (Lance, 1987;Kofron, 1990).Clutch and egg sizeEgg size and egg number per clutch arespecies dependent but increase with the sizeand age of the female, with younger femaleslaying small eggs from which fewer, smallerand more slowly growing hatchlings are produced.Hormonal controlThe hormonal control of the reproductivecycle and factors influencing this controlhave been described by Lance (1987). A sexsteroid-binding protein, seasonally presentin the plasma of female American alligatorsand probably other crocodiles as well, preventsthe delivery of free steroid to targetorgans outside the breeding season (Ho et al.,1987).OvulationAll follicles are normally ovulated togetherover a period of a few hours (personal communication,V.A. Lance, San Diego, 2000),but according to Youngprapakorn (1990b)sometimes some follicles ovulate prematurelyand proceed through the oviduct tothe uterus in advance of the others.Fertilization takes place in the infundibulumor upper oviduct before the albumen andshell are secreted in the glandular part of theoviduct (uterus). The eggs are then stored inthe muscular part (vagina) until they arelaid. In the American alligator the eggs arestored in the vagina for 3–3.5 weeks beforethey are laid (Lance, 1989). During this time,before oviposition, initial embryonic developmentis already taking place, to the 15–17somite stage and occasionally further (personalcommunication, V.A. Lance, San Diego,2000). Oestradiol liberated during ovulation

increases plasma calcium levels for the productionof the eggshells, but, unlike birds,crocodiles do not deposit calcium in theirbones before ovulation (Elsey and Wink,1986).Nesting and incubationNesting habits vary from species to species.Forest-dwelling species build nest moundsfrom leaves scooped up from the forest floorand depend on the heat produced by composting,rather than on the sun, to incubatethe eggs. Swamp dwellers make nest mountsfrom swamp vegetation and rely on compostingheat as well as on the sun for theincubation of the eggs, while crocodiles livingin rivers nest in the sandy banks abovethe flood level and rely on the sun for incubationheat. All eggs in the clutch aredeposited into the nest at the same time andcovered again with nesting material. The incubation period depends on the species aswell as on incubation temperature, decreasingwith rising temperature, and rangesroughly from 60 to 90 days.Cross-breedingSeveral species of crocodiles cross-breedvoluntarily, producing fertile hybrids(Youngprapakorn, 1990a; Thang, 1994). Asthese hybrids tend to be more vigorous, theyare sought after by some farmers. Escapedhybrids, however, can pollute existing wildpopulations and this constitutes a considerabledanger to the conservation of certaincrocodile populations.CirculationBlood flowThe flow of blood transports oxygen andnutrients to the organs and tissues, and CO2and end-products of metabolism from theorgans and tissues to the lungs and kidneys.In addition, it can speed up the transport ofheat from the skin to the internal organswhen the crocodile is basking, move whiteblood cells and antibodies to infection sites,and hormones to targeted organs. A 70 kgAmerican alligator at 28°C has a blood flowof 0.2 l min_1, a stroke volume of 6.3 ml, acirculation time of 27 min and 4% of themetabolic rate of a person of equal mass(Coulson et al., 1989).Heart rateThe heart rate depends on the size of the animaland on the temperature. In 57- to 78-cmlongAmerican alligators at an ambienttemperature of 22–25°C it was 18.7 beats perminute (Huggins et al., 1971). In anaesthetizedAmerican alligators from 1.5 to4.3 kg live mass it ranged from 10 beats perminute at 10°C to 30 beats per minute at30°C (Campos, 1964). At 38°C and aboveirreversible damage occurred through overheating(Wilber, 1960).At 28°C a 70 kg American alligator had ablood flow of 0.2 l min_1, a stroke volume of6.3 ml and a circulation time of 27 min(Coulson et al., 1989).Bradycardia

Diving bradycardia occurs when crocodilesdive after a sudden fright. Under these circumstancesthe heart rate decreases fromaround 30 beats per minute to 2–5 beats perminute, but not during voluntary (shortterm)dives (Gaunt and Gans, 1969; Smith etal., 1974). During bradycardia the blood pressureis maintained by peripheral vasoconstriction(Jones and Shelton, 1993).ShuntUnder certain conditions, venous (deoxygenated)blood can become mixed with arterial(oxygenated) blood through the foramenof Panizza, through direct release into theright aorta and through the anastomosesbetween the right and left aortas (see p. 23).This mixing of venous and arterial blood isreferred to as a left-to-right shunt. Jones andShelton (1993) described a biphasic systolicpressure curve in the right ventricle of restingcrocodiles in which the first phase suppliesthe pulmonary artery and the secondphase supplies the right aorta. This shuntdiverted 15–25% of the venous blood awayfrom the pulmonary circulation. They speculatedthat in addition to the respiratoryrequirements for a shunt during forced diving,the alkaline wave caused by the productionof HCl in the stomach after a meal alsonecessitated a flow of venous (acidic) bloodto the digestive viscera.VasoconstrictionVasoconstriction is mediated hormonallyand by nervous stimuli and can occur locally,e.g. as a response in thermoregulation (seebelow), or systemically. Adrenaline wasfound to produce a stronger vasoconstrictionin the American alligator than noradrenaline(Akers and Peiss, 1963). Angiotensin I of theAmerican alligator was found to be closelyrelated to that of the chicken (Takei et al.,1993).

Nervous activityThe brain of the crocodile is larger and betterorganized than that of other reptiles, but it isstill relatively small in relation to the crocodile’sbody mass. Many functions are thereforedelegated to centres in the spinal cord.During hypothermia (2–4°C) in restrainedAmerican alligators (size not stated, butprobably large as their sex was stated), theelectrical activity of cerebrum and opticlobes decreased, whereas it increased in thecerebellum (Parsons and Huggins, 1965).Compared with that of other reptiles, aswell as with that of many birds and mammals,the hearing of crocodiles is very acute,particularly in the middle range but less sofor high and low tones (Wever, 1971). On thefarm, the crocodiles rely mainly on smell andhearing to recognize the person usuallyworking with them. In nature they recognizeeach other by the excretions of their skinglands (see p. 52). Therefore, captive orfarmed crocodiles may fail to recognize, and

thus be disturbed by, people who occasionallywear perfume.ThermoregulationCrocodiles are exothermic reptiles, unable tomaintain a constant internal body temperatureindependently of the environment.However, they try to achieve and then maintaintheir temperature within a preferredrange, and they do this by making use ofthermogradients in the environment. Thesegradients exist between sun and shade,warm surface water and cool deep water.Some species also make use of burrows inwhich their rate of cooling during winternights would be slower than in cold water(Pooley, 1962) or which protect them fromheat and dehydration during aestivation(Christian et al., 1996).During cooling the blood circulation tothe body surface is restricted, thus reducingthe rate of cooling. During warming theblood flow to the skin is increased and thewarmed blood transports the heat to theinternal organs (Johnson, 1974; Grigg andAlchin, 1976; Johnson et al., 1976; Drane et al.,1977; Johnson and Voigt, 1978; Smith andAdams, 1978; Smith et al., 1978; Smith, 1979).When basking in shallow water, the bloodsupply to the submersed skin is reduced,while it is increased to the skin exposed tothe air and sun (Johnson, 1974). The osteodermsmay also play a role in thermoregulationas heat collectors (Seidel, 1979). Gapingincreases evaporation and thereby contributesto cooling, which at certain times ofthe day appears to be preferred to going intothe water. However, gaping may also beused to increase the temperature-exchangesurface. Consequently Nile crocodiles havebeen observed gaping while basking on acold African winter morning (own observation).The preferred temperature depends onthe crocodile’s activity: fasting crocodilesprefer cooler and feeding ones select highertemperatures (Lang, 1979). Endogenous(metabolic) heat plays a role only in verylarge crocodiles with a low surface area tomass ratio (Smith, 1979).In very cold winter weather Americanalligators remain close to the surface, withonly the nostrils protruding from the water.In this position, called ‘icing’, they are safefrom suffocation when the water freezes over(Hagan et al., 1983; Lee et al., 1997). However,even such specimens do not survive if theirinternal temperature falls below 4.5°C(Brisbin et al., 1982). A released Americanalligator in a swamp in Pennsylvaniaappears to have survived at least six coldwinters before it was shot (Barton, 1955).Nile crocodiles tolerate a minimum internaltemperature of 10°C.Exposing juvenile farmed crocodiles tovarying temperature regimes, Turton et al.(1994) found that high temperatures aremore stressful than lower ones, and that temperature

changes are always accompaniedby increased corticosterone levels. Certainfarming conditions tend to expose the crocodilesperiodically to overheating as well as towidely fluctuating day–night temperatures,a situation that is obviously to be avoided. Itis my belief that for their well-being crocodilesneed to be able to thermoregulateactively along a thermogradient within therange of preferred temperatures. They should also be strictly protected from forcedoverheating, i.e. being exposed to high temperatureswithout any means of avoiding theheat or being able to cool down.The growth rate of juvenile crocodiles isclosely related to the temperature at whichthey are kept, with the fastest growth seen inthose kept closest to the preferred maximumtemperature, and particularly in those withthe highest preferred temperature (Lang,1987). As the preferred temperature is influencedby the incubation temperature, one canactively select for a higher temperature preference,and thus for faster growth, by incubatingat a higher temperature (Lang, 1987).ImmunityCrocodiles can react to infections by developingantibodies and thus becomingimmune to the agent in question. The whiteblood cells that play a role in this systemhave been described above (p. 25). Inresponse to a stimulus, lymphocytes are producedin the thymus and spleen. An activespleen increases in size very rapidly, but asthe tough fibrous capsule resists this rapidgrowth, the active tissue buds out throughthe capsule, giving the hypertrophic spleenan irregular, knobby appearance (Fig. 1.46).Unlike other reptiles, crocodiles are capableof an anamnestic response. YoungAmerican alligators immunized with 50 mghaemocyanin had antibodies in their bloodafter 20 days. However, when given a secondinjection of 2.5 mg, antibodies becamedetectable after only 2 days (Lerch at al.,1967). An immunoglobulin with two IgG-likelight chains was isolated from American alligatorsby Saluk et al. (1970). Turton et al.(1994) isolated an immunoglobulin fromjuvenile Indo-Pacific crocodiles and identifiedit as IgG, with a molecular weight of218 kDa and heavy and light chains of 57and 27 kDa, respectively.

American alligators did not have any isohaemagglutination,but their serum containedthree agglutinins, one for all humancells, another similar to the _-agglutinin andone similar to the _-agglutinin of humanserum (Bond, 1940).InflammationExudationInflammation is a reaction by the body tolocalize, isolate and fight a local infection orother injury. The first step is a congestion ofthe local capillaries, allowing serum to seepinto the tissue and cause oedema. In mammals,

the liquid from this oedema is filteredwith the lymph through lymph nodes, butcrocodiles, like birds, lack lymph nodes. Toprevent the drainage of pathogens away fromthe inflammation site directly into the generalcirculation, they exude fibrin into the inflamedsite, which immobilizes all the pathogens andprevents their escape. This is a very successfulstrategy, with the result that crocodiles andbirds rarely contract septicaemia from woundinfections. However, if the inflammatory cellsare unable to remove the invading pathogens,the exudation process continues and ultimatelycan lead to serious problems.Abscess, fibriscessFibrin also inhibits the movement of leucocytesand prevents the liquefaction ofnecrotic tissue, with the result that trueabscesses filled with liquid pus cannot beformed. Hard swellings forming at the site ofan infected wound consist of sheets of fibrinbetween the tissues and these are extremelydifficult to remove surgically. Veterinariansoften refer to this exudate erroneously as‘inspissated pus’. Since such a fibrin-filledswelling cannot be classified as a trueabscess, it should rather be called fibriscess(Huchzermeyer and Cooper, 2000).Cellular reactionsAfter subcutaneous injection of turpentineinto juvenile American alligators, Mateo et al.(1984b) observed oedema followed by granulocytemigration in the first 3 days. Latermonocytic cells predominated, includingvacuolated macrophages. From 14 daysonwards zones of necrotic debris (most likelyexudate, see above) were surrounded by palisadesof vacuolated multinucleated giantcells and capillary-laden immature fibrousconnective tissue.FeverFever results from a higher setting in an animal’sthermoregulatory system in reaction toan infection or similar event. In endothermsthe increase in temperature is achieved metabolically,but crocodiles, as ectotherms, haveto adjust their temperature behaviourally byselecting a higher temperature on the environmentalgradient (Lang, 1987) (see p. 44).DiseaseIn a holistic view, a healthy animal lives in astate of balance with its natural environment.Within certain ranges it can respond to allphysical, chemical and biological challenges.These challenges may act singly or in combination(Fig. 1.47). The responses are thedefences. An animal that is unable to defenditself adequately against any such single orcombined challenge slides into a state ofimbalance that is referred to as disease(Wedemeyer et al., 1976).Captive, farming and ranching conditionsare usually different from natural conditionsand often far from ideal. Often they increasethe severity of the challenges while limitingthe animal’s ability to respond. Such conditions

can easily cause an animal to becomeimbalanced and diseased.From the holistic definition of disease it isclear that such a state cannot be diagnosedmerely by examining the affected animalalone, nor by the laboratory analysis of certainspecimens taken from the diseased ordeceased animal. In addition, all factors inthe environment, as well as nutrition, haveto be taken into consideration.

Crocodilian BehaviourNormal behaviour is characteristic of ahealthy animal. Not to be able to act in accordancewith its behavioural requirements in acaptive or farm situation may severely stressan animal. Non-domesticated animals, particularly crocodiles, are very sensitive to thiskind of stress. It is therefore important forcrocodile farmers and veterinarians to beaware of the behaviour patterns and requirementsof their crocodiles.Embryonic learningIt has been found that the food selection ofIndo-Pacific crocodiles can be influenced bypainting flavours on to the crocodile eggsduring incubation (Sneddon et al., 1998). Thiskind of embryonic learning may also influenceother aspects of hatchling behaviour. Itis possible that by urinating on the nest themother primes the hatchlings to recognizeher when they hatch.Parental careParental care has been observed in manycrocodile species and it is presumably therule in all crocodilians. It includes guardingthe nest, helping the hatchlings out of theegg, carrying them from the nest to the water,guarding them there, responding to the distresscalls of young ones in danger and occasionallymoving the pod to new nurseryareas. All this is done mainly by the female,but, where the adults live in pairs or wherethe female has disappeared, males have beenseen either helping or taking over the care ofthe hatchlings (Alvarez del Toro, 1968).ImprintingImprinting is known from birds, which athatch are imprinted with the image of theirmother and thereby recognize their parentsand later in life choose their sexual mate intheir parents’ image. Human imprintingin intensively reared ostrich chicks can leadto behavioural disturbances, sometimeswith serious consequences (Huchzermeyer,1996a).It is difficult to explain the complex hatchling–parent interactions of crocodilians withoutthinking of the possibility of imprinting.A suspected case of human imprinting offarmed Nile crocodiles in South Africa wasreported by Huchzermeyer (1998b). It is postulatedthat when the mother drives thehatchlings away before the new broodhatches (see below), a behavioural switch isoperated, inducing the juveniles to avoidlarger crocodiles from then on. When farmreared

juvenile crocodiles are released intothe wild, often many of them are eaten byolder crocodiles. Possibly their lack ofimprinting made them unable to see the dangerposed by larger members of the samespecies.Dispersal of the youngBefore the new clutch hatches, in somespecies possibly long before, the juvenilesleave their mother, and there are some indicationsthat, in fact, they are being activelychased away (Hunt, 1977; Hunt andWatanabe, 1982). After leaving their parentsthey may still stay together in pods, evenpods consisting of several clutches (Allstead,1994), or they may disperse individually. Inmany species the different age groupsoccupy different habitats, primarily for agespecificprey requirements, but also keepingthe different sizes apart and thereby minimizingcannibalism.CannibalismCannibalism occurs quite commonly in crocodilesand may be regarded as a populationregulatory mechanism, allowing more juvenilesto reach adulthood in a depleted adultpopulation (Hutton, 1989). However, highlosses of released juveniles (Rootes andChabreck, 1993) could also be caused by thefact that the released farm-reared animalswere non-imprinted and therefore did nothave the behavioural switch enabling themto avoid larger members of the same species(see above). Where the size classes are segregatedin the wild, it would be important torelease juveniles into the correct habitat orniches to avoid excessive cannibalism.The fact that many sporulated coccidiansporocysts often become sequestered in different organs of crocodiles (see p. 187) couldalso be an indication that cannibalism is anormal occurrence in crocodiles in the wildand that this parasite, at least, has adoptedthis as a mechanism for its transmission.Hunting and feed selectionCrocodiles are nocturnal animals and huntor forage actively during the night.However, they will also take prey during theday if the opportunity should arise. Howmuch their behaviour is affected by diurnalfeeding in captive and farm situations is notknown. However, crocodiles might becomemore interesting for zoo visitors to observe ifthey were shown in a night display.All crocodiles prefer live, moving food,but they easily adapt to inert feed, such asfresh or boiled mince. Under suitable stressfreeconditions they also will take pelletedfeed without any problems. While small fish,tadpoles, frogs and toads were readily recognizedas prey by Nile crocodile hatchlingspreviously fed with mince, lizards (Agamastellio) were left untouched (Morpurgo et al.,1991).Social behaviourSocial interactions revolve around sexual,territorial and food competition, and the

establishment of a ranking hierarchy in apopulation. For this the crocodiles useacoustic and visual signalling as well asaggression. The signals vary to some extentfrom species to species, and so does aggressiveness.In juvenile farmed Nile crocodiles,aggression was found to be linked to feedingand the type of feed, to stocking density andto size variation. Inert feed, low stockingdensity and removal of the larger individualswere all conducive to low aggression levels(Morpurgo et al., 1993b).TerritorialityTerritorial behaviour varies from species tospecies and is most marked in adult crocodilesduring the breeding season. In general,one can say that swamp-inhabiting crocodilesare stricter about establishing territories,while riverine species tend to be moregregarious or tolerant of a higher populationdensity in a breeding area. This may haveimplications for the establishing of breedingcolonies on crocodile farms.Although apparently severe wounds maybe inflicted during territorial fights betweenmales, the fights are more of a ritualizednature (Plate 4) and much less severe thanthose between females over nesting sites.Sexual behaviourThe sexual behaviour of crocodiles consistsof courtship displays, mating and defendingthe ‘harem’. Here also the details differ fromspecies to species. Often, while the dominantmale is occupied with one particular female,some of the other females of his ‘harem’ willseek out and copulate with other males. Thisleads to multiple fatherhood of particularclutches and has the benefit of a widerspread of genes.In large breeding colonies on Nile crocodilefarms one aims to provide distinct territoriesfor several dominant males by thedisposition of islands and other visual barriers(Fig. 1.48).NestingDepending on the characteristic habitat ofthe species, crocodiles either build nestmounds from the substrate (vegetation matteror sand) or dig holes in the sand. It isbelieved that the rotting vegetation in thenest mound contributes to the creation of thecorrect incubation temperature, particularlyin dense forest, where the nest mounds cannotbe exposed to the sun. In areas whereflooding occurs, crocodiles choose highground for their nesting sites. For this reasonit is important when designing breedingcolonies on Nile crocodile farms to have allthe nesting sites at the same level, to avoidcompetition for the more elevated ones(Fig. 1.49).ThermoregulationAll crocodiles like to maintain a constantinternal body temperature, dependingsomewhat on their activities and the time ofday. To achieve this they make use of the

environmental thermogradient, which consistsof sun (radiation) and shade (air temperature),as well as warm surface and cooldeep water. To some extent they can alsomake use of evaporative cooling, although itis unlikely that that is the only purpose ofgaping (Fig. 1.50). During gaping the gularvalve remains closed, whereas it opens duringyawning.Many species make use of burrows toescape excessive heat (aestivation) or excessivecold. This means of maintaining thtemperature is usually not provided oncrocodile farms. In autumn, Chinese alligatorsdig particularly elaborate burrows, withone or two openings usually facing south,one or two tunnels, one to three chambers, asleeping platform and a pool. They use theirsnout, fore limbs, body and tail for diggingand moving the soil (Bihui et al., 1990).We also know the pleasures of thermoregulatorybehaviour, lying on a beach andsoaking up the warm sunshine, then divinginto the cold water to cool down, and thenback into the sun again and so on. Crocodileslying in the sun the whole morning are notjust lying there, they are busy thermoregulating.We should always keep in mind that toprevent crocodiles in a captive or farm situationfrom being able to thermoregulate andachieve their desired temperature can causevery severe stress.Optimal core temperatures are between28 and 33°C. Temperatures above 35°C arelethal (once the internal temperature rises tothose levels), and several systems cease tofunction below 25°C. However, Americanalligators are known to survive very lowtemperatures. If, during a cold spell, theirwater freezes over, they keep their nostrilsout of the water (ice), while the rest of thebody remains submerged. This behaviour iscalled ‘icing’ (