| Varanus indicus | |
|---|---|
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Sauropsida |
| Order: | Squamata |
| Suborder: | Lacertilia |
| Family: | Varanidae |
| Genus: | Varanus |
| Subgenus: | V. (Euprepiosaurus) |
| Species: | V. indicus |
| Binomial name | |
| Varanus indicus (Daudin, 1802)[1] |
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Varanus indicus, also known as the Mangrove Monitor[2], Mangrove Goanna, or the Western Pacific monitor lizard, is a member of the monitor lizard family with an enormous distribution from northern Australia and New Guinea to the Solomon Islands, Marshall Islands, Caroline Islands, and Marianas Islands. It grows to lengths of 3.5 to 4 feet (1.1 to 1.2 m).
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Varanus indicus was first described by the French herpetologist François Marie Daudin in 1802.[1] Daudin's original holotype of a subadult specimen was collected on Ambon, Indonesia, and has since disappeared from the museum in Paris. Daudin's original name for the species was Tupinambis indicus, an appellation it would carry for 75 years until being renamed as a Varanus.[3]
The generic name Varanus is derived from the Arabic word waral (ورل), which translates to English as "monitor."[4] Its specific name, indicus, is Latin for the country of India, however, in this instance it relates to Indonesia or the East Indies, where the animal was first described.[3]
Due to its large geographic range, Varanus indicus is considered a cryptic species complex of at least four species: Varanus indicus, Varanus doreanus,Varanus spinulosus, and Varanus jobiensis.[5] More research is being done on possible future species within this complex, not surprising since it has had over 25 different scientific names since it was first described.[3]
The Mangrove monitor's range extends throughout Australia, New Guinea, the Solomon Islands, the Marshall Islands, the Caroline Islands and the Marianas Islands, where it inhabits damp forests near coastal rivers, mangroves, and permanent inland lakes. It also occurs on the Moluccan islands of Morotai, Ternate, Halmahera, Obi, Buru, Ambon, Haruku, and Seram. Within this range of thousands of miles across hundreds of islands there is a huge amount of variation in size, pattern and scalation.[3] The monitors have also been introduced to Japan since the 1940s.[6] Like the introduction to Japan, some herpetologists believe this animal's dispersal from the East Indies to smaller Pacific islands was facilitated by Polynesians in order to provide a meat supply. However, other scientists maintain this is not likely as the monitors would compete with man for food, grow slowly, and yield little meat.[7]
The monitor's body is dark green or black in color and covered with golden yellow spots, with light coloration on the top of the head and a solid, cream colored belly lacking dark markings.[3] It has a distinct dark purple tongue and serrated teeth.[3][8] The mangrove monitor attains different sizes in different parts of its range, but seldom if ever exceeds 1.5 meters in total length.[3] Australian herpetologist Harold Cogger gives a total length of 100 cm for Australian specimens.[9] The tail is almost two times the length of the body and laterally compressed to aid in swimming. Like the rest of the lizard's body it is covered with small, oval, keeled scales.[8]
This monitor has the ability to increase the size of the mouth by spreading the hyoid apparatus and dropping the lower jaw in order to eat large prey, a process similar in appearance to that of snakes, although the jaw of the mangrove monitor remains rigid.[10] Mangrove monitors possess a Jacobson's organ which they use to detect prey, sticking their tongue out to gather scents and touching it to the opening of the organ when the tongue is retracted.[8]
The mangrove monitor is one of only two species of monitor lizard, the other being Varanus semiremex, that possess salt-excreting nasal glands, which enable them to survive in saltwater conditions and to consume marine prey.[7] The presence of this gland probably enabled the monitors to reach new islands and aid in its dispersal throughout the Pacific.[8]
The Mangrove monitor is an opportunistic carnivore feeding on the eggs of reptiles and birds, mollusks, rodents, insects, crabs, smaller lizards, fish, and carrion.[9][11][12] Mangrove monitors are the only monitor capable of catching fish in deep water.[13] In some parts of its range it is known to eat juvenile crocodiles.[3]
A study done in 1993 showed that mangrove monitors in the Southern Mariana Islands shifted major prey classes when their regular prey began declining.[6] The monitors were known for eating shrews on Guam,[4] but the introduction of the brown tree snake (Boiga irregularis) led to decreased shrew numbers, prompting the monitors to switch to eating invertebrates and foraging through human garbage.[4][6]
The species lacks distinct sexual dimorphism, however mature male monitors on Guam have been reported to be three times the mass of mature females.[4] Males fight for females and in one observation, after mounting the female, the male used his chin to rub the dorsum of the female's head and forequarters.[6] It was also observed that while mounted and oriented head to head, the male and female slowly rotated in a clock-wise direction through 360 degrees with the male remaining superior.[6]
Female mangrove monitors lay 2-12 eggs that measure 3.5 to 5 cm in length. The oblong eggs are white, and hatch in about 7–8 months.[8]
The first successful captive breeding of this species was at the Philadelphia Zoo in 1993.
The Reptilian Zoo in Vlissingen, the Netherlands [14], reports that they have successfully hatched eggs by a female animal that was not in any contact with a male of the same species.
Humans have introduced the mangrove monitor to a number of Pacific Islands since the 1930s.[12][15] They have been present on Ifaluk in the western Caroline Islands since the Second World War.[15] The Japanese introduced the lizards to the Marshall Islands prior to World War 2 to eliminate rats; the lizards flourished and soon began to raid the local chicken houses.[12] When American troops arrived, the locals asked them for help in getting rid of the mangrove monitors. The US response was to introduce the marine toad (Bufo marinus) which proved toxic to the lizards.[12] As the monitor population dropped, however, the rat population began to rise.[12] Marine toads were introduced to the Palau Islands for a similar reason, and the demise of the mangrove monitors led to an increase in numbers of beetles known to be harmful to coconuts.[15]
The mangrove monitor is hunted in many places for its skin which is used for leather in making drum heads.[16][17] Although international trade in this species is small, Mertens referred to it as one of the most heavily exploited monitor lizards.[17] In 1980 trade in over 13,000 monitors was declared. However in many remote places they are used as a food source and are killed because of their reputation for preying on domestic animals.[8] There is an ethnic group on Guam that eats the monitors as a traditional food and a business exists there that sells monitors for food.[6]
The United States Department of Agriculture’s Animal and Plant Inspection Service division, announced that it will use a combination of two poisons, diphacinone and brodifacoum, to kill off the rodents on Cocos Island. They will also attempt to lower the mangrove monitor population on Cocos Island by 80%, using a myriad of trapping methods proposed by herpetologist Seamus Ehrhard, as the lizard is believed to prey upon the endangered Guam Rail (Gallirallus owstoni). Most locals, however, do not see the monitor as an invasive species and a few activists are opposed to the attempt to lower the population on Cocos Island.
Mangrove monitors are often kept in zoos and private collections as they are an active and alert animal that generally can be handled if tamed properly.[11] Most specimens defecate on their handlers when stressed.[8] With proper care they can live up to 20 years in captivity.[11]
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