Varanus niloticusNile Monitor, Water Leguaan

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Geographic Range

Varanus niloticus is found throughout most of sub-Saharan Africa. It is absent from the arid regions of the north and the southwest, but does reach Egypt along the Nile River. (Branch, 1998; Steel, 1996)

Habitat

Nile monitors inhabit a wide variety of habitats including woodland, dry savanna, scrub, evergreen thickets, swamps, and mangroves. Nile monitors are usually found near water, either temporary or permanent, but especially rivers, lakes, and pans. Their lifestyle is both terrestrial and aquatic and they are both superb climbers and swimmers which allows for great adaptability to different environments. Exposed, open areas are crucial habitat components as they require sufficient basking locations. This species is known to bask on open rooftops and streets. Young Nile monitors often lie on branches overhanging rivers or pools, and if disturbed will drop into the water. When alarmed and when water is not available, Nile monitors use their sharp claws and strong legs to climb trees up to a height of 5 to 6 meters. They are also known to flee down a hole or rocky crevice, or into a termite nest. Nile monitors live in burrows which they will excavate themselves or expand an existing one. A softer, sandy substrate is necessary to construct these burrows. This species has been documented at elevations from 0 to 1,600 m above sea level. (Branch, 1998; Steel, 1996)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • temporary pools
  • Range elevation
    0 to 1600 m
    0.00 to 5249.34 ft

Physical Description

Nile monitors may be Africa's largest lizards, though a former subspecies, now recognized as the species Varanus ornata, may be as big or bigger. Hatchlings are about 30 cm long and weigh about 26 g, but adults may grow to lengths of 2.4 m and have an impressively powerful physique. Sharp claws and strong muscular legs make it easy for them to climb trees. A deep, stoutly constructed skull, blunt crushing posterior teeth, and a bowed lower jaw make it easy for Nile monitors to eat hard-shelled prey such as mollusks. The tough skin is covered by bead-like scales. They have rounded nostrils which are located slightly nearer to the eye than to the snout. The toes are strong and moderately long. The tail of Nile monitors is compressed laterally with a dorsal keel and a very low double-toothed crest. (Branch, 1998; Cowles, 1928; Steel, 1996)

Adult Nile monitors are brownish or greenish-gray, with darker reticulation and yellowish spots or stripes on the back and limbs and yellowish-green spots on the head. Ventral surfaces are yellowish with blackish cross-bands. Juveniles are black, with the head bearing yellow cross-lines with black and yellow vertical bars on the lips. The neck features yellow lines and the back has a transverse series of yellow spots. The tail of juveniles has alternating black and yellow bars. This species exhibits no sexual dimorphism. (Branch, 1998; Steel, 1996)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    2.4 (high) m
    7.87 (high) ft

Development

Nile monitor eggs may require 10 months of incubation, though eggs in captivity have hatched within as few as 129 days. Within the eggs the embryos are coiled with the snout being forced against the shell wall, ready to rupture the integument and emerge. Hatching takes from 15 minutes to 7 hours. Juveniles lack the crushing and bowed lower jaws of adults, but that most likely reflects the shift from an insectivorous to a molluscivorous diet. Maturity is reached in three to four years. Most wild populations exhibit steady, indeterminate growth throughout their lives, but some heavily exploited populations may undergo significant growth up until 30 months of age and then cease. (Branch, 1998; Buffrenil and Hemery, 2002; Cowles, 1928; Steel, 1996)

Reproduction

Nile monitors are a polygynandrous species and will mate promiscuously. Males may reportedly fight each other in violent "wrestling matches," presumably due to competition for mating opportunities. (Branch, 1998; Steel, 1996)

Varanus niloticus breeds once annually, with the season starting in June and ending in October range-wide. Mating and egg laying usually follows the rainy season, which varies regionally. This season correlates with the development of the gonads. The testicles of males are enlarged from June until September while the females are being sought. Regression of the gonads subsequently occurs until January and then the cycle recommences.

After spring rains (August through September), the female excavates a hole in the ground or in an active termite nest and lays 20 to 60 eggs. This may take 2-3 days to complete. If she lays her eggs in a termite nest, the termites will repair the hole in their nest, and the monitor eggs develop inside. Eggs may take up to 1 year to hatch. The small young weigh an average 26 g upon hatching. After hatching, the young may need to wait for rain to soften the hard nest, or reportedly the mother monitor may return at the right time and open the nest to free the hatchlings. Once they have emerged, however, the baby Nile monitors are on their own. (Branch, 1998; Steel, 1996)

  • Breeding interval
    Varanus niloticus breeds once a year.
  • Breeding season
    Varanus niloticus breeds from June until October.
  • Range number of offspring
    20 to 60
  • Average number of offspring
    35
  • Range age at sexual or reproductive maturity (female)
    3 to 4 years
  • Range age at sexual or reproductive maturity (male)
    3 to 4 years

Parental investment for Nile monitors does not extend far past initial egg fertilization and laying. The female will excavate a secure place to lay her clutch, either in the ground or in an active termite nest. The mother monitor may return to the clutch sometime after the young hatch and open the buried nest to free her hatchlings. Once the young have emerged, however, they are on their own. (Branch, 1998; Cowles, 1928; Steel, 1996)

  • Parental Investment
  • no parental involvement
  • female parental care
  • pre-fertilization
    • provisioning
    • protecting
      • female

Lifespan/Longevity

Estimates vary, but the expected lifespan in captivity is reported at between 10 and 20 years. (Bartlett and Bartlett, 1996; Buffrenil and Castanet, 2000; Steel, 1996)

  • Typical lifespan
    Status: captivity
    10 to 20 years

Behavior

Nile monitors are usually solitary, with only occasional intraspecies interaction outside the mating season. When inactive, Nile monitors bask or rest on waterside vegetation, trees, logs and rocks, often in a prominent position. In the colder parts of South Africa, it hibernates in big rock cracks or burrows. When walking and foraging, this monitor holds its body well off the ground, and uses its long, forked tongue to sense potential food sources. Nile monitors are usually wary and if approached, will run away or jump into water, often from a considerable height. They are powerful swimmers. If cornered and threatened, a Nile monitor will arch its back and stand at full stretch on its legs, hissing as the body inflates and flicking the tail sideways. The lashing tail can inflict a painful wound. As a last resort a cornered Nile monitor will bite and eject foul-smelling musk from the cloaca to deter attack or throw off a pursuer. (Bartlett and Bartlett, 1996; Sprawls, et al., 2002; Steel, 1996)

Home Range

Nile monitors are active foragers and appear to have a fairly large home range, but specific figures appear to be unavailable. (Sprawls, et al., 2002)

Communication and Perception

Nile monitors use both visual and olfactory cues when sensing the environment. The long forked tongue supplies the vomero-nasal organ in the roof of the mouth. Their eyesight appears to be good. They perceive their environments through visual, tactile, auditory and chemical stimuli.

If cornered, these lizards will inflate the throat and hiss loudly, raise themselves up high, stiff-legged and lash with their tails to threaten the attacker. (Bartlett and Bartlett, 1996; Sprawls, et al., 2002; Steel, 1996)

Food Habits

Nile monitors are stealthy hunters, concluding their predatory quests by pouncing with lightning speed on their prey. The teeth can administer a painful crushing bite, although this is likely to be lethal to only relatively small victims. While holding an adversary in its jaws, a Nile monitor can cause severe injuries with its claws. They often use a combination of jaws and powerful forelimbs to break the backs of their prey. They eat a variety of prey that includes frogs, toads (even poisonous ones of the genus Breviceps), rodents, fish, lizards, small turtles, birds and their eggs, beetles, orthopterans, crabs, caterpillars, spiders, millipedes, earthworms, and slugs. They have been known to rob crocodile nests for eggs. They may occasionally work in pairs to rob a crocodile nest, with one lizard distracting the guarding mother crocodile while the other races in to grab eggs. Even after hatching, newly emergent crocodiles are not safe. Hard-shelled mollusks are a conspicuous feature of Nile monitors' diets and a thickened skull, blunt crushing teeth, and bowed lower jaw make them well-equipped for breaking open thick shells. Land snails of the genus Achatina are Nile monitors' favorite prey. (Bartlett and Bartlett, 1996; Branch, 1998; Sprawls, et al., 2002; Steel, 1996)

  • Primary Diet
  • carnivore
    • eats terrestrial vertebrates
  • Animal Foods
  • birds
  • mammals
  • amphibians
  • reptiles
  • fish
  • eggs
  • carrion
  • insects
  • terrestrial non-insect arthropods
  • mollusks
  • terrestrial worms
  • aquatic crustaceans

Predation

Nile monitors have few predators, but pythons prey on these lizards most often. Young Nile monitors make easy prey for a large python but even larger Nile monitors are at risk. A 4.5 m long African rock python has been reported to seize a 1.4 m long Nile monitor, and within half and hour had swallowed it head first. Crocodiles also take monitors on occasion. A defensive Nile monitor will arch its back and stand at full stretch on its legs, hissing as the body inflates and flicking the tail sideways. As a last resort a cornered Nile monitor will bite savagely and eject foul-smelling matter from the cloaca to deter attack or throw off a pursuer. (Bartlett and Bartlett, 1996; Branch, 1998; Sprawls, et al., 2002; Steel, 1996)

  • Known Predators

Ecosystem Roles

Nile monitors are both predators (on invertebrates and small vertebrates) and scavengers. Their persistent predation on crocodile nests may serve as a control on crocodile numbers.

Termite mounds make a perfect place for Nile monitors to lay their eggs, except during the dry season the hard clay makes it very difficult to burrow in. When the rain comes the females dig holes in the sides of termite mounds and lays their eggs. The termites quickly repair the gaping hole which protects the eggs during development. (Cowles, 1928; Sprawls, et al., 2002; Steel, 1996)

Species Used as Host
  • Termites (Eutermes trinervius)

Economic Importance for Humans: Positive

Nile monitors are among the most exploited species of tetrapods in Africa where it is used for food and leather. In certain areas, Nile monitors are hunted to obtain fat from the reproductive organs. It is locally believed that when melted down, this fat is a cure for ear aches as well as a protection against lightning.

Nile monitor lizards consume crocodile eggs and may act as population control in some areas.

Nile monitors are occasionally kept as pets by reptile hobbyists, but these large and aggressive lizards are a poor choice as a pet except perhaps for very experienced keepers who can provide large enclosures. (Bartlett and Bartlett, 1996; Branch, 1998; Buffrenil and Castanet, 2000; Sprawls, et al., 2002; Steel, 1996)

Economic Importance for Humans: Negative

Nile monitors have been known to acclimate to human towns, farms, and homesteads, and occasionally attack livestock such as chickens. (Sprawls, et al., 2002; Steel, 1996)

Conservation Status

In 1975, some north African states replaced the Wild Animals Law created in 1963 with a new law that gives protection to crocodiles, pythons, and monitor lizards. Nile monitors are also listed under the Endangered Species Decree of 1985, which means that international trade of the species in prohibited. This species is listed under CITES, Appendix II. (Anadu, 1987; Bartlett and Bartlett, 1996; Branch, 1998)

Other Comments

Nile monitors are sometimes known as "water monitors", or "water leguaan" in South Africa. (Branch, 1998)

Contributors

Kyle Szczepaniuk (author), Michigan State University, James Harding (editor), Michigan State University, Rachelle Sterling (editor), Special Projects.

Glossary

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

acoustic

uses sound to communicate

carnivore

an animal that mainly eats meat

carrion

flesh of dead animals.

chemical

uses smells or other chemicals to communicate

diurnal
  1. active during the day, 2. lasting for one day.
drug

a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

freshwater

mainly lives in water that is not salty.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nomadic

generally wanders from place to place, usually within a well-defined range.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

scent marks

communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

suburban

living in residential areas on the outskirts of large cities or towns.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

visual

uses sight to communicate

References

Anadu, P. 1987. Progress in the Conservation of Nigeria's Wildlife. Biological Conservation, 41: 237-251.

Bartlett, R., P. Bartlett. 1996. Monitors, Tegus, and Related Lizards. Hauppauge, New York: Barron's Educational Series, Inc..

Branch, B. 1998. Field Guide to Snakes and Other Reptiles of Southern Africa. Sanibel, Florida: Ralph Curtis Books.

Buffrenil, V., J. Castanet. 2000. Age Estimation by Skeletochronology in the Nile Monitor (Varanus niloticus), a Highly Exploited Species. Journal of Herpetology, 34: 414-424.

Buffrenil, V., G. Hemery. 2002. Variation in longevity, growth, and morphology in exploited Nile monitors (Varanus niloticus) from Sahelian Africa. Journal of Herpetology, 36 (3): 419 - 426.

Cowles, R. 1928. The Life History of Varanus niloticus. Science: New Series, 67: 317-318.

Sprawls, S., K. Howell, R. Drewes, J. Ashe. 2002. A Field Guide to the Reptiles of East Africa. San Diego, California: Academic Press.

Steel, R. 1996. Living Dragons. UK: Ralph Curtis-Books.