23 January 2011

Monitors of Singapore: Who's the Rarest of Them All?

(Photo taken from Varanus.nl)

Singapore has 3 species of monitor lizard, and 2 of them, the Malayan water monitor (Varanus salvator) and clouded monitor (Varanus bengalensis nebulosus), are commonly encountered.

Previously, I wrote about the Malayan water monitor, corrected some misconceptions and discussed some threats faced by monitors, and also shared information about the clouded monitor. In this last post of the series, we take a look at our 3rd monitor lizard, a rare and elusive species that was once thought to be extinct in Singapore.

Dumeril's monitor

The 3rd and final monitor lizard found in Singapore is very rarely encountered; in fact, up until 2008, it was thought to be locally extinct.

(Photo from The Ultimate Reptile Shop)

Unlike the previous 2 species, very little is known about Dumeril's monitor (Varanus dumerili) in the wild. It is rare and elusive, and sparsely distributed. Its apparent preference for undisturbed forests, combined with a shy nature and cryptic habits (when not hunting, it spends a lot of time hidden away in burrows or tree hollows), mean that finding one is in itself a challenge, let alone observing and studying its behaviour and ecology.

However, it is a species in great demand for the exotic pet trade, largely because of the bright orange head and attractive patterns seen in hatchlings.

(Photo taken from Geckos Unlimited forums)

Unfortunately, these markings soon fade away, although adults still retain some stripes and mottling.

(Photo by Michael Cota)

(Photo taken from Pro Exotics Reptiles)
This species is reported to grow up to 1.5 metres in length, although no wild specimens have measured more than 1.3 metres.

(Photo by D. Kiehlmann, taken from Varanus.nl)
Like the clouded monitor, Dumeril's monitor has nostrils that are midway between the eyes and the tip of the snout. In fact, it is believed that these 2 species are closely related to each other, with the Malayan water monitor as a more distant relation.

In the wild

Dumeril's monitor seen in Phuket (Photo by Phamon Sumphanthamitr, from Khao Phra Thaew Ecological Sustainability Project)

Dumeril's monitor is believed to frequent forested environments near water, including evergreen rainforest, mangroves, and peat swamps. It appears to be quite similar to the water monitor in terms of habits, having a strong affinity for water, as well as being an active climber and digger.

Dumeril's monitor in Khao Sok National Park (Photo by Valts & Vija, taken from Paddle Asia)

Observations of wild and captive specimens indicates that this species specialises on crabs. Both young and adult individuals feed on crabs by violently shaking and dismembering their prey; the pincers and legs are broken off before the carapace is crushed into pieces by the jaws and swallowed. Insects have also been recorded as prey. Despite its size, this species has not been recorded eating smaller vertebrates in the wild.

(Photo taken from Varanus.nl)

While captive Dumeril's monitors will adjust to feeding on other prey such as crickets and rodents, one Japanese owner actually went to the extent of purchasing and preparing freshwater crabs as food.

Captive Dumeril's monitor feeding on prawns (Video by ScientistRyan)

One possible reason why it is difficult to find Dumeril's monitor is that it may be nocturnal to some degree. The recent record from Singapore (which will be discussed later) involved encountering a large monitor lizard foraging at night, which is behaviour very much unlike that of monitor lizards in general, which are almost entirely diurnal.

Another encounter with Dumeril's monitor, this time in Thailand, was similar in that the lizard was seen out in the open at night. One possible explanation for why this species is more active at night could be because the freshwater and terrestrial crabs that it specialises on are nocturnal in the first place.

Male Dumeril's monitor caught while active at night. (Photo by Tanya Chan-ard)


So far, records indicate that it occurs from southern Myanmar and Thailand to Peninsular Malaysia, Sumatra, Borneo, and nearby islands such as the Riau archipelago, Belitung and Bangka. A specimen reported in 1935 is the most substantial piece of evidence that the range of Dumeril's monitor once included Singapore. Due to a lack of subsequent records, this species was declared locally extinct in the mid-1990s.

However, one night in January 2008, a large monitor lizard was spotted and filmed in the Nee Soon Swamp Forest. Based on the markings and the position of the nostrils, it is believed that this particular lizard is a Dumeril's monitor, and the first local record of this species in 75 years.

Screen captures from video taken of the monitor lizard seen in Nee Soon Swamp Forest.

That such a large lizard could go undiscovered for so long goes to show that Singapore's biodiversity continues to harbour many surprises, and also speaks of the value of conserving the Nee Soon Swamp Forest; it is the last refuge for many species, in particular those endemic to our original freshwater habitats.


Unlike the water monitor and clouded monitor, it seems that there is no demand for Dumeril's monitor in the leather and meat markets. It appears that one of the greatest threats to its survival would be excessive collection for the pet trade. This species was once regularly seen offered for sale in pet stores in Europe and North America. However, not many were successfully bred in captivity, and the species is now more rarely available, although it is still available in Japan and North America. Since 1997, the European Union imposed import restrictions from Indonesia for Dumeril's monitor and 3 other monitor lizard species, due to a lack of scientific justification for export quotas. Until now, the original conservation concerns for these species remain unaddressed and the EU import restrictions are still in place. In 2009, a confiscation of more than 1,000 clouded monitors in Pahang also turned up 2 Dumeril's monitor.

Trade restrictions, as well as possibly overexploitation in range countries, have meant that this species is now less commonly encountered in the trade, although dedicated captive breeding could rectify the situation. In any case, the species is so rarely encountered in the wild that the great majority of photos online are of captive specimens.

Captive Dumeril's monitor in enclosure (Photo by Jume)

Even if collection for the pet trade lessens due to captive breeding, Dumeril's monitor still faces the threat of habitat destruction; its apparent preference for undisturbed rainforest and peat swamps means that it is more vulnerable to deforestation than the other 2 monitor species. It seems that so little is known about this species in the wild that it is not even listed on the IUCN Red List.

In Singapore, its continued survival would most definitely depend on our ability to ensure that the Nee Soon Swamp Forest receives adequate protection and minimal disturbance.


Monitor lizards are an iconic element of our biodiversity. We have lost much of our megafauna, but so far, our 2 common species of monitor have proven themselves capable of adapting to living in close proximity to people. The challenge is whether we are able to accept their presence, and to be able to grant them the habitat that they need to survive, even in our urban areas.

There are currently 73 known species of monitor lizard, and there are almost certainly more waiting to be discovered. The numerous islands of the Philippines and Indonesia have proven to be a hub for discovery of new species of monitor lizards; 4 new species were described in 2010 alone.

While we most probably will not be discovering any new monitor lizard species here, there is still a lot of room for further study.

(Photo by Marcus)
It would be essential to determine if the large numbers of Malayan water monitors in Sungei Buloh are negatively affecting bird populations, and whether culling might be necessary to keep their numbers in check. On the other hand, predation by water monitors might be helping to protect the wetlands from invasion by non-native animals such as red-eared sliders and American bullfrogs. At the same time, the return of the smooth otters and estuarine crocodiles, both potential predators and killers of water monitors, may help play a role in controlling their numbers. Elsewhere, monitor lizards continue to serve as top predator in many of our waterways and coastal areas, and it would be useful to find out to what extent their scavenging activities prevent excessive pollution of our waterways.

(Photo by Johnny Wee)
While clouded monitors are closely tied to forest habitats, it may be possible for studies to be conducted to see how tolerant they are of recreational park use. There is great potential for clouded monitors to colonise many of our wooded areas and scrubland via park connectors, and it might also be important to make sure that they are not poached like they are in other countries.

(Picture from the now-defunct Roughneckmonitors.com)
As for the much rarer Dumeril's monitor, more studies are needed to assess the size of the population, and whether it is able to adapt to similar forested habitats elsewhere in Singapore. While it is unlikely that poaching for the pet trade will occur here, it is essential that the Nee Soon Swamp Forest is protected from unnecessary intrusion and habitat alteration.

(Photo by Michael Cota)
One more species of monitor lizard, the rough-necked monitor (Varanus rudicollis), is known from neighbouring countries, although it has not been recorded from Singapore. Like its close relative Dumeril's monitor, it too has a preference for undisturbed forests. While it has never been recorded from Singapore, and we may have too few patches of primary and mature secondary rainforest, the recent rediscovery of Dumeril's monitor might lend some credence to the possibility that the roughneck monitor once lived in or still lives in Singapore.

There are many resources available on monitor lizards; several books about the group as a whole include the following titles:

Online websites with plenty of information about the various species of monitor lizard include the following:

The International Varanid Interest Group publishes Biawak, a quarterly journal dedicated to nothing but monitor lizards.

The monitor lizards evolved during the Mesozoic, at a time when the dinosaurs were still the dominant group of animals on Earth. May Singapore continue to be a home for these real-life dragons.


Joseph Lai Tuck Kwong said...

Thanks Ivan. Marvellous posting you have here about the monitor lizards. Cheers : )

Unknown said...

Excellent article on local monitors. Thank you for sharing. I've gained new knowledge about our animal species in sg.
- Animal enthusiast

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