Nile Crocodiles (Crocodylus niloticus & C. suchus) Fact Sheet: Behavior & Ecology

Active at day and night

• Active at night, most often (Huchzermeyer 2003)
• Spend night in water
• Bask in the sun or cool off in the shade in daylight
• Crocodiles use behavior (and physiological adaptations) to regulate body temperature (Seebacher et al 2003)
• Winter activity pattern, in South Africa (Downs et al 2008; Kofron 1990)
• Leave water to bask around 10 am
• Remain in the water longer on overcast, foggy, or misting days
• Researchers typically conduct census counts in the morning when animals are onshore
• 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

Resting behavior

• Submerge in water for long periods
• May remain underwater for up to two hours when not moving
• Made possible because they are adapted to high levels of lactic acid in their blood (from Britton 2003)
• Such lactic acid levels would kill other vertebrates
• At the water surface, only the eyes, ears, and nostrils are exposed (Frey & Salisbury 2000)
• Mouth gaping
• Often rest with an open mouth
• Behavior may help to reduce body or head temperature (Cott 1961; Loveridge 1984)
• Some studies suggest 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

Shoreline territories

• Males defend bands of water and shoreline (from Modha 1967)
• Shoreline stretches of c. 60 m (197 ft) in length
• Possibly extending 50 m (164 ft) into the water,  study at Lake Rudolph in Kenya

Gregarious

• Tolerate nearness of other Nile Crocodiles (Huchzermeyer 2003)
• Use colonial nests in the wild (from Cott 1975)
• Several crocodile families may use a suitable nesting area
• Many former colonial nesting sites abandoned due to human interference
• Females stay in nesting area to first guard nest, then to protect young
• Males uncommonly guard hatchlings (Britton 2003)

Social hierarchy

• Largest crocodiles are usually dominant
• Dominance and territorial behaviors are not evident when the crocodiles congregate at basking sites in Gonarezhou National Park, Zimbabwe (Kofron 1990)
• Other researchers observed dominant breeding bulls guarding their territories and basking separately in Kenya's Lake Turkana (Modha 1967; Cott 1961)
• Sexes form separate dominance hierarchies
  
• Males establish a dominance hierarchy as they fight to establish a mating territory (Pooley & Gans 1976)
• Females establish hierarchies among themselves (Garrick & Lang 1976)

Territorial

• Males display some territorial behavior
• Systematically patrol a territory (from Garrick & Lang 1976)
• Behavior also noted for Crocodylus acutus which patrols along coastal habitats of the Caribbean Sea
• American alligators (Alligator mississippiensis) in the southeastern U.S. do not patrol territories
• Territorial male may chases out intruding males trying to mount females in their territory (from Modha 1967)
• Do not attempt to "herd" or keep females within a territory
• Intruding males that are sub-dominant 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
• Fights occur at times
• Begin with a face off and vigorous hissing
• May last up to 45 minutes; until one male swims away in defeat

Aggression

• Fights typically between males
• Males receive many injuries in fights while establishing territories and access to females (Cott 1961)
• Female aggression
• Mothers become very aggressive when guarding hatchlings at the water's edge (from Cott 1975)
• Known to attack humans and boats

Uncommon (from Dinets 2015)

• Object play
• Toss objects, including food items, repeatedly into the air
• May spin, jump, and splash around with the object
• Social play
• Young crocodiles (siblings) tussle/wrestle with one another

Complex systems of communication

Visual Signs and Displays

• Signals adapted to open water habitats
• In contrast to alligators' vocal signals that may be more advantageous in marshy, more closed habitats (Garrick & Lang 1977)
• Sign of submission or appeasement (from Pooley & Gans 1976; Garrick & Lang 1977)
• Snout lifting
• Behavior performed by subordinate males
• Also made by females to all males
• By comparison, female American Alligators pair bond without any submissive behavior by either sex (Garrick & Lang 1977)
• Raise the head to expose the throat
• Aggressive signals (from Britton 2000 unless otherwise noted)
• Mouth gape
• Function possibly for social signaling to others as well as having a cooling function (Kofron 1990)
• Inflate (puff up) the body
• Lunge
• Generally short movements of the body
• 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)
• Bite
• Threat vocalizations often accompany such displays
• Courtship displays (from Modha 1967 unless otherwise noted)
• Males display in dramatic fashion
• Splash display
• Jaws smacked on water's surface, producing a loud noise
• Fountain display
• Snout held below the water, air expelled through nostrils
• Produces a jet of water

Vocalization

• Crocodiles are highly vocal
• Young vocalize even before hatching (Campbell 1973)
• Call up to 30 minutes before hatching
• May help coordinate hatching 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.
• Juvenile vocalizations
• Serve to help young communicate with siblings and adults (Britton 2001)
• Threat calls
• Larger juveniles make a hiss much like the adult call that confronts an intruder
• Adult calls
• Threat hisses
• Often accompany visual signals (gaping, body inflation, short lunges, and attempts to bite)
• Annoyance calls
• Loud, high intensity vocalizations
• Perhaps in attempt to intimidate an attacker
• Distress calls
• Acoustic signal is similar to hatchling calls, though much louder and longer in duration
• Serve to attract other adults (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)
• Roar
• Loud sound given by territorial males roar to attract females
• Female specific calls (from Trutnau & Sommerlad 2006)
• Mother vibrates muscles around her rib cage to warn young of danger
• Young respond by diving into the water

Olfaction/Scent Marking

• Good sense of smell
• Detect prey by smell (Pooley & Gans 1976).

Aquatic movements

• Travel through the water most often
  
• Swimming
• Move quickly through the water
• Torpedo-shaped body reduces drag (Trutnau & Sommerlad 2006)
• Strong tail provides main forward thrust for swimming
• Capable of launching the body out of the water in pursuit of prey (Trutnau & Sommerlad 2006)
• Propulsion achieved by a undulating wave from head to tip of tail
• Front limbs are tucked under body; hind limbs drag behind
• Diving (from Frey & Salisbury 2000)
• Slowly withdraw the body backwards, most often
• From a floating position, release air from nostrils and mouth and sink down gently
• Also powered backwards dive with help from hind limbs and tail
• Forward diving is rarely observed
• Bottom walking (from Frey & Salisbury 2000)
• Frequently used in shallow water

Terrestrial movements

• Become exhausted easily on land
• High walk
• Body held off the ground with the legs and tail
• Do not drag body across ground, crawling like most lizards
• Legs erect under the body and tail dragging
• Achieve speeds of c. 3 km/h (1.9 mi/h)
• "Run" with a sprawling gait
• Faster movement
• Front and rear limbs coming together on one side as the body curves in that direction
• Gallop (from Webb and Manolis 1989; Cott 1975)
• Fastest form of terrestrial movement
• Reach speeds of 18 km/hr (11mi/hr) for short distances
• Front limbs moving out and forward as hind limbs thrust the body forward
• Resembles bounding run of squirrels
• Typical in the young of some crocodile species

Interactions with hippos (from Cott 1975; BBC Wildlife 2009; Kofron 1990)

• Hippos are dominant, most often
• 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

Animal associates (Cott 1975)

• 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) (from Trutnau & Sommerlad 2006)
• 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

Humans

• Nile Crocodiles kill many humans (from Crocodile Specialist Group 1996)
• Cause more human deaths than any other crocodile species
• In Zimbabwe they cause more human fatalities than do all other animals combined

Image credit: © Steve Garvie from Flickr. Some rights reserved.

BBC Wildlife Magazine (2009)
Britton (2001)
Britton (2003)
Campbell (1973)
Cott (1975)
Cott (1975)
Crocodile Specialist Group (1996)
Dinets (2015)
Downs (2008)
Frey & Salisbury (2000)
Garrick & Lang (1977)
Huchzermeyer (2003)
Kofron (1990)
Loveridge (1984)
Modha (1967)
Modha (1968)
Pooley & Gans (1976)
Romero (1983)
Seebacher et al (2003)
Spotilla et al (1977)
Trutnau & Sommerlad (2006)
Webb & Manolis (1989)