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Varanus bengalensisBengal Monitor

By Kathleen Farmer and Eric Wright

Geographic Range

Bengal monitors or common Indian monitors (Varanus bengalensis) occur across much of southern Asia. Compared to other varanid lizards, Bengal monitors have a much larger geographic range, where they are considered less restricted both geographically and environmentally. This species is widely distributed from Afghanistan to Java, including southeastern Iraq, Iran, and Afghanistan, Pakistan and India, southern Nepal, Bhutan, and China, North and South Vietnam, Laos, and islands in the Strait of Malacca and the Greater Sunda Islands. In Iran they are generally restricted to southeastern regions in close proximity to rivers that drain into marshes or shallow lakes, rather than seas, they are particularly common along the River Gartatab. In Afghanistan, Bengal monitors are limited to the Kabul River Valley in the extreme southeastern part of the country. (Auffenberg, 1994; Pianka, 1995)

Habitat

Unlike other varanid lizards, Bengal monitors have the ability to cope with a broad range of environments, from deserts to rainforests to habitats having seasonally snowy winters. However, generally they are found in areas with continuously warm climates, with mean annual air temperatures of approximately 24 C. Most of southern Asia experiences seasonal monsoons and wind patterns influenced by neighboring seas and mountains. Thus, precipitation across much of the range for Bengal monitors is highly variable. Some habitat areas are relatively arid, with mean precipitation less than 200 mm per year. Other habitats are considerably more humid, with annual rainfall reaching 2,200 mm per year. The most common tropical forest habitats for Bengal monitors are deciduous, semi-deciduous, evergreen tropical forests, and thornbrush. (Auffenberg, 1994)

  • Range elevation
    0 to 1500 m
    0.00 to 4921.26 ft

Physical Description

Adult Bengal monitors are generally grey or greenish-grey in color, with a ventral pattern of grey to black crossbars from the chin to the tail. These markings are generally darkest in the western parts and lightest in the eastern parts of the geographic range. These ventral markings typically become lighter, and the ground color darker, with age. Thus, adults display a less pronounced, less contrasting pattern than younger Bengal monitors.

In the wild, the heaviest recorded male Bengal monitor weighed 7.18 kg, though captive individuals have been reported to reach 10.2 kg. In the wild, males generally weigh 42% more than females. Males of the same snout to vent length (SVL) as females are typically 9.2% heavier. Young Bengal monitors, on average, weigh 0.078 kg. (Auffenberg, 1994)

  • Sexual Dimorphism
  • male larger
  • Range mass
    7.18 (high) kg
    15.81 (high) lb
  • Range length
    61 to 175 cm
    24.02 to 68.90 in

Development

Development in Bengal monitors begins with a variable length incubation period. Laboratory investigations have shown this incubation period to range from 70 to 327 days. The length of incubation depends largely on mean egg temperature. However, even within a single brood, there can be variations of up to 105 days from first to last hatching. High incubation temperatures typically lead to shorter development times, but also may skew sex ratios or cause developmental defects.

Bengal monitors are relatively long-lived varanids. As such, this species does not reach sexual maturity until 2.5 to 3 years. Most produce one clutch of offspring each year for the remainder of their lives. Environmental influences play an important role in body size and overall length in Bengal monitors. In general, longer individuals are found in areas with greater soil moisture, such as marsh environments, whereas shorter individuals often occur in surrounding forests. In addition, those found on the small islands in both the South China Sea and the Gulf of Thailand have been found to become sexually mature at a much smaller size than those from the nearby mainland, reaching reproductive maturing with SVLs as low as 23.3 cm. (Auffenberg, 1994)

  • Development - Life Cycle
  • temperature sex determination

Reproduction

Chemical cues play an important role in the ability of males to recognize receptive females. These chemical cues are produced by the female, from glands located in skin of the abdomen. In captivity, females show the greatest chances of successful copulation by mating with only one or two individual males in successive years, though they still may be courted by several other males. (Auffenberg, 1994)

In females, the reproductive cycle is annual. Follicles mature only during one part of the year, shortly before ovulation. Follicles and ovaries reach their largest size during the months of July and August for those individuals in the western part of the species range, and from October to December for those in more southern areas such as India and Sri Lanka. Yolk deposition in an egg has no correlation with the ovulatory phase in females, but it does correlate with fat accumulation. (Auffenberg, 1994)

There are three major phases in the reproductive cycle of female V. bengalensis: previtellogenesis, recrudescence, and ovulation. During the previtellogenesis phase, the ovaries are small and inactive. This stage usually occurs during the fall months, usually September through March. Females forage less during this time for individuals in more southern regions, while those in more northern regions do not forage at all. A return to foraging in the spring initiates oogenesis. This phase is also characterized by regressed oviducts and nonsecretory epithelial and gland cells, which are used to attract mates. In the second phase of the reproductive cycle, called recrudescence (or true vitellogenesis), the ovarian follicles will fully mature with the completion of yolk deposition. This phase occurs in the premonsoon period, from April to June. In a short time, the ovaries increase in size and change from a pearly white to a deep yellow color. A mature preovulatory ova has a mean diameter of 17.8 mm. The oviducts will also increase in width and secretions will start to flow into the oviductal lumen. The last phase of the reproductive cycle, ovulation, is characterized by the movement of the egg from the upper part of the oviduct to the lower portion after fertilization has occurred. Once the egg reaches the lower portion of the oviduct, a shell will form around each egg. Ovulation begins in June, but reaches full force in July. The most successful copulation occurs slightly before or after ovulation has reached its peak. Egg laying will occur two weeks after copulation, usually during the months of July, August, and early September. By the last week of October, both sexes are largely inactive with size and sperm production heavily reduced in the male and new follicles for the next year appearing in the ovaries of the female. (Auffenberg, 1994)

Usually, mature females of V. bengalensis will produce only one clutch annually. However, in some areas where the environment experiences two monsoon seasons, some females may lay two clutches annually. If two clutches are laid, there are 23 to 30 days between the first egg laying and breeding for the second clutch. Data also suggests that those females from more mesic environments have a higher proportion of pregnancies than those from xeric areas. This may be due to the longer breeding periods in more mesic environments as well as higher food abundance, which has an effect on fat production. In captive species, day length also had an effect on courtship and breeding patterns. When day length was artificially lengthened, combat among males occurred as early as April and courtship initiation and breeding began earlier. The average number of eggs laid per year is 20, of which about 80% typically hatch. This results in about 16 young per female per year. Additionally, because V. bengalensis has a large clutch size relative to most tropical lizards, neonates are subject to relatively high predation rates. Because of predation, roughly half of the offspring do not live past the age of two. (Auffenberg, 1994)

Both males and females become sexually mature at approximately 2.5 to 3 years of age, both in the wild and in captivity. In both sexes, the onset of sexual maturity is linked to a body mass greater than 0.4 kg. In female Bengal monitors, reproductive efforts occur throughout most of their life span. After reaching sexual maturity, females remain reproductively active for the remainder of their lives, which may extend to 27 years. (Auffenberg, 1994)

  • Breeding interval
    Female Bengal monitors that live in environments that experience two monsoon seasons may lay two clutches annually while those in one monsoon areas lay just one clutch annually.
  • Breeding season
    Individuals mate in June, July, and early August and the shelled eggs are laid any time from July through early September. The incubation period is from 4 to 8 months.
  • Average number of offspring
    20
  • Range gestation period
    4 to 8 months
  • Range age at sexual or reproductive maturity (female)
    2.5 to 3 years
  • Range age at sexual or reproductive maturity (male)
    2.5 to 3 years

In varanid species, the bulk of parental investment occurs through the materials and energy supplied prior to hatching. This includes resources provided for egg, embryonic, and initial post-hatching development. For example, during embryonic development the yolk supplies the fetus with nutrients required for growth. Females typically create and spend a large amount of time in the nest. She devotes energy to ensuring eggs are protected from predators, such as other monitors, and have proper incubation conditions in order to increase the offsprings' chances of survival. After their lengthy incubation period, however, neonates have very little yolk remaining when they hatch from the egg. This means that new hatchlings must locate food resources quickly and independently. It is interesting to note that females in captivity will frequently retain the eggs longer, spending additional time searching for an appropriate nesting medium. This makes the egg shells thicker than usual. Thicker shells require greater movement and strength on the part of the offspring to break out of the egg. Thus, small yolk food reserves also affect hatchling success. There are no reports showing sufficient evidence that female varanids provide additional care for offspring after laying eggs and hiding them in the nest. In fact, both males and females often eat eggs of other monitors. There also is evidence that females sometimes eat their own eggs. Data suggest that mean egg size and hatchling size are reduced in xeric habitats. (Auffenberg, 1994)

  • Parental Investment
  • no parental involvement
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

Like many other large predators, V. bengalensis is relatively long-lived. This species is relatively unaffected by drought or daily variations in rainfall, so population sizes remains fairly stable from season to season. Mortality rates are highest for neonates, due to predation, with only about half surviving past the age of two and reaching sexual maturity. For captive individuals, the longest recorded life span was about 22 years. (Auffenberg, 1994)

Age estimates in reptiles are obtained by counting bone layers. Reptiles, including V. bengalensis have annual cyclic bone growth that can be estimated by staining methods. (Auffenberg, 1994)

  • Average lifespan
    Status: captivity
    22 years

Behavior

In the wild, Bengal monitors are almost completely solitary. Much of the daytime is spent in constant movement, searching for food. Bengal monitor are more likely to interact with one another during the peak breading season, when males compete for mates. (Auffenberg, 1994)

Communication and Perception

Like most varanids, Bengal monitors use primarily scent as their main method of communication and perception. They “taste” the environment around them by constantly flicking their highly sensitive tongues while moving their head from side to side. This is useful in tracking prey and mates and in signaling between monitors of the same species. It has been documented in the wild that V. bengalensis spends large amounts of time examining the droppings of other Bengal monitors that have passed through their territory. Even though they are solitary creatures, scent messages in feces are said to be important in communication. The scent perceived by one monitor from another can inform of hostile intentions or to stay away from the particular territory. (Auffenberg, 1994)

There is a diverse range of intraspecific communication exhibited by V. bengalensis through touching, biting, clawing and wrestling. Being solitary predators, roughly three quarters of encounters begin as purely investigatory and the remaining quarter are for the purpose of sex and courtship. Conflict between males, whether over food or mating, usually results in an initial investigation through acquiring each others scents and their intent. Conflict typically involves vocalization which is usually a hissing noise accompanied by the monitor inflating its upper body to appear larger. Tail-slapping and whipping is also common behavior between males and sometimes females to establish dominance. Encounters between males can lead to wrestling in which case both males stand on their hind legs and embrace each other while thrashing their heads and upper bodies. Occasionally biting and clawing can occur during wrestling but it is usually collateral damage rather than intentional. (Auffenberg, 1994)

Food Habits

The diet of Bengal monitors is almost strictly carnivorous. They consume almost anything that is smaller than themselves and that they can easily overpower. They are known to scavenge carcasses of previously felled animals. Their documented observed prey species list is considerable, containing roughly 200 species. Common prey include: annelids, insects, amphibians, smaller reptiles, birds, small mammals, and eggs. Cannibalism of eggs, hatchlings, and even adults has been noted, although predation on adults is rare. As with most varanids, they swallow prey whole but are also capable of ripping and tearing flesh from larger animals and carcasses. At smaller body sizes for Bengal monitors, various beetles species represent the largest portion of their diet, averaging 52.8%. The second largest component of their diet is made up of orthopteran insects at 9.5%. The remainder of their diet is made up of other insects, crabs, rodents, reptiles, spiders, birds and almost any other animal they can reasonably consume. (Auffenberg, 1994)

  • Animal Foods
  • birds
  • mammals
  • amphibians
  • reptiles
  • carrion
  • insects
  • terrestrial non-insect arthropods

Predation

Predation on V. bengalensis does occur despite the fact that they themselves large predators. Species that prey upon V. bengalensis include other Bengal monitors, pythons and other large snakes, eagles, mongooses, wild and domesticated dogs, feral cats, and even humans. Most predation occurs early in life as eggs, hatchlings, and juveniles, while only a small portion of predation involves fully grown adults. (Auffenberg, 1994)

Ecosystem Roles

Bengal monitors are primary predators of many smaller animals in the ecosystems they inhabits. Juveniles are preyed upon by larger predators, including other monitors. There are four tick species known to infect Bengal monitors, including: Aponommon gervaisi, A. varanensis, A. laeve, and Amblyomma helvolum. In addition, trematodes, cestode worms, nematodes, filarial worms, and sporozoan protozoans are known to infect these monitors.

Commensal/Parasitic Species

Economic Importance for Humans: Positive

Bengal monitor population declines for this species are largely due to the commercial exploitation of their skins for leather products. In addition, various parts of their bodies are used in some village medicines. Monitors in general, also are eaten by human populations in some parts of Asia, Africa, and Australia. (Auffenberg, 1994)

Economic Importance for Humans: Negative

There are no noted negative impacts of Varanus bengalensis on humans. Bengal monitors are not large enough to attack any livestock nor do they eat any human cultivated crops. They may eat any small mammals that they can easily catch, so may pose a threat to small domestic animals. (Auffenberg, 1994)

Conservation Status

According to the IUCN Red List of Threatened Species, V. bengalensis is a species of Least Concern. This is based on its wide geographic range. However, there are increasing pressures on the species. They are hunted for their meat, skins, and for use in medicine. Due to expanding human habitation and urbanization, the range threats to their population are likely to increase in the future. (Papenfuss, et al., 2010)

Contributors

Kathleen Farmer (author), Radford University, Eric Wright (author), Radford University, Christine Small (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

Glossary

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

fertilization

union of egg and spermatozoan

food

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

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

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.

insectivore

An animal that eats mainly insects or spiders.

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).

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.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

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.

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

scavenger

an animal that mainly eats dead animals

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

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

Auffenberg, W. 1983. Notes on feeding behaviour of Varanus bengalensis (Sauria: Varanidae). Journal of the Bombay Natural History Society, 80/2: 286-302.

Auffenberg, W. 1994. The Bengal Monitor. Gainesville, Fl: University Press of Florida.

Auffenberg, W. 1981. Combat behaviour in Varanus bengalenisis (Sauria: Varanidae). Journal of the Bombay Natural History Society, 78/1: 54-72.

Auffenberg, W. 1979. Intersexual Differences in Behavior of Captive Varanus bengalensis (Reptilia, Lacertilia, Varanidae). Journal of Herpetology, 13/3: 313-315. Accessed February 04, 2011 at http://www.jstor.org/stable/1563325.

Auffenberg, W. 1983. The food and feeding of juvenile Bengal monitor lizards (Varanus bengalensis). Journal of the Bombay Natural History Society, 80/1: 119-124.

Bohme, W. 2003. Checklist of the living monitor lizards of the world (family Varanidae). Leiden, 341/25: 4-43.

Duengkae, P., Y. Chuaynkem. 2009. Observations of basking in Varanus bengalensis nebulosus from northeastern thailand. Biawak, 3/3: 88-92.

Gupta, D., A. Sinha. 2001. Notes on the burrows of Varanus bengalensis in and around Agra. Zoos' print journal, 16/12: 651-654.

Lauprasert, K., K. Thirakhupt. 2001. Species diversity, distribution and proposed status of monitor lizards (Family Varanidae) in southern Thailand. The Natural History Journal of Chulalongkorn University, 1/1: 39-46.

Loop, M. 1974. The Effect of Relative Prey Size on the Ingestion Behavior of the Bengal Monitor, Varanus bengalensis (Sauria: Varanidae). Herpetologica, 30/2: 123-127.

Losos, J. 1988. Ecological and evolutionary implications of diet in monitor lizards. Journal of the Linnean Society, 1988/35: 379-407.

Papenfuss, T., S. Shafiei Bafti, M. Sharifi, D. Bennett, S. Sweet. 2010. "Varanus bengalensis" (On-line). IUCN Red List of Threatened Species. Accessed April 04, 2011 at http://www.iucnredlist.org/apps/redlist/details/164579/0.

Pianka, E. 1995. Review: Lizards observed. Science, 268/5217: 1636.

Sweet, S., E. Pianka. 2003. The Lizard Kings. Natural History, 112/9: 40-45.

Wikramanayake, E. 1992. Energy and water turnover in two tropical varanid lizards, Varanus bengalensis and V. salvator. Copeia, 1992/1: 102-107. Accessed February 04, 2011 at http://www.jstor.org/stable/1446540.

Wikramanayake, E., G. Dryden. 1993. Thermal Ecology of Habitat and Microhabitat Use by Sympatric Varanus bengalensis and V. salvator in Sri Lanka. Copeia, 1993/3: 709-714.

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