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

* 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)

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

Dimorphic in size

• Males larger than females
  

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

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

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

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)