Varanus niloticus
(Nile monitor)

Pictures

Picture	Title	Caption	Copyright
Title Adult in habitat Caption Nile Monitor (Varanus niloticus), Lake Manyara Park, Tanzania. June 2009. Copyright ©D. Gordon E. Robertson - CC BY-SA 3.0	Adult in habitat	Nile Monitor (Varanus niloticus), Lake Manyara Park, Tanzania. June 2009.	©D. Gordon E. Robertson - CC BY-SA 3.0

Identity

Preferred Scientific Name

• Varanus niloticus (Linnaeus 1766)

Preferred Common Name

• Nile monitor

Summary of Invasiveness

V. niloticus, the Nile monitor, is a large, mainly aquatic, lizard. It has been introduced to Spain (Pleguezuelos, 2004), and in the United States in Massachusetts (Cardoza et al., 1993) and Florida (Dalrymple, 1994; Duquesnel, 1996; Campbell, 2003, 2005; Enge et al., 2004). All known introductions are via the pet trade invasion pathway (Kraus, 2009; Krysko et al., 2011a).  This species is established (i.e. has reproducing populations) only in Florida, where it was first illegally introduced in ca. 1990 (Krysko et al., 2011a, b).

The distribution of V. niloticus in Florida covers hundreds of square kilometers and is expanding, especially along the enormous network of man-made canal and levee banks in southern Florida. The main ecological impacts are currently on native animals that may be consumed as prey, including rabbits (Sylvilagus sp.), burrowing owls (Athene cunicularia), waterfowl, American alligators (Alligator mississippiensis), American crocodiles (Crocodylus acutus), frogs, snakes, terrestrial and freshwater turtles, and sea turtles, many of which are protected species. Additionally, non-native species may be consumed such as goldfish (Carassius auratus), Cuban treefrogs (Osteopilus septentrionalis), Cuban brown anoles (Anolis sagrei), and domestic animals such as poultry (Bennett, 1995; Cowles, 1930), cats (McGraw, 1992; Bennett, 1995), and dogs (Enge et al., 2004). V. niloticus may pose a threat to human health, especially when cornered or captured because it will vigorously defend itself by biting, clawing, or tail whipping (Cowles, 1930; Branch, 1998; Faust, 2001; Enge et al., 2004).

 In Florida, V. niloticus is considered a pest species and is currently listed as a ‘conditional species’ by the Florida Fish and Wildlife Conservation Commission. Although conditional species are not allowed to be acquired for personal possession, reptile dealers, researchers and public exhibitors may purchase an annual permit for its importation or possession. Residents who owned a V. niloticus before 1 July 2010 may keep the animal for the remainder of its life; however, pet owners must maintain a valid Reptile of Concern license. Outside of Florida, regulations on ownership and possession of V. niloticus are highly variable and ownership may possibly be prohibited altogether. Since 1975, this species has been listed as Appendix II of CITES, which allows monitoring of the pet trade and skin industry.

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Chordata
•             Subphylum: Vertebrata
•                 Class: Reptilia
•                     Order: Sauria
•                         Family: Varanidae
•                             Genus: Varanus
•                                 Species: Varanus niloticus

Description

V. niloticus is the largest lizard in its native range of Africa. It is dark green or brownish, occasionally almost black, with yellow spots or patterns along its body and tail. It attains a total length up to 243 cm (TL) (Faust 2001), but median size is 155 cm TL (ca. 5 kg) for adult males and 134 cm TL (ca. 3 kg) for adult females (de Buffrénil et al. 1994).

Distribution

V. niloticus is native to sub-Saharan Africa excluding desert regions; it occurs along desert fringes, in grasslands, rainforests, rivers, swamps, ponds, lakes and seashores (Luxmoore et al., 1988, Faust, 2001, Bayless, 2002; Spawls et al., 2002). It is currently established in at least three of these areas of Florida (Cape Coral, West Palm Beach, and Homestead) where it is expanding its range (Enge et al., 2004; Krysko et al., 2001b).

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

V. niloticus has been introduced to Spain in 1994 and 1998 and Massachusetts, but has not become established in either location (Cardoza et al., 1993; Pleguezuelos, 2004; Kruas, 2009). Established introduced populations occur only in Florida, where it was introduced in the 1990s and has been continually illegally introduced to at least seven locations. It is currently established in at least three of these areas (Cape Coral, West Palm Beach, and Homestead), where it is expanding its range (Enge et al., 2004; Krysko et al., 2001b).

Introductions

Habitat

V. niloticus inhabits human areas and cultivations, often foraging around rubbish dumps (Bennett, 1995; Faust, 2001) where it seeks refuge in crevices or burrows, including those made by other animals (Edroma and Ssali, 1983). This species is mainly aquatic. It may hide in dense vegetation in shallow water or use bare tree trunks, logs, rocks, or overhanging branches as basking sites (Edroma and Ssali, 1983; Lenz, 1995; Muhigwa, 1998; de Buffrénil and Hémery 2002; Spawls et al., 2002). When alarmed, it will quickly dive into the water and swim far away or remain submerged for more than 20 minutes (Cowles, 1930; Bennett, 1995; Branch, 1998; Spawls et al., 2002). In Florida, it is found in mangrove swamps, tidal creeks, edges of freshwater and saltwater marshes, and banks of rivers, canals and lakes (Enge et al., 2004).

Habitat List

Biology and Ecology

Reproductive Biology

V. niloticus is a prolific species (Western, 1974). In its native range, females oviposit in burrows or termite mounds between August and January (Branch and Erasmus, 1982; Bennett, 1995; de Buffrénil and Rimblot-Baly, 1999; Spawls et al., 2002). In Mali and Chad, females reach sexual maturity at 24 months of age or ca. 36 cm snout-vent length (SVL), and ca. 50% of mature females reproduce each year (de Buffrénil and Rimblot-Baly, 1999). Smaller females usually oviposit 12–13 eggs (3 x 5 cm), but larger (>71.5 cm SVL) females may oviposit 53–60 eggs (de Buffrénil and Rimblot-Baly, 1999; Spawls et al., 2002). Eggs may take 6–12 months to hatch in the wild (Cowles, 1930; Faust, 2001; Spawls et al., 2002). Neonates have been observed in January in southern Kenya, and in July in northern Kenya and northern Tanzania (Spawls et al., 2002).

Little reproduction data has been documented in its invasive range in Florida. Enge et al. (2004) found eight well-developed eggs, possibly an incomplete clutch, inside a female (52 cm SVL), collected in Florida.

Longevity

V. niloticus can live for more than 10 years in the wild and 15 years in captivity (de Buffrénil and Hémery, 2002).

Activity Patterns

V. niloticus is mainly aquatic during activity periods, at times hiding in dense vegetation in shallow water (de Buffrénil and Hémery, 2002).

Population Size and Structure

V. niloticus can attain high population densities (Western, 1974). Despite V. niloticus having the widest distribution of any monitor lizard species in Africa (Bayless, 2002; Spawls et al., 2002), little is known regarding population structure in its native range and no study has addressed the origins of invasive populations in Florida.

Nutrition

V. niloticus is an opportunistic generalist predator in its native range that hunts or scavenges for arthropods, crabs, crayfishes, mussels, gastropods, fishes, anurans, lizards, turtles, snakes, young crocodiles, eggs, birds and small mammals, including domestic cats (Cowles, 1930; Cott, 1960; Cissé, 1972; Losos and Greene, 1988; McGraw, 1992; Lenz, 1995; Branch, 1998; Bennett, 2002). Nile monitors also feed readily upon human food scraps and carrion (Edroma and Ssali, 1983). Juveniles are typically better climbers than adults and prey primarily upon invertebrates (Bennett 1995, Lenz 1995, Faust 2001, Bennett 2002). Skull and tooth morphology changes with age (Rieppel and Labhardt 1979), allowing older monitors to crush molluscs and crustaceans, which are heavily preyed upon where available (Bennett 1995, Branch 1998).

Associations

Little is known of possible symbionts or similar floral/faunal associations.

Environmental Requirements

In its native range, V. niloticus is found in a vast array of habitat types, excluding deserts. These include along desert fringes, in grasslands, rainforests, rivers, swamps, ponds, lakes, and seashores (Luxmoore et al., 1988, Faust, 2001, Bayless, 2002; Spawls et al., 2002). In its introduced range in Florida, it is found in mangrove swamps, tidal creeks, edges of freshwater and saltwater marshes, and banks of rivers, canals, and lakes (Enge et al., 2004).

Natural Food Sources

Climate

Latitude/Altitude Ranges

Water Tolerances

Notes on Natural Enemies

Notes on Natural Enemies

Predation on V. niloticus has not yet been documented in its invasive range of Florida. However, possible predators on small monitors include birds, snakes, American alligators (Alligator mississippiensis) and American crocodiles (Crocodylus acutus).

Means of Movement and Dispersal

Natural Dispersal

V. niloticus is found along the network of man-made canal and levee banks in southern Florida. Because these canal systems occupy hundreds of square kilometers this invasive species has the potential to disperse throughout much of the state. It has also dispersed naturally to the Upper Florida Keys by following man-made roadways and saltwater mangroves fringes.

Accidental Introduction

V. niloticus has likely been accidentally introduced to some parts of Florida, escaping from nearby enclosures of pet owners.

Intentional Introduction

V. niloticus has been intentionally introduced to some parts of Florida by pet dealers trying to establish populations to exploit at a later time for resale into the pet trade (Enge et al., 2004). Additionally, V. niloticus may be illegally released by reptile dealers when they are too damaged or sick to sell (Enge et al., 2004).

Pathway Causes

Pathway Vectors

Impact Summary

Economic Impact

The invasion of V. niloticus in Florida is relatively recent and economic damages have not yet been examined. In Florida, this species is considered a pest species and is currently listed as a ‘conditional species’ by the Florida Fish and Wildlife Conservation Commission. Although conditional species are not allowed to be acquired for personal possession, reptile dealers, researchers and public exhibitors may purchase an annual permit for its importation or possession. Residents who owned a V. niloticus before 1st July 2010 may keep the animal for the remainder of its life; however, pet owners must maintain a valid Reptile of Concern license. This law likely does not have much effect on pet dealers as hatchlings retailed for as little as $10 (Enge et al., 2004). Outside of Florida, regulations on ownership and possession of V. niloticus are highly variable and  it is possibly prohibited altogether. Since 1975, this species has been listed as Appendix II of CITES, which allows monitoring of the pet trade and skin industry.

Economic Value

Since 1975, V. niloticus has been listed as Appendix II of CITES, which allows monitoring of the extensive pet trade and tanned skin industry. Its meat and skins have not yet been utilized from its invasive range in Florida.

Environmental Impact

Impact on Habitats

V. niloticus could have negative impacts on man-made habitats such as canal and levee banks, where it makes burrows.

Impact on Biodiversity

The main ecological impacts are currently on native animals that may be consumed as prey, including rabbits (Sylvilagus sp.), burrowing owls (Athene cunicularia), waterfowl, American alligators (Alligator mississippiensis), American crocodiles (Crocodylus acutus), frogs, snakes, terrestrial and freshwater turtles and sea turtles, many of which are protected species. Additionally, non-native species may be consumed such as goldfish (Carassius auratus), Cuban treefrogs (Osteopilus septentrionalis), Cuban brown anoles (Anolis sagrei), and domestic animals such as poultry (Bennett, 1995; Cowles, 1930), cats (McGraw, 1992; Bennett, 1995), and dogs (Enge et al., 2004).

Threatened Species

Social Impact

Fear of large lizards is mostly irrational; however, V. niloticus can potentially harm humans who are attempting to harm, capture or hold it as it can defend itself by biting, clawing or tail whipping (Cowles, 1930; Branch, 1998; Faust, 2001). Bites can result in tendon, bone and tissue damage, and may lead to septicemia (Enge et al., 2004). Large V. niloticus can kill domestic cats, dogs, and birds, and thus acquire considerable negative views by residents.

V. niloticus has been part of the pet trade view of demanding exotic and ferocious live animals. Bennett (1995:210) stated that ‘very few of the people who buy brightly colored baby Nile monitors can be aware that, within a couple of years, their purchase will have turned into an enormous, ferocious carnivore, quite capable of breaking the family cat’s neck with a single snap and swallowing it whole.’

Risk and Impact Factors

• Proved invasive outside its native range
• Has a broad native range
• Abundant in its native range
• Highly adaptable to different environments
• Is a habitat generalist
• Pioneering in disturbed areas
• Tolerant of shade
• Capable of securing and ingesting a wide range of food
• Highly mobile locally
• Benefits from human association (i.e. it is a human commensal)
• Long lived
• Fast growing
• Has high reproductive potential

• Negatively impacts human health
• Negatively impacts livelihoods
• Threat to/ loss of endangered species
• Threat to/ loss of native species

• Interaction with other invasive species
• Predation

• Highly likely to be transported internationally deliberately
• Highly likely to be transported internationally illegally
• Difficult to identify/detect in the field
• Difficult/costly to control

Uses

Economic Value

Since 1975, V. niloticus has been listed as Appendix II of CITES, which allows monitoring of the extensive pet trade and tanned skin industry. Its meat and skins have not yet been utilized from its invasive range in Florida.

Social Benefit

V. niloticus has been part of the pet trade view of demanding exotic and ferocious live animals. However, Bennett (1995:210) stated that ‘very few of the people who buy brightly colored baby Nile monitors can be aware that, within a couple of years, their purchase will have turned into an enormous, ferocious carnivore, quite capable of breaking the family cat’s neck with a single snap and swallowing it whole.’

Uses List

General

• Botanical garden/zoo
• Pet/aquarium trade
• Sociocultural value

Human food and beverage

• Meat/fat/offal/blood/bone (whole, cut, fresh, frozen, canned, cured, processed or smoked)

Materials

• Skins/leather/fur
• Tanstuffs

Similarities to Other Species/Conditions

Until recently, two subspecies of V. niloticus were recognized: the Nile monitor, V. niloticus niloticus (Linnaeus, 1758), and the ornate Nile monitor, V. niloticus ornatus (Daudin, 1803). Based on morphology, Böhme and Ziegler (1997) elevated both to species status: V. niloticus and V. ornatus. V. Niloticus is diagnosed by having 6–11 (mean = 7) light-colored body bands and a bluish black tongue, and V. ornatus by having 3–6 (mean = 5) body bands and a pinkish tongue (Böhme and Ziegler 1997; Faust, 2001).

Prevention and Control

SPS Measures

There does not appear to be quarantine or certification measures associated with importation of V. niloticus. In Florida, ownership is regulated by its listing as a 'conditional species' by the Florida Fish and Wildlife Conservation Commission. Outside of Florida, regulations on ownership and possession of V. niloticus are highly variable and it is possibly prohibited altogether. Since 1975, this species has been listed as Appendix II of CITES, which allows monitoring of the pet trade and skin industry.

Early Warning Systems

There is no dedicated early detection and rapid response system for V. niloticus in Florida. However, the Everglades Cooperative Invasive Species Management Area (CISMA) has been involved in the early detection and rapid response of the West Palm Beach population of V. niloticus, in southeast Florida.

Public Awareness

There have been no public awareness programs dedicated to V. niloticus in Florida. However, several Pet Amnesty Days are scheduled around the state each year where the public is allowed to surrender unwanted pets with no questions asked. There is also a hotline for pet owners to surrender them at any time in Florida.

Eradication

There is no dedicated eradication program for V. niloticus in Florida. Most individual animals are removed opportunistically by humans.

Control and Sanitary Measures

There are no known control methods or sanitary measures for V. niloticus in Florida.

Monitoring and Surveillance (incl. Remote Sensing)

There is no dedicated monitoring or surveillance for V. niloticus in Florida.

Gaps in Knowledge/Research Needs

Although little is known about V. niloticus in its invasive range in Florida, studying these animals in the wild there should be secondary to dedicated eradication attempts. Nonetheless, in areas where eradication is unfeasible, studies should be conducted on gathering life history data to help manage this pest species, as well as determine the origin in its native distribution, via genetic testing, to create a thorough phylogeographic study.

References

Bayless MK, 2002. Monitor lizards: a pan-African check-list of their zoogeography (Sauria: Varanidae: Polydaedalus). Monitor lizards: a pan-African check-list of their zoogeography, 29:1643-1701.

Bennett D, 1995. A little book of monitor lizards. Glossop, UK: Viper Press, 207 pp.

Bennett D, 2002. Diet of juvenile Varanus niloticus (Sauria: Varanidae) on the Black Volta River in Ghana. Diet of juvenile Varanus niloticus on the Black Volta River in Ghana, 36:116-117.

Branch B, 1998. Field guide to snakes and other reptiles of southern Africa. Sanibel Island, Florida, USA: Ralph Curtis Books, 399 pp.

Branch WR; Erasmus H, 1982. Notes on reproduction in the South African water monitor Varanus niloticus niloticus, 28:4-9.

Buffrénil Vde; Chabanet C; Castanet J, 1994. Preliminary data on the size, growth and longevity of the Nile monitor (Varanus niloticus) in Lake Chad. (Données préliminaires sur la taille, la croissance et la longévité du varan du Nil (Varanus niloticus) dans la région du lac Tchad.) Preliminary data on the size, growth and longevity of the Nile monitor in Lake Chad, 72:262-273.

Buffrénil Vde; Hémery G, 2002. Variation in longevity, growth, and morphology in exploited Nile monitors (Varanus niloticus) from Sahelian Africa. Variation in longevity, growth, and morphology in exploited Nile monitors from Sahelian Africa, 36:419-426.

Buffrénil Vde; Rimblot-Baly F, 1999. Female reproductive output in exploited Nile monitor lizard (Varanus niloticus L.) populations in Sahelian Africa. Female reproductive output in exploited Nile monitor lizard populations in Sahelian Africa, 77:1530-1539.

Böhme W; Ziegler T, 1997. A taxonomic review of the Varanus (Polydaedalus) niloticus (Linnaeus, 1766) species complex. A taxonomic review of the Varanus niloticus species complex, 7:155-162.

Campbell TS, 2003. Species profile: Nile monitors (Varanus niloticus) in Florida. Nile monitors in Florida, 10:119-120.

Campbell TS, 2005. Eradication of introduced carnivorous lizards from Southwest Florida. 1120 Connecticut Avenue NW, Suite 900, Washington, DC 20036, USA: National Fish and Wildlife Foundation, 30 pp.

Cardoza JE; Jones GS; French TW; Halliwell DB, 1993. Exotic and translocated vertebrates of Massachusetts, 6. Massachusetts Division of Fisheries and Wildlife, Westborough, Massachusetts, USA: Fauna of Massachusetts Series.

Cissé M, 1972. The feeding of Varanidae in Senegal (L'alimentation des Varanidés au Sénégal), 34:503-515.

Cott HB, 1960. Scientific results of an enquiry into the ecology and economic status of the Nile crocodile in Uganda and northern Rhodesia. Ecology and economic status of the Nile crocodile in Uganda and northern Rhodesia, 29:212-356.

Cowles RB, 1930. The life history of Varanus niloticus (Linnaeus) as observed in Natal South Africa. The life history of Varanus niloticus as observed in Natal South Africa, 22:1-31.

Dalrymple GH, 1994. Non-indigenous amphibians and reptiles in Florida. In: An assessment of invasive non-indigenous species in Florida's public lands [ed. by Schmitz, D. C. \Brown, T. C.]. Tallahassee, Florida, USA: Florida Department of Environmental Protection, 67-78.

Duquesnel J, 1996. Scaly visitors, 40. Tallahassee, Florida, USA: Florida Department of Environmental Protection Resource Management Notes.

Edroma EL; Ssali W, 1983. Observations on the Nile monitor lizard (Varanus niloticus, L.) in Queen Elizabeth National Park, Uganda. Observations on the Nile monitor lizard in Queen Elizabeth National Park, Uganda, 21:197-201.

Enge KM; Krysko KL; Hankins KR; Campbell TS; King FW, 2004. Status of the Nile monitor (Varanus niloticus) in southwestern Florida. Status of the Nile monitor in southwestern Florida, 3:571-582.

Faust RJ, 2001. Nile monitors. Hauppauge, New York, USA: Barron's Educational Series, 95 pp.

Kraus F, 2009. Invading nature: Springer series in invasion ecology 4. Springer, 563 pp.

Krysko KL; Burgess JP; Rochford MR; Gillette CR; Cueva D; Enge KM; Somma LA; Stabile JL; Smith DC; Wasilewski JA; Kieckhefer GNIII; Granatosky MC; Nielsen SV, 2011. Verified non-indigenous amphibians and reptiles in Florida from 1863 through 2010: Outlining the invasion process and identifying invasion pathways and stages. Verified non-indigenous amphibians and reptiles in Florida from 1863 through 2010, 3028:1-64.

Krysko KL; Enge KM; Moler PE, 2011. Atlas of Amphibians and Reptiles in Florida. Tallahassee, Florida, USA: Florida Fish and Wildlife Conservation Commission, 524 pp.

Lenz S, 1995. The biology and ecology of Nilwarans, Varanus niloticus (Linnaeus 1766), in The Gambia, West Africa. (Zur biologie und ökologie des Nilwarans, Varanus niloticus (Linnaeus 1766) in Gambia, Westafrika.) The biology and ecology of Nilwarans, Varanus niloticus, in The Gambia, West Africa, 5:1-256.

Losos JB; Greene HW, 1988. Ecological and evolutionary implications of diet in monitor lizards, 35:379-407.

Luxmoore R; Groombridge B; Broads S, 1988. Significant Trade in Wildlife: A Review of Selected Species in CITES Appendix II. Volume 2: Reptiles and Invertebrates. Significant Trade in Wildlife [ed. by Luxmoore, R. \Groombridge, B. \Broads, S.]. Cambridge, UK: IUCN Conservation Monitoring Centre, 306 pp.

McGraw M, 1992. Monitor lizard eats cat, 16:30-31.

Muhigwa JB, 1998. Diel activity and biotope choices of the Nile monitor lizard in western Kenya, 36:271-275.

Pleguezuelos JM, 2004. Introduced species of amphibians and reptiles. In: Atlas and Red Book of amphibians and reptiles of Spain. (Las especies introducidas de anfibios y reptiles. In: Atlas y libro rojo de los anfibios y reptiles de España.) Introduced species of amphibians and reptiles [ed. by Pleguezuelos, J. M. \Márquez, R. \Lizana, M.]. Madrid, Spain: General Directorate of Nature Conservation, Association of Spanish Herpetology, 502-532.

Spawls S; Howell K; Drewes R; Ashe J, 2002. A field guide to the reptiles of East Africa. San Diego, USA: Academic Press, 543 pp.

Western D, 1974. The distribution, density and biomass density of lizards in a semi-arid environment of northern Kenya, 12:49-62.

Wilcove DS; Rothstein D; Dubow J; Phillips A; Losos E, 1998. Assessing the relative importance of habitat destruction, alien species, pollution, over-exploitation and disease, 48:607-616.

Distribution Maps