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Nile Crocodiles (Crocodylus niloticus & C. suchus) Fact Sheet: Physical Characteristics

Physical Characteristics

Attribute Males & Females
Weight 70-100 kg (154-220 lb) *
Total Length 2.0-3.3 m (6.6-11 ft) **
Skull Length 1/2-2/3 total length

* Adults weighing 1,000 kg (2,205 lb) have been reported
** Maximum recorded body length from snout to tip of tail is 5 m (16 ft)
(from Pooley & Gans 1976)

Body Mass

  • Weight data for animals above TL (Total Length) of 3.5 is not commonly collected.  A weighbridge is required and these animals require significantly more effort to handle.  (Grigg 2015)

General Appearance

Body shape

  • Large bodied
    • Size
      • Largest crocodile in Africa
      • 61-63 vertebrae
    • Tail
      • Long and powerful
    • Limbs
      • Forelimbs
        • 5 digits on each forefoot
          • Lack webbing
      • Hindlimbs
        • 4 visible toes on each hindfoot
          • 5th is underdeveloped and not visible externally
          • 3 hind toes are clawed with webbing between them
      • Hip and ankle joints
        • Highly flexible (Trutnau & Sommerlad 2006)
        • Ankle bones (from Carroll 1988)
          • 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")
            • A diagnostic characteristic of all crocodilians
  • Head
    • Narrow
    • Eyes, ears, and nostrils set on top of the head
      • Eyes are green
    • 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)
    • Mouth
      • Jaws
        • Upper and lower jaw of similar width
          • 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)
      • Teeth
        • 64-68
          • Enlarged 4th lower tooth fits into a socket in upper jaw and is visible when jaws are closed
        • Lost and replaced throughout lifetime
        • Egg tooth (carnucle) helps break shell at hatching time
      • Gular fold (a flap of tissue) helps can keep water out of throat and lungs when mouth is open

Adult appearance

  • Body scaled
  • Back
    • Dark olive-green, bronze to brown, or gray
    • Dark cross-bands dark
    • Rows of scutes run down the length of the back
    • Coloration often obscured by a layer of mud and dust (Britton 2003)
    • True albino Nile Crocodiles reported from a game farm in South Africa (Huchzermeyer 2003)
      • True albinos lack dark markings; white crocs with dark markings do not exhibit albinism
  • Belly
    • Yellowish

Juvenile appearance

  • Back dark olive brown
    • Black cross bands on tail and body
      • Bands more distinct than those of adults

Skin

  • Texture
    • Leathery scales
  • Structure
    • Geometrical arrangement of scales
      • Many with a bony core (osteoderm) made of calcium phosphate and calcium carbonate
      • Scales over the head the plates are fused to the skull
    • Keels present on some
      • Flank scales that are closest to the upper (dorsal) scales (Trutnau & Sommerlad 2006)
  • Osteoderms
    • Barely visible on belly (ventral) scales, as do alligators and caiman (Trutnau & Sommerlad 2006)
    • Small oval osteoderms on the sides and throat (Brazaitis 1989)
  • Sensory organs in the skin (from Britton 2009)
    • Dermal Pressure Receptors (DPRs)
      • Also known as integumentary sense organs
      • Located on the upper and lower jaws
      • Detect water pressure
    • Similar sensory pits found on rest of the crocodile body
      • Their function not well understood
      • Presence of sensory pits on the belly distinguishes crocodiles from alligators
        • Alligators and caimans lack these extra sensory pits; only have DPRs on the jaws

Sexual Dimorphism

Dimorphic in size

  • Males larger than females

Other Characteristics

Adaptations

  • Bird and mammal like features
    • Hindbrain
      • Structure resembles that of birds and mammals
        • Cerebellum may be "an evolutionary precursor" of strongly folded gray matter seen in the hindbrains of birds and mammals (Trutnau & Sommerlad 2006)
    • Heart (circulatory system) (from Koshiba-Takeuchi et al 2009; Summers 2005)
      • Four-chambered
        • Functions as a three chambered heart when under water
          • Enables crocodiles to save oxygen
        • Complete separation of the two ventricles (pumping chambers) of the heart
          • Enables application of different pressures depending whether blood goes to the lungs or the rest of the body
          • Independently evolved (convergent) 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)
    • Stomach (digestive system) (from Trutnau & Sommerlad 2006)
      • Stomach resembles that of birds, complete with gizzard
        • Gizzard not present in lizards, snakes, and turtles
    • Mitochondrial DNA rate of change similar to that of mammals (from Janke & Arnason 1997)
      • Faster than that of seen in birds
      • Previously, a rapid evolutionary rate was believed to apply only to "hot blooded" mammal, not to "cold blooded" reptiles (Kumazawa & Nishida 1995)
    • Complex vocalizations
      • Vocalizations by parent and prehatchling crocodiles similar to those of birds (Ferguson 1985)
  • Tolerant to salt water
    • Salt glands on the tongue secrete salt
      • Glands help maintain electrolyte balance in saline environments (Franklin & Grigg 1993; Leslie and Taplin 2001)
        • Crocodiles tolerate salt water better than alligators
          • Alligators have the salt glands too, though they are nearly non-functional (Britton 2009)
            • Alligators inhabit primarily fresh water but can sometimes be found in higher saline environments
      • Glands become larger or smaller with changes in salinity of water (from Leslie and Taplin 2001)
        • Given time to adjust, crocodiles can adapt and thrive despite changes in the salinity of their surroundings
    • Crocodiles are not as adapted to sea water as fully marine reptiles such as turtles, marine iguanas, and sea snakes
      • True marine tolerant animals can drink sea water; crocodiles must get their drinking water from their food when in a salty environment
  • Sense of hearing
    • Sense sounds in and out of the water
      • A tympanic membrane in the side of the crocodile skull conveys sound vibrations to the inner ear (Shute & Bellairs 1955)
      • Unique, movable ear-flaps cover and protect these membranes (Shute & Bellairs 1955)
    • Sound perception
      • Hear frequencies between 100 and 4,000 hertz (cycles per second) (Pooley & Gans 1976)
  • Vision
    • Broad visual field (from Britton 2003)
      • Good vision to the side and front, though not fully binocular (with a 25° overlap)
      • Overall vision with 270° range
    • Bony eyelids protect and cover the eyes (from Britton 2009 unless otherwise noted)
      • Eyes can be pulled in to streamline the head when in water
      • Bony eyelids also present in the extinct ornithiscian dinosaurs such as ankylosaurs, the pterosaurs, and modern alligators (Coombs 1972)
    • Nictitating membranes (third eyelid
      • Provides extra protection to the eyes
    • A Harderian glands (from Trutnau & Sommerlad 2006)
      • Secretions empty into the eye through ducts
        • Produce the "crocodile tears" often seen when the crocodiles eat a large piece of food
  • Respiratory system
    • Adapted to increase intake of oxygen (from Pooley and Gans 1976)
      • Push liver against the lungs to improve exhalation of breath
        • Shift live away from the lungs to improve intake of air

Ectotherms

  • Behavioral mechanisms regulate body temperature (Seebacher et al 1999)
    • Body temperature relatively stable
      • Temperature range more narrow than in other reptilians, based on results of a study of East and Central African crocodiles (Cott 1975)
        • Preferred temperature range for crocodiles and alligators is 25 -35 °C (77-95 °F). (Lance et al 2001)
      • Behavioral and biochemical processes regulate temperature
        • Bask in sun or retreat to water
        • Increase or decrease rates of enzyme activity
          • Able to alter enzyme activities according to seasonal changes in air and water temperatures, results of one recent study (Seebacher et al 2003)
    • Suggests an endothermic ancestor for crocodiles (Seymour et al 2004)
      • Not all researchers agree with this idea (Hillenius & Ruben 2004)

Dwarfism

  • Noted in several populations (Trutnau & Sommerlad 2006)
    • Likely due to lack of adequate food supply
      • Typically these smaller individual have been forced by hot, dry weather to spend time inactive in burrows

Nile Crocodile

Nile Crocodile

The Nile Crocodile has a broad visual field. It can view a 270o range.

Image credit: © Joachim Muller from Flickr. Some rights reserved.

Page Citations

Brazaitis (1989)
Britton (2003)
Britton (2009)
Carroll (1988)
Cott (1975)
Franklin & Grigg (1993)
Franklin & Axelsson (1994)
Grigg (2015)
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)

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