Nile Crocodile, Crocodylus niloticus
TAXONOMY & NOMENCLATURE
(Benton 1985) (Brochu 2001) (Brochu & Densmore 2001) (Carroll 1988) (Harper 2001) (Hekkala et al 2009) (Janke & Arnason 1997, 2005) (King & Burke 1997)
(Parker 2009) (Summers 2005)(Sereno 2001)(Seymour et al 2004)
(Trutnau & Sommerlad 2006)
Describer (Date): Laurenti: 1768. Spec. Med. Synops. Rept. 53
Order: Crocodylia (8 genera, 22 species)
Suborder: Eusuchia (modern crocodilians)
Family: Alligatoridae (7 species)
Family: Crocodylidae (crocodiles and relatives; includes extinct members)
Subfamily: Crocodylinae (13 species, 2 genera) - True crocodiles.
Species: Osteolaemus tetraspis (African Dwarf Crocodile)
Species: Crocodylus niloticus
Subfamily: Tomistominae (1 species) - False Gharial
Gavialinae (1 species) - Indian Gharial
Taxonomic History and Nomenclature
Common name: Nile Crocodile. Mamba in Swahili, Garwe in Shona. (Harper 2001)
Scientific name: Crocodylus niloticus is derived from the Greek kroko ("pebble") referring to the rough texture of the skin and deilos ("worm"); niloticus means "of the Nile River (Africa) ". Herodotus used "krokodilos" to describe a lizard in the Ionic dialect of Greek. Later, Ionians applied this term to creatures in Egypt they thought similar. (Parker 2009)
- Taxonomy above according to King and Burke (1997) except for position of Indian Gharials which, according to recent molecular studies (Janke et al 2005), should be placed within the Crocodile family, not in a separate family.
- Brochu & Densmore (2001) detail some of the discussions of how to classify the Indian Gharial.
- In 1827 two crocodile mummies from Egyptian tombs were named by French naturalist Geoffroy Saint Hillaire - Crocodylus lacunosus and Crocodylus complanatus. (Trutnau & Sommerlad 2006)
- All crocodiles are diapsid or "two arched" reptiles
- Have two skull openings behind the eye socket, each of which is roofed by a bony arch
- Other vertebrates such as mammals are synapsids (one opening) or anapsids such as turtles (no openings)
- By modern classification systems, birds are also considered diapsids, although their skull have been highly modified.
- Subdivisions of diapsids:
- Archosauria ("ruling replies") - includes modern birds and crocodiles as well as extinct dinosaurs, pterosaurs, and crocodile relatives)
- Lepidosauria - lizards, snakes
- Seven subspecies of Nile Crocodile have been proposed but are not officially recognized. (Britton 2009)
- Geographical races distinguished mainly by features of their scales (numbers of, shape, ossification). (Trutnau & Sommerlad 2006).
- Newly rediscovered dwarfed crocodiles in Mauritania had been previously considered extirpated; these crocodiles were believed to be Nile Crocodiles, but Schmitz et al (2003) consider them distinct from Crocodylus niloticus after study of their DNA
- Crocodylus suchus is the proper name for these dwarf crocodiles, according to Schmitz, who suggested the name given by Geoffroy Saint Hillaire in 1807 should be used.
(Trutnau & Sommerlad 2006)
- Modern crocodiles descend from ancestors such as the small, active, and bipedal land-based predator Saltoposuchus that appeared some 225 million years ago (Late Triassic) in Europe, South America, and South Africa.
(Summers 2005) (Seymour et al 2004)
- These active, warm-blooded, land-living ancestors later adapted a largely aquatic way of life, leaving terrestrial habitats to the dinosaurs.
- Birds diverged from crocodile-like animals 254 million years ago (Janke & Arnason 1997).
- A distant crocodile-relative, Sarcosuchus, lived in Africa around 110 million years ago (Cretaceous); it reached lengths up to twice that of the largest living crocodiles or 12 meters (about 40 ft). (Sereno 2001)
- Around 65 million years ago crocodiles, alligators, and caiman diverged; this phylogeny is supported by both fossils and molecular data.
- Since 65 million years ago, crocodylids have existed on every continent except Antarctica.
- Crocodiles and alligators diverged around 140 million years ago (Jurassic/Cretaceous boundary). (Janke et al 2005)
- Modern species of crocodiles began to be distinct around 5-7 million years ago. (Late Miocene). (Brochu 2001)
- The genus Crocodylus is known from 5-7 million year-old fossils in Africa.
- Fossil record of Nile Crocodile (Crocodylus niloticus) dates to 2.5-3.5 million years ago (Late Pliocene) in North and East Africa. (Trutnau & Sommerlad 2006)
- Modern crocodiles' closest living relatives are birds (they are only distantly related to lizards).
(Janke & Arnason 1997).
- Molecular data shows similar protein coding genes and much other evidence. (Janke & Arnason 1997)
- Crocodiles' second closest relatives are the lepidosaurs (spiny lizards such as Tuatara of New Zealand) plus other lizards and snakes. (Benton 1984)
- With the application of modern genetic testing, an altered view of Nile Crocodiles:
(Hekkala et al 2009)
- Modern populations have "highly significant geographic structuring" that may represent "distinct evolutionary lineages" within this species.
- Traditionally, Nile Crocodiles seen as one species adapted to a wide geographic range.
DISTRIBUTION & HABITAT
(Brochu 2001) (Dessauer et al 2002)(Grenard 1991) (Hekkala et al 2009) (Pooley & Gans 1976)
(Ross 1998) (Trutnau & Sommerlad 2006)
- Formerly Nile Crocodiles lived all across Africa, extending even in historical times to Israel and along southern Mediterranean coast as far west and north as Tunisia
and Syria. (Pooley & Gans 1976) (Trutnau & Sommerlad 2006)
- Eliminated (extirpated) 1810-1820 on Comoros Island off Africa's East Coast.
- In early 1900's extirpated in Israel.
- After the Aswan dam project in Egypt in 1970's Nile Crocodiles moved back into areas in Egypt they once inhabited.
- Today, native to western Madagascar and widespread throughout Africa south of the Sahara desert; in eastern Africa up to Lake Nassar in Egypt. Absent from the southern tip of Africa.
- Range of Nile Crocodile occupies parts of 47 African countries. (Hekkala et al 2009)
- Distribution centered around seven geographical areas, each with distinct populations:
(Hekkala et al 2009)
- Four major drainages of Eastern Africa
(Kenya's Lake Turkana,
Ruaha River in Tanzania, Zambezi River in Zimbabwe, Limpopo River in South Africa)
- Two groups in Madagascar.
- One Sahara/West Africa population.
- Can be locally rare, with populations fragmented due to human land use and a drying climate. (Ross 1998)
- Fossil and modern distribution records suggests some lineages dispersed across marine barriers.(Brochu 2001) (Dessauer et al 2002))
- Distribution of the genus Crocodylus world-wide can't be explained by continental drift, land bridges, sea levels, or any other geological phenomena
- Today's Madagascar population probably founded by individuals who crossed 250 miles of marine waters from Africa
- Several physiological mechanisms of crocodiles probably helped them to colonize new lands via marine waters.
- Lakes, rivers, but also brackish coastal swamps.
- Nile Crocodiles are more salt-tolerant than previously believed, in spite of their primarily freshwater habitats. (Brochu 2001)
- In spite of preference for fresh water, Nile Crocodiles colonized Madagascar, some 250 miles across open ocean from East Africa
- Nile Crocodile also found in caves of western Madagascar.
- Presence of Nile Crocodiles in western Madagascar implies they swam across open ocean from Africa. (Grenard 1991)
- During hot and dry seasons in Somalia and Tanzania, stay in burrows. (Trutnau & Sommerlad 2006)
- Juveniles known to occupy burrows for protection from adults.
(Brazaitis 1989)(Britton 2003, 2009) (Carroll 1988) (Cott 1975) (Franklin & Grigg 1993) (Franklin & Axelsson 1994) (Huchzermeyer 2003)(Koshiba-Takeuchi et al 2009) (Kumazawa & Nishida 1995) (Lance et al 2000) (Leslie 2001) (Leslie & Taplin 2001) (Pooley and Gans 1976) (Seebacher et al 1999) (Seymour et al 2004) (Shute & Bellairs 1955) (Summers 2005)
(Trutnau & Sommerlad 2006)
Body Weight: 70 - 100 kg (154-220 lb); adults weighing 1,000 kg (2,205 lb) have been reported. (Pooley & Gans 1976)
Total Body Length: 2 - 3.3 m (6.6-10.8 ft); maximum recorded body length from snout to tip of tail is 5 m (16 ft) (Pooley & Gans 1976).
Skull length: About one half to two thirds of total body length.
- A large, narrow- headed scaly reptile with rows of scutes down the back, a long powerful tail and eyes, ears, and nostrils on top of the head.
- Juveniles dark olive brown, black cross bands on tail and body. Bands less distinct in adults.
- Adults' skin olive-green, bronze to brown; crossbands dark.
- Eyes are green.
- Underbelly is yellowish; upper body dark olive, brown, or gray.
- Coloration often obscured by a layer of mud and dust. (Britton 2003)
- Skin is leathery and made up of a geometrical arrangement of scales, many with a bony core (osteoderm) made of calcium phosphate and calcium carbonate; over the head the plates are fused to the skull.
- Nile Crocodiles have barely visible osteoderms on the lower (ventral) scales, as do alligators and caiman. (Trutnau & Sommerlad 2006)
- On Nile Crocodile flanks, scales that are closest to the upper (dorsal) scales are strongly keeled (ridged). (Trutnau & Sommerlad 2006)
- Nile Crocodiles have small oval osteoderms in the sides and throat regions of the skin. (Brazaitis 1989)
- Sense organs in skin (Dermal Pressure Receptors or DPRs; previously called Integumentary Sense Organs) on upper and lower jaws detect water pressure (Britton 2009)
- Similar sensory pits found on rest of the crocodile body but their function not well understood.
- Alligators and caimans have these sensors only on the jaws.
- In crocodiles, presence of these pits on the belly skin is one way to distinguish them from alligators.
- True albino Nile Crocodiles reported from a game farm in South Africa. (Huchzermeyer 2003)
- White crocodiles that still have dark markings aren't albinos.
Other Physical Characteristics
- Males are larger than females.
- Largest crocodile in Africa.
- Crocodiles are the most advanced living reptiles; have some features in common with birds and mammals.
- A hindbrain that much resembles that of birds and mammals; structures in the cerebellum may be "an evolutionary precursor" of strongly folded gray matter of bird and mammal hindbrains.
(Trutnau & Sommerlad 2006)
- A four-chambered heart (but it functions as a three chambered heart when under water, thus saving oxygen).
(Koshiba-Takeuchi et al 2009)
- Complete separation of the two ventricles (pumping chambers) of the heart so that different pressures can be applied depending whether blood goes to the lungs or the rest of the body.
- This feature evolved independently in birds, mammals, and crocodiles.
- Laboratory studies reveal that the functioning of the crocodile heart is "perhaps the most complex of all vertebrates" (Franklin & Axelsson 1994).
- Mitochondrial DNA rate of change in crocodile family similar to that of mammals (and faster than that of birds). (Janke & Arnason 1997)
- Previously, a rapid evolutionary rate was believed to apply only to "hot blooded" mammal, not to "cold blooded" reptiles. (Kumazawa & Nishida 1995).
- Vocalizations by parent and prehatchling crocodiles similar to those of birds. (Ferguson 1985)
- Stomach resembles that of birds, complete with gizzard. (Trutnau & Sommerlad 2006)
- Lizards, snakes, and turtles don't have a gizzard.
- Crocodiles are usually described as simple ectotherms (use behavior to regulate body temperatures that may fluctuate widely).
(Seebacher et al 1999)
- A study, however, in East and Central Africa showed crocodile body temperatures maintained within a narrower range than that of many other reptiles. (Cott 1975)
- A recent study of thermoregulation in alligators suggests they use both behavior and biochemical processes (such as rates of enzyme activity) to help control body temperature. This ability to alter basic enzyme activities according to seasonal changes in air and water temperatures had not been previously recognized. (Seebacher et al 2003)
- Crocodiles may have had ancestors that were endothermic (narrow range of tolerable temperatures; body temperature regulated by metabolic processes) (Seymour et al 2004) although some researchers disagree with this idea (Hillenius & Ruben 2004).
- Preferred temperature range for crocodiles and alligators is 25 -35 °C (77-95 °F). (Lance et al 2001)
- All true crocodiles in the family Crocodylidae, including the Nile Crocodile, have salt glands
on their tongues that secrete salt and help maintain electrolyte balance in salt-water environments. (Franklin & Grigg 1993) (Leslie and Taplin 2001)
- Salt glands become larger or smaller with changes in salinity of water; given time to adjust, crocodiles can adapt and thrive despite changes in the salinity of their surroundings.(Leslie and Taplin 2001)
- Crocodiles are not as adapted to sea water as fully marine reptiles such as turtles, marine iguanas, and sea snakes that can drink sea water (crocodiles must get their drinking water from their food).
- Alligators have the salt glands too, but they are nearly non-functional. (Britton 2009)
- Crocodiles tolerate salt water better than alligators do.
- Alligators inhabit primarily fresh water but can sometimes be found in higher saline environments.
- A tympanic membrane in the side of the crocodile skull conveys sound vibrations to the inner ear; crocodiles have unique movable ear-flaps that can cover and protect this membrane. (Shute & Bellairs 1955)
- Sound perception between 100 to 4,000 hertz (cycles per second). (Pooley & Gans 1976)
- Have bony eyelids; eyes can be pulled in when in water to streamline the head. (Britton 2009)
- Bony eyelids also present in the extinct ornithiscian dinosaurs such as ankylosaurs, the pterosaurs, and modern alligators. (Coombs 1972)
- Good vision to the side and front; not full binocular vision (a 25° overlap), overall vision with 270°
range. (Britton 2003)
- Nictitating membranes (third eyelid) give extra protection to eyes.
- A Harderian gland empties into the eye through ducts; these produce the "crocodile tears" often seen when the crocodiles eat a large piece of food. (Trutnau & Sommerlad 2006)
- A flap of tissue, the gular fold, can keep water out of throat and lungs when mouth is open.
- Snout flattened side to side, making little resistance to quick movement in water. (Pooley & Gans 1976).
- Nostrils pinch shut to prevent flooding of nasal passages when submerged. (Pooley & Gans 1976).
- Breathing helped by movement of the liver which is pushed against the lungs for exhalation and shifted away from the lungs for air intake. (Pooley and Gans 1976)
- 61-63 vertebrae
- 64-68 teeth, replaced throughout lifetime.
- An enlarged fourth lower tooth fits into a socket in upper jaw and is visible when equally wide jaws are closed.
- Alligators by contrast have an upper jaw that is wider than the lower jaw, so that when the mouth is closed only the upper teeth show. (Britton 2009).
- Egg tooth (carnucle) helps break shell at hatching time.
- Forelimbs - five digits with no webbing.
- Hindlimbs - 4 visible toes and an undeveloped 5th toe.
- Three hind toes are clawed with webbing between them.
- Great agility in crocodiles is aided by flexible hip joints and ankle bones. (Trutnau & Sommerlad 2006)
- Ankle bones are arranged so that astragalus moves with the lower leg and the calcaneum moves with the foot; the calcaneum has a socket for the pegged astragalus which allows great twisting and turning movements.
- This ankle anatomy is called crurotarsal (a "crocodile normal ankle"); it is a diagnostic characteristic of all crocodilians. (Carroll 1988)
- Dwarfism has been noted in several populations of Nile Crocodiles. (Trutnau & Sommerlad 2006)
- Typically these smaller individual have been forced by hot, dry weather to spend time inactive in burrows; lack of adequate food may explain the small size.
BEHAVIOR & ECOLOGY
(BBC Wildlife Magazine 2009) (Britton 2001,2003) (Campbell 1973) (Cott 1961, 1975) (Crocodile Specialist Group 1996) (Downs 2008) (Frey & Salisbury 2000)
(Garrick & Lang 1977) (Huchzermeyer 2003) (Kofron 1990)
(Loveridge 1984) (Modha 1967, 1868)
(Pooley & Gans 1976) (Romero 1983) (Seebacher et al 2003) (Spotilla et al 1977) (Trutnau & Sommerlad 2006) (Webb & Manolis 1989)
- Crocodiles are primarily nocturnal. (Huchzermeyer 2003)
- Crocodiles use behavior (and physiological adaptations) to regulate body temperature.
(Seebacher et al 2003)
- Spend night in water; days mostly basking in sun and staying cool in shade or water when necessary.
- Basking behavior studied in Nile Crocodiles in South Africa in winter. (Downs et al 2008)
- Leave water to bask around 10 am (if day is overcast, foggy, or misting , remain in water longer)
- Return to water in afternoon, before air temperature begins cooling.
- In hottest part of day may use mouth gaping to promote body heat loss. (Cott 1975)
- Temperature sensors implanted showed continuous variations in body temperature rather than reaching a preferred body temperature target.
- Researchers counting crocodiles should work after the animals come ashore in the morning to bask in order to get more accurate counts.
- Can submerge for up to two hours when not moving, a feat possible because they are adapted to levels of lactic acid in their blood that would kill other vertebrates. (Britton 2003)
- Nile Crocodiles often rest with an open mouth; gaping has been explained as a behavior to reduce body or head temperature (Cott 1961, Loveridge 1984) but other studies hypothesize it is used most often as a threat (Kofron 1990).
- Gaping occurs at night and in winter, when the animals are not usually overheating.
- Kofron (1990) observed 45 threat displays using gaping in a sample of 54 observations of basking Nile Crocodiles in Kenya's Lake Turkana.
- Males defend a territory along a shoreline; about 60 m (197 ft) in length and possibly extending 50 m (164 ft) into the water at Lake Rudolph in Kenya. (Modha 1967)
- Nile crocodiles are a gregarious species; tolerate nearness of other Nile Crocodiles. (Huchzermeyer 2003)
- In wild, colonial nesting habits bring together several females into one suitable area. (Cott 1975)
- Many former colonial nesting sites abandoned due to human interference.
- Females stay in nesting area to first guard nest, then to protect young.
- Males in wild may guard hatchlings but this is not typical behavior. (Britton 2003)
- Males are seasonally monogamous. (Pooley and Gans 1976)
- Largest crocodiles are usually dominant.
- Males establish a dominance hierarchy
as they fight to establish a mating territory. (Pooley & Gans 1976)
- Females establish hierarchies among themselves. (Garrick & Lang 1976)
- Dominance and territorial behaviors are not evident when the crocodiles congregate at basking sites in Gonarezhou National Park in Zimbabwe (Kofron 1990); other researchers observed dominant breeding bulls guarding their territories and basking separately in Kenya's Lake Turkana. (Modha 1967, Cott 1961).
- Nile Crocodile males systematically patrol a territory (also true for Crocodylus acutus along coastal habitats of the Caribbean Sea).
(Garrick & Lang 1976)
- Male alligators (Alligator mississippiensis) in the southeastern U.S., by contrast, do not patrol their territories.
- Territorial male may chases out intruding males trying to mount females in their territory, but do not attempt to "herd" or keep females. (Modha 1967)
- Intruding males that are subdominant usually swim away and come out on shore, followed by the snapping territorial male.
- The pursuing male half emerges from the water, roars, snaps at the fleeing male, and slides back into the water.
- Actual fight also occur, sometimes beginning with a face off and vigorous hissing. Fights may last up to 45 minutes until one male swims away, defeated.
- Males receive many injuries in fights while establishing territories and access to females. (Cott 1961)
- Adult females guarding hatchlings on the water's edge become very aggressive, attacking humans, boats. (Cott 1975)
Visual Signs and Displays
- Snout lifting: submission or appeasement of a subordinate male: raised head and exposed throat; also made by females to all males. (Pooley & Gans 1976)
(Garrick & Lang 1977)
- By comparison, female American alligators pair bond without any submissive behavior by either sex. (Garrick & Lang 1977)
- Threat vocalizations also employ visual displays of mouth gaping, body inflating, short lunges, and attempts to bite. (Britton 2000)
- Mouth gaping has been explained as a behavior for reducing body temperature, but field studies disproved this idea and suggested instead that it did reduce the temperature of the animal's head. (Spotilla et al 1977)
- Males first challenge intruders in their territory by lifting the head and arching the tail; chases, lunges, mock or real fighting may follow. (Garrick & Lang 1977)
- When courting, male Nile Crocodiles have two dramatic displays:
- Courtship splash display - jaws smacked on water's surface
- Fountain display - male's snout held below water, air expelled through nostrils, jet of water rises.
- Visual signaling in crocodiles appears to be adapted to open water habitats in contrast to alligators' vocal signals that may be more advantageous in marshy, more closed habitats. (Garrick & Lang 1977).
- A gaping mouth may be a social signal to others as well as having a cooling function. Kofron 1990)
- Crocodiles are highly vocal, even before hatching.
- Juvenile vocalizations serve to help young communicate with siblings and adults (Britton 2001)
- Hatchlings calls: up to 30 minutes before hatching, these may help coordinate hatchings or serve to call for adult assistance in digging open the nest, and in helping break open the eggs.
- Hatchlings appear to actively seek adult's attention after hatching, perhaps to be picked up and transported to water
- Hatchlings in adult's mouth continue to call, possibly with less intensity, which may ensure they aren't swallowed. (Pooley & Gans 1976).
- How does the hatchling get air while making in-the-egg vocalizations? Some researchers speculate by using the egg tooth to tear the inner membrane of the egg, allowing air to diffuse through the porous shell.
- Threat calls: Larger juveniles will make a hiss much like the adult call that confronts an intruder.
- Threat hisses usually accompanied by visual signals of gaping, inflating the body, short lunges, and attempts to bite.
- Annoyance calls: made by loud, high intensity vocalizations perhaps in attempt to intimidate an attacker.
- Distress calls: acoustic signal is similar to hatchling calls, but much louder and longer; serve to attract adult (male and female) and warn nearby juveniles. Adults that respond to a distress call may produce a responding call and rush to drive away a predator. (Romero 1983) (Trutnau & Sommerlad 2006)
- Territorial males roar to attract females.
- Young are warned of danger by their mother vibrating muscles around her rib cage; young respond by diving into water. (Trutnau & Sommerlad 2006)
- Good sense of smell; can detect prey by smell. (Pooley & Gans 1976).
- Tails provide main forward thrust for swimming; propulsion achieved by a undulating wave from head to tip of tail.
- A torpedo-shaped body and strong tail provide forward movement at "lightning speed" and to thrust out of the water in pursuit of prey. (Trutnau & Sommerlad 2006)
- Swimming crocodiles tuck under the front limbs and let the rear limbs drag behind.
- Usually crocodiles avoid travel on land; they become exhausted easily.
- They don't drag body across ground, crawling like most lizards; instead hold body off ground with legs and tail.
- "High walking" with legs erect under the body and tail dragging achieves a speed of about 3 km/h (1.9 mi/h)
- Faster movement on land achieved by a sprawling gait with front and rear limbs coming together on one side as the body curves in that direction.
- Some crocodile species' young known to gallop with front limbs moving out and forward as hind limbs thrust the body forward up to speeds of 18 km/hr (11mi/hr) for short distances; resembles bounding run of squirrels.
(Webb and Manolis 1989) (Cott 1975)
- Diving is usually accomplished backwards; forward diving is rarely observed. (Frey & Salisbury 2000)
- Diving accomplished from a floating position either slowly by releasing air from nostrils and mouth and sinking down gently or by a powered backwards dive with help from hind limbs and tail.
- Bottom-walking frequently used in shallow water. (Frey & Salisbury 2000)
- While resting in water, only the eyes, ears, and nostrils are exposed. (Frey & Salisbury 2000)
- Hippos and Nile Crocodiles occupy the same water and land habitats; hippos are dominant. (Cott 1975)
(BBC Wildlife 2009)
- Hippos may push aside a crocodile basking on land or knock it into the water
- A female hippo with a calf or others in the herd will drive out all crocodiles from their pool of water; crocodiles are killed by hippos if they stray too close to hippo calves.
- Several birds associate with the Nile Crocodile:
- Spurwing Plover (Uganda) and Water Dikkop (Uganda, South Africa) make shrill alarm calls to which crocodiles respond; plovers have also been reported to enter crocodile mouths to clean off parasites.
- Common Sandpiper, Egyptian Plover (Egypt), and Blacksmith Plover (Zambia) pick ectoparasites from the crocodiles.
- African softshell turtles (Apalone) lay their eggs near Nile Crocodile nests; when female Nile Crocodiles carry their young safely to water after hatching, they also may carry young turtles that have synchronized their hatching to that of the young crocodiles. (Trutnau & Sommerlad 2006)
- Nile Crocodiles cause more human deaths than any other crocodile species; in Zimbabwe they cause more human fatalities than do all other animals combined. (Crocodile Specialist Group 1996)
DIET & FEEDING
(Brazaitis 1969) (Cott 1975) (Grenard1991) (Pooley & Gans 1976)
- Crocodiles are active hunters; prey changes as crocodiles grow.
- Nile Crocodiles will also scavenge carcasses.
- Youngest juveniles are insectivorous: prey water-bugs, dragonfly nymphs, beetles, spiders, mole crickets, frogs, snails.
- Older juveniles eat freshwater crabs, snails, toads, frogs, turtles, small birds and rodents.
- Adults can kill and eat almost anything that moves; may feed on smaller prey when available - frogs, crabs, and fish; fish are mainstay of diet.
- Prey caught on land often killed by drowning and dismemberment in water. (Pooley & Gans 1976).
- May ambush prey in overhanging tree branches by launching themselves out of water.
- Attack from water of an animal on the shore:
- Swims underwater to get close
- May surface to check position
- Makes a lunge that may carry the crocodile several times its own length, upward onto land.
- Attack birds in reeds:
- Crocodile uses tail to bend reeds down
and flip birds into water
- Cooperative predation
- Normally a large animal is consumed by the crocodile taking hold and rotating in the water until a piece is torn away.
- When feeding on an animal that is small enough that it turns in the water along with the crocodile, a second crocodile often helps to hold the prey and they both feed. (Pooley & Gans 1976)
- Two crocodiles have been observed walking over land, carrying a carcass of a nylala antelope between them. (Pooley & Gans 1976)
- Young crocodiles have often been seen forming a semi-circle in a flowing stream, facing oncoming water with fish, snapping up the fish and not fighting among themselves for this prey. (Pooley & Gans 1976)
- In Kruger National Park (South Africa) feed on impala, bushbuck, water buck, giraffes, buffalo, young hippos, hyenas, wild dogs, porcupines, lions. (Grenard 1991)
- Eat up to 20 percent of body mass in one meal.
- More humans killed by crocodiles than by all other predators combined (including snakes) in Kruger National Park.
- Crocodiles ingest rocks and other non-food objects (gastroliths) that may add close to one percent of body weight.
- Possibly for achieving buoyancy that would allow lying on the river bottom
- For adding weight that would help subdue struggling prey.
- For lowering the center of gravity to gain stability in the water.
- Youngest crocodiles do not yet have stomach stones.
- Crocodiles can go for long periods without eating; nesting females generally fast.
REPRODUCTION & DEVELOPMENT
(Bishop 2009) (Britton 2003) (Cott 1961, 1975) (Leslie 2001)(Ferguson 1985) (Hutton 1987) (Modha 1967)
(Rhodes & Lang 1996) (Trutnau & Sommerlad 2006)
Basic crocodile breeding system is polygynous (multiple females, one male); short-term pair bonding has been described for Nile Crocodiles and other species. (Britton 2003)
- Multiple paternity (one female, many males) has been reported for some species of Crocodylus. (Britton 2003)
- Courtship, mating, and nesting occurs over a period of several months for Nile Crocodile. (Britton 2003)
Courtship (Modha 1967)
- Males on Lake Rudolph in Kenya approached a female and exhibited a courtship splash display.
- Male arches tail upwards with tip touching the water.
- Male raises head and neck with lower jaw touching the water
- The neck appears to swell
- Bubbles of water emerge on both sides of the male's body.
- Male splashes water by lashing tail sideways and opening and closing the jaws.
- Water "boils" near the snout and tail.
- Other adults, presumably females, come from near and far towards the male.
- The male begins to follow one female
- Male and female lift heads and rub jaws, gape the jaws widely, but do not bite
- When male and female begin to swim parallel, male may draw the female near with his forelimb, mount the female, and copulate.
- Mating takes place in shallow water.
- Larger females mate earlier in the season than smaller females and they mate with the largest males. (Ferguson 1985)
- Reported interval between copulation and egg laying about one month (Ferguson 1985); other authorities report up to 5 months between copulation and egg laying for Nile Crocodile (Pooley & Gans 1976)
- Females store sperm until conditions are good for nesting.
- Stressed females may retain eggs (Ferguson 1985)
- Nile Crocodiles usually breed once a year in South Africa. (Cott 1961) (Leslie et al 2001)
- In the past, breeding Nile crocodiles gathered in large groups along rivers and lakes; communal nesting much more difficult in modern times due to conflict with humans for these habitats.
- Female occupied with nesting and hatchling care for a total of nearly 6 months. (Cott 1975)
- Nesting sites are communal; nests are close enough to almost touch each other.
- Nests built on river flats, lake-side beaches, sand-spits, dried up watercourses. (Cott 1975)
- Female builds a hole-type nest (not a mound) near permanent water in dry season.
- Nest - 40 by 60 cm (1.3 by 2 ft)
in diameter. (Ferguson 1985)
- Nest dug cm (8 -18 in) deep.
- Nest is oblique burrow leading into a wide egg-chamber.
- After eggs are deposited, hole is filled in so that it is flush with surrounding ground and not obvious. (Cott 1975)
- On an island in Lake Rudolph, Kenya, Nile Crocodile females covered eggs with a mixture of soil and grass. (Modha 1967)
- Suitable nesting sites have shade in regions where air temperature might become excessively hot, water close by, good sandy soil for digging.
- Female returns to same nest site year after year. (Pooley & Gans 1976)
- Female digs hole with hind limbs and snout, buries the eggs 30-45 cm ( in) deep.
- Eggs laid between January and December, depending on rains and local climate. (Ferguson 1985)
- Ideally, eggs are in nest during last of dry season, hatching occurs at onset of rains. (Cott 1975)
- Female lays 55- 60 eggs
- Eggs are about 7.5 cm long (3 in) and weigh about 110 g (4 oz)
- Eggs kept at ave. temperature of 31°C (88°F); these reptiles have a narrow range of optimal incubation temperatures — 28-34°C
- Eggs incubated below 27°C (81°F) never hatch (Hutton 1987)
- Sex of embryos determined by nest temperature; clutches may be biased for one sex or another; half of all clutches are made up of both male and female offspring. (Rhodes & Lang 1996)
- Only females hatch at temperatures between 28°C and 31°C. ( 82-88°F) (Hutton 1987)
- Temperatures of 34°C (93°F) result in 81% male hatchlings.
- Other species have slightly different temperature sensitivities.
- Female guards nest 10-14 weeks (mostly by lying over the nest) and defends it against egg snatching predators.
- Mammals (mongooses, water-mongooses, baboons, hyenas, warthogs, bush-pigs), vultures, marabou storks, and especially varanid lizards like the Nile Monitor cause high egg mortality. (Cott 1975) (Ferguson 1985)
- Female does not eat, or eats rarely while nesting.
- Eggs are incubated 84 to 90 days.
- Nest humidity must be kept high for embryos to develop normally, either from a soil or plant cover, or local climate, or behavior of mother (urination, splashing water on nest).
- Flooding of the nest causes embryo death.
Hatching (Ferguson 1985) (Modha 1967) (Pooley and Gans 1976) (Cott 1975)
- At time of hatching soil over eggs has become quite compacted.
- Baby crocodiles utter distress calls from within the egg below ground.
- Sound can be heard up to 20 m ( ft) away.
- Females are strongly attracted to the calls and will make every effort to reach the nest if she's some distance away.
- Female moves soil off the nest to allow young to escape.
- A nest deserted by the mother will have young calling for several days; if the eggs then are unearthed the young burst from the eggs. (Cott 1975)
- Female opens mouth, depresses her tongue to make space, gathers hatchlings in her mouth, and moves from nest to nearby water where hatchlings are released by swinging her head from side to side.
- Young do not head for water on their own as do baby turtles; left to themselves, they seek a place to hide, even turning away from the water in that search.
- Male crocodiles have also been observed aiding in hatchling births by breaking eggs to free the young (rolls egg back and forth between his tongue and palate), a behavior which shows great muscular control and tactile sensitivity.
- Hatchlings are about 28 cm (11 in) long
and weigh about 80 g (2.8 oz).
- Hatchlings from a single clutch stay together, often with members of other clutches, guarded by adult females and males
- Place chosen is near floating vegetation or a log fallen into water; young remain guarded for up to 3 months before dispersing.
- At end of clutch or crèche
phase the young gradually disperse to areas where no adults or subadults are found.
- Young grow in length about 30 cm/yr (1 ft/yr) (Cott 1975)
- Old crocodiles only grow around 2.5 cm/yr (1 in/yr)
- While the young previously sought adult protection, now they avoid it.
- Young crocodiles dig tunnels in river banks as shelter from predators and possibly to keep warm.
- Young use tunnels protection for their first five years.
- Youngest crocodiles eat insects.
- Sexual maturity when males are over nine feet in length, females eight feet.
- Sexual maturity at 12 to 19 yrs. in wild (Ferguson 1985)
- Sexual maturity in captivity can be much younger than in wild. (Ferguson 1985)
- Peak reproductive years for females: 15-30
- Nile Crocodiles are long lived: 50 - 80+ years (Bishop et al 2009)
- Hatchling and juvenile Nile Crocodiles are killed by crabs and large fish, other reptiles, herons and storks, and the mongoose and hyena. (Pooley & Gans 1976).
- Hippos are known to attack and kill adult crocodiles that threaten their calves. (Cott 1975)
DISEASES AND PATHOLOGY
(Bishop et al 2009)
(Dozma et al 2008) (Foggin 1987) (Huchzermeyer 2002 a&b, 2003)
(Kirchhoff et al 1997)
- Few infectious diseases are a problem; most mortality in farmed crocodiles from stress. (Huchzermeyer 2002b)
- Wild populations susceptible to diseases from introduced crocodiles from game farms.
- Captive populations susceptible to infectious agents from wild-caught breeding stock. (Huchzermeyer 2002a)
- Wild populations have several crocodile-specific diseases and parasites
(Huchzermeyer 2002a, 2003) (Kirchhoff et al 1997)
- Chlamydiosis - a common and serious disease caused by bacteria; probably comes from wild populations, transmitted by contaminated water.
- Crocodile pox affects hatchling and juvenile individual; caused by a parapoxvirus.
- Adenoviral hepatitis; virus also may infect lizards and snakes.
- Mycoplasmoisis; an arthritic condition coupled with pneumonia. (Kirchhoff et al 1997)
- Coccidiosis - caused by a spore-forming, parasitic protozoa
- Blood-sucking flies (tsetse flies)
- Captive populations
susceptible to diseases of wild crocodiles plus additional health problems: (Huchzermeyer 2002 a&b, 2003) (Foggin 1987):
- Embryos in eggs can drown if eggs are so wet as to interfere with air exchange through the shell.
- Fungal lesions from infected eggs and faecal contamination.
- Generalized acute bacterial infections caused by stressful situations.
- Newcastle disease (paramyxovirus); electron microscopy study showed captive Nile Crocodiles had been exposed to the Newcastle disease virus of birds; they possibly act as carriers but do not get sick themselves.
- Skin sores ("winter sores") in cold weather.
- Thermal stress affects immune systems, leads to many infections and diseases; metabolic disorders such as anorexia also caused by inadequate temperature exposures.
- Salmonellosis - Many infections of embryos, hatchlings and adults caused by Salmonella bacteria.
- Trichinellosis - caused by parasitic nematode roundworm.
- Nutritional diseases:
- Osteomalacia, or "rubber jaws", abnormal backbone curvature, "glassy teeth" from lack of bone in diet and also from infections due to stress.
- Osteoporosis, or bone thinning from lack of calcium being stored in bone tissue.
- Abnormal fat cell death and hardening of fat tissue from lack of vitamin E
- Gout from eating when ponds are too cold and possibly vitamin A deficiency.
(Bell 2001) (Bishop et al 2009) (Foggin 1987) (Huchzermeyer 2002 a, b)
- Zoos, Animal Parks
- Ancient Egyptians revered the Nile Crocodile.
- Sobek was a crocodile deity, depicted as a crocodile-headed man or as a crocodile.
- A bejeweled live crocodile, Petsuchos (Greek for "son of Sobek"), was kept in captivity in the ancient city of Crocodilopolis until it died and was replaced by another living crocodile. (Bell 2001)
- Mummified Petsuchos crocodiles were given the great honor of being buried pharaohs.
- All crocodiles in captivity are quite susceptible to stress which damages their immune systems
and metabolism (Huchzermeyer 2003)
- Hatchling and juvenile crocodiles need cover under which to hide; they fear being seen
- Capture, handling, sudden noises and movements in managed care all cause stress.
- In 2009 nearly 300 Nile Crocodiles are currently on exhibit in zoos and animal parks world-wide.
- Other Captive Breeding
- Nile crocodiles and many other crocodiles and alligators world-wide are raised in farms for their skins.
- Ideal conditions for intensive rearing have not yet been established.
- Problems in farmed animals due to poor sanitation, low water temperature, poor diet and general stress.
- Crocodiles must be able to reach a core temperature of at least 32° C once a day and not more than 34° C. (Huchzermeyer 2003)
- Pathogens such as Salmonella and Chlamydia are difficult to control.
POPULATION AND CONSERVATION STATUS
(Bishop et al 2009)(Britton 2009) (Craig 1992) (Crocodile Specialist Group 2009) (Dzoma et al 2008) (Hekkala et al 2009) (IUCN-SSG/CSG 2009) (Leslie & Spotila 1998) (Ross 1998)
(Simbotwe & Matlhare 1987)
- Few population survey data available for central and western Africa; populations in southern and eastern Africa better known (IUCN-SSG/CSG 2009)
- Two thirds of African countries have little information about populations status of their Nile Crocodiles. (Britton 2009)
- Nile Crocodiles have been one of the top commercially utilized species of crocodiles in the world. (Ross 1998)
- When hunting was allowed, the Ocavanga Delta between Botswana and Namibia lost 48,000 adult Nile Crocodiles between 1957-1968.
- 80,000 skins traded in 1993 on world market, most from Zimbabwe and South Africa from ranching and captive breeding.
- Because of commercial ranching efforts, illegal trade is not thought to be a significant conservation problem.
- Ranching efforts involve collecting wild eggs that normally experience high mortality; in theory the impact of wild populations is minimized; juveniles are raised in captivity for commercial use. (Hekkala et al 2009).
- Estimated 250,000 to 500,000 individuals in wild. (Britton 2009)
- ISIS captive population
Threats to survival
- 1960's: Hunted to near extinction in Egypt.
- 1973: CITES ban on trade of products from wild C. niloticus; enacted in 1975.
- 1983: Commercial ranching begun for Okavnago Delta population to supply international demand for hides.
- After quotas established, around 1050 adults and 14,000 eggs removed to game farms.
- Nest surveys recorded a 50% reduction in active nests in wild by 1987. (Simbotwe & Matlhare 1987)
- The Nile Crocodile population today (2009) in this basin is only partially recovered.
- 1982: Vulnerable
- 1986: Vulnerable
- 1988: Vulnerable
- 1990: Vulnerable
- IUCN Status:
Low Risk/Least Concern, possibly threatened in some parts of range
- CITES Status: I Except:
- Zimbabwe Appendix II (ranching)
- Botswana, Cameroon, Congo Republic, Kenya, Madagascar, Malawi, Mozambique, Republic of Tanzania, Sudan, and Zambia, Appendix II
- CITES quotas permit ongoing removal of egg clutches and breeding females. (Bishop et al 2009)
- U.S. Endangered Species Act: Threatened (2009)
- The population status of C. niloticus, and C. cataphractus, O. tetraspis is now a major focus of the CSG.
- In Africa, a Crocodile Conflict Working Group presently documents crocodile attacks on humans and livestock. (Britton 2009)
- Human/crocodile conflict
- Nile Crocodiles occupy scarce water resources needed by humans.
- Human fatalities often result.
- A significant decline in this species' effective population size has resulted from major exploitation in mid and late 20th century
(effective population size takes into account the animal's genetic diversity and actual breeding potential) (Bishop et al 2009)
- This long-lived species may be vulnerable to extinction despite their apparent partial recovery.
- A non-native composite shrub (Chromolaena odorata) invades shoreline habitats in Greater St. Lucia Wetland Park, South Africa and poses a threat of local extinction of Nile Crocodiles. (Leslie & Spotila 1998)
- The shrub shades nest sites, making them cooler than normal, which results in only females hatching from the eggs (crocodile embryos are sensitive to nest temperatures: too hot = all males, too cool = all females)
Important Web Resources:
- SSC/IUCN Crocodile Specialist Group (CSG)-- Worldwide network of biologists, wildlife managers, government officials, independent researchers, NGO representatives, farmers, traders, tanners, fashion leaders, and private companies actively involved in the conservation of the world's 23 living species of Alligators, Crocodiles, Caimans, and Gharials in the wild
- Crocodile Specialist Group Newsletter [PDF] -- Abundant information on conservation status, news and current events relating to crocodilians.
- Center for North American Herpetology -- Promotes preservation and conservation of North American Amphibians, crocodilians, reptiles and turtles.
- Crocodilian, Tuatara, and Turtle Species of the World: An Online Taxonomic and Geographic Reference -- Annotated checklist of species of crocodiles by 42 biologists.
- Bibliography of Crocodilian Biology -- Over 2500 references for biology of crocodiles and caimans.
- Crocodilians: Natural History and Conservation -- Website hosted by Florida Museum of Natural History and IUCN-SSC Crocodile Specialist Group. Information on taxonomy, distribution maps, biology and behavior, captive care, and wide selection of audio recordings of crocodile vocalizations.
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