16 January 2011

More on Monitors

Many of us are familiar with the Malayan water monitor (Varanus salvator), a large lizard commonly found near water, from urban drains and canals to reservoirs, as well as in coastal areas such as beaches and mangroves. This species was the focus of 2 earlier posts, one discussing its biology, and another post about how this common species is so often misidentified or feared, as well as some of the threats it faces. However, this is not the only species of monitor lizard found in Singapore, and another relatively common species will be covered in this post.

Clouded monitor

Another monitor lizard that is widely distributed across tropical South and Southeast Asia is the species known as Varanus bengalensis. There are 2 subspecies:

Bengal monitor in Corbett National Park, India (Photo by Shailee Shah)
The nominate subspecies, known as the Bengal monitor (Varanus bengalensis bengalensis), is found in the Indian subcontinent, from eastern Iran, Afghanistan and Pakistan, to India, Sri Lanka, Nepal, Bangladesh, and Myanmar.

Clouded monitor in Hindhede Nature Park (Photo by Siyang)
The other subspecies is the clouded monitor (Varanus bengalensis nebulosus), and it is found in Southeast Asia, from southern Myanmar, Thailand and Indochina to the Malay Peninsula, Singapore, Sumatra and Java.

One species, or two?

Although the clouded monitor is usually treated as a subspecies of the Bengal monitor, work by herpetologists Wolfgang Böhme and Thomas Ziegler, who specialise in monitor lizards, suggests that the clouded monitor may warrant being split off as an entirely separate species. Under such classification schemes, the clouded monitor is known as Varanus nebulosus.

Bengal monitor in Yala National Park, Sri Lanka (Photo by Niall Corbet)

Clouded monitor in Pulau Ubin (Photo by Daniel Koh)

As can be seen from the photos, the 2 forms differ somewhat in terms of coloration. Morphologically, there are also subtle differences in the scales; the clouded monitor has enlarged scales above the eyes, which the Bengal monitor lacks, and the Bengal monitor generally has a higher number of scales than its eastern counterpart.

(Photo by Shirley Ng)

There are also differences in behaviour; the clouded monitor is supposedly more arboreal than the Bengal monitor, and prefers resting in tree hollows, whereas the latter is more likely to inhabit burrows.

Clouded monitor basking on tree at Lower Peirce Reservoir (Photo by Johnny Wee)

On one hand, it is said that the ranges of the Bengal and clouded monitors overlap in parts of Myanmar and northern Thailand without hybridisation taking place, suggesting that they are in fact separate and well-differentiated species, while there are also reports of captive hybrids between the 2 (sub)species, indicating that at the same time, they are very closely related. Whatever the case, determining whether the clouded monitor is sufficiently different from the Bengal monitor to be considered a distinct species requires more research, and also probably depends on whether the scientists working on this issue are lumpers or splitters.

Less dependent on water

(Photo by Chan Kwok Wai)

Compared to the Malayan water monitor, the clouded monitor is more fully terrestrial, and is not so dependent on watercourses and coastal habitats. In Indochina, the clouded monitor is found far inland, whereas the water monitor is restricted to the coasts. The species as a whole is tolerant of a wide range of habitats, from the edges of rainforests to deciduous forests and agricultural land. Its Bengal cousin is even found in arid savannas and desert fringes in the Indian subcontinent.

(Photo by Siyang)

The clouded monitor specialises more on terrestrial invertebrates; the bulk of its diet is made up of insects such as grasshoppers and beetles, although it will raid nests for eggs, and snatch up smaller vertebrates such as frogs, smaller lizards and rodents.

Clouded monitor foraging at Bukit Timah Nature Reserve (Video by Mei Lin)

Here in Singapore, I've seen clouded monitors basking on trees or digging through the leaf litter for food in the forests of Bukit Timah Nature Reserve, or basking on the ground along the many dirt paths on Pulau Ubin.

Clouded monitor at Singapore Zoo (Photo by MardyFox)

Clouded monitors are often spotted foraging on the grounds of the Singapore Zoo; these are not free-ranging captives, but truly wild lizards that have established territories within the compound. The zoo's open concept and lush vegetation, contiguous with the surrounding forests of the Central Catchment Area, mean that a lot of native wildlife lives in and around the zoo.

Clouded monitor on footpath at Singapore Zoo (Photo by Dan Robertson)

Sun Chong Hong has a clouded monitor living in a tree within his condominium grounds, and has contributed a post and video documenting its behaviour.

(Video by Sun Chong Hong)

Clouded monitor seen at MacRitchie Reservoir (Video by samisupidupi)

How do they get along?

While the clouded monitor and Malayan water monitor are sympatric in many parts of their range, it is unknown to what extent they interact with and compete with each other. Water monitors are known to feed on juvenile Bengal monitors, and in the most comprehensive work ever done on the Bengal monitor, herpetologist Walter Auffenberg writes,
"In my experience with mixed collections of both species in captivity, adult salvator often attacked and severely injured even full-grown bengalensis... - so much so that I stopped placing them together."

Studies in Sri Lanka have found that the Bengal monitor and water monitor are active at different times of day; the water monitor is at its most active in the morning and late afternoon, while the Bengal monitor is most active during the midday afternoon. Hence, there is little overlap in timing of peak activity, and direct competition between the 2 species is avoided. However, it is also possible that such segregation in activity periods occurs because the Bengal monitors are deliberately avoiding competition with the larger, more aggressive water monitors.

Clouded monitor basking on rock in Pulau Ubin (Photo by Alexander Lovegrove)

Given that the Bengal monitor will inhabit wetlands and even drier parts of mangroves in areas where the water monitor is absent, it is possible that the latter aggressively competes with and excludes the former from such habitats. It would also be interesting to see if such differences in behaviour can be seen between the clouded monitor and water monitor; as far as I know, no one seems to have witnessed any interaction between the 2 species here in Singapore.

(Photo by Shirley Ng)

Clouded or Water?

It appears that in Singapore at least, the Malayan water monitor is more widely distributed than the clouded monitor, being found in and close to coastal areas, reservoirs and urban waterways. The clouded monitor is more likely to be found in inland forests, where the water monitor is absent. However, there are some areas where both species may be found. Based on sightings, these areas include the Central Catchment Area, Sungei Buloh, and Pulau Ubin. There are times when the 2 species might be confused for one another, even by nature lovers. So, how do you tell them apart in the field?

Young clouded monitor seen at Sungei Buloh (Photo by Ria)

The clouded monitor is typically smaller than the water monitor; the maximum length for this species in Malaysia is 1.6 metres, although it appears that most individuals are much smaller. Personal anecdotes do not quite count as data, but based on what I have observed offhand, large water monitors reaching 1.5 metres in length are not uncommon (just visit Sungei Buloh), but I have yet to see a clouded monitor that size.

Coloration also helps provide a rough guide to telling the 2 apart.

(Photo by Shirley Ng)
Like the water monitor, the baby clouded monitor is adorned with attractive patterns that fade as it reaches adulthood.

Clouded monitor at Hindhede Nature Park (Photo by Johnny Wee)
As an adult, the clouded monitor retains dense yellow specks on a greyish-brown base; the water monitor's skin pattern is very different, and tends to be a much darker grey. Even in those water monitors that have not completely lost the markings on their skin, the pattern of spots is quite dissimilar from that seen in the clouded monitor.

Malayan water monitor at Sungei Buloh (Photo by Ria)

However, the easiest way to differentiate the 2 species is by looking at the shape of the head and the placement of the nostrils.

Malayan water monitor at Sungei Buloh (Photo by Marcus)
The Malayan water monitor has a long and slender snout, almost crocodilian in profile, and its nostrils are placed close to the tip of the snout.

(Photo by Shirley Ng)
Whereas the clouded monitor has a shorter snout, and its head is more triangular or wedge-shaped. Its nostrils are located midway between the eyes and the tip of the snout.


The clouded monitor is at risk from many of the same threats as those faced by the water monitor. I feel that one of the biggest threats locally is a general lack of understanding, resulting in fear and paranoia when people encounter clouded monitors. This is an example of how a clouded monitor that wandered into someone's backyard caused a little ruckus, which in my opinion was a little unnecessary, unless the person had small pets at risk. In any case, this particular lizard, nicknamed Spotty, was caught and relocated by the Animal Concerns Research and Education Society (ACRES). Such fear also manifests itself in the form of ugly behaviour in our parks, as witnessed by Marcus.

(Photo by Shirley Ng)

However, within the region, the greatest threat comes from hunting; in nearly all the countries where it is present, the clouded monitor is hunted on a massive scale, for meat, leather, and medicinal purposes. While it is likely that healthy populations can support some level of harvesting, the sheer numbers that are caught must take a heavy toll in some areas. Despite being protected, large quantities of clouded monitors are often discovered and confiscated by the Malaysian authorities. Recent cases that have been reported include:
  • 4.3 tonnes of assorted reptiles (among them clouded monitors) seized from a lorry parked near the border with Thailand; December 2010. (Link)

  • 422 confiscated while being transported in a lorry in Johor; September 2010 (Link)

  • 1,202 found in the back of a lorry in Pahang, and after obtaining information from this seizure, a raid on a nearby home turned up an additional 34 individuals; April 2009. (Link)

  • 2,330 discovered in a raid on a workshop garage in Pahang; January 2009. (Link)

  • 7,144 confiscated in 2 separate raids on storage facilities in Johor; November 2008. (Link)

These are just a few of the more than 7,000 clouded monitors confiscated in raids in November 2008; perhaps the more disturbing find from this case was that of the plucked and frozen carcasses of 917 owls belonging to 5 different species. (Picture taken from article in The Star)

Based on just these few reports, it shows that the clouded monitor is very heavily exploited by the wildlife trade, presumably for markets throughout Southeast Asia and China. Apparently, the flesh of the clouded monitor is more highly prized than that of the water monitor, and this species as well as its Bengal relative have declined or even been extirpated in areas where hunting pressure is heavy. Currently, the species as a whole is considered to be of Least Concern in the IUCN Red List, although the general population trend is said to be decreasing.

Combined distribution of Bengal and clouded monitor (Taken from IUCN Red List)

Clouded monitor at market in Laos (Photo by Shirley Ng)

Hopefully, poaching of clouded monitors does not occur on a similar scale here in Singapore. In any case, like all other reptiles, the clouded monitor is fully protected under the Wild Animals and Birds Act.

(Photo by Johnny Wee)

Unlike the Malayan water monitor, the clouded monitor is not as commonly seen in the pet trade, although it apparently adapts well to captivity. However, both species cannot compare to another species of monitor lizard in terms of value and desirability among hobbyists. This is the 3rd and last monitor lizard recorded in Singapore, and will be the focus of the 4th and final post in this series on the monitor lizards of Singapore.

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