31 December 2010

Magnificent Monitors

Today is the last day of 2010, and to conclude the International Year of Biodiversity, I decided it would be a good idea to talk about an iconic member of Singapore's native fauna.

Where it comes to scaly encounters, almost all of us would be familiar with the little house geckos that scurry about on walls and ceilings. Many of us would also be familiar with the changeable lizards (often misidentified as chameleons) that are often seen basking on trees and fences in parks and gardens. But these lizards are featherweights compared to their massive cousins, the monitors.

What are monitor lizards?

The monitor lizards belong to a family known as the Varanidae, and are distributed throughout Africa, the Middle East, tropical Asia, Australia, and some islands in the Pacific.

Why are they called monitor lizards? One reason states that these lizards would sound a warning when crocodiles were nearby. Another more likely reason stems from the fact that some species will rear up on their hind legs to survey their surroundings, and apparently "monitor" what is happening around them. This page has another plausible explanation that comes from linguistic confusion; the Arabic word for lizard is ouaran (often written as varan). This led to confusion with the German word warnen, which means "to warn", and the lizards were termed Warn-eidechsen, which means "warning lizard".

In Australia, monitor lizards are more commonly known as goannas, due to early confusion with iguanas, an entirely separate family of lizards. In South Africa, monitor lizards are known as leguaan, from the Dutch word for iguana. And in Malay, monitor lizards are simply called biawak, which is also the title of the journal of the International Varanid Interest Group.

One distinctive feature about a monitor lizard is that like a snake, it has a forked tongue, which it constantly flicks in and out of its mouth; this allows the monitor lizard to literally taste the air and detect the scent of prey. In fact, one of the Chinese terms for monitor lizards directly translates as 'four-legged snake', probably because of this similarity.

You can clearly see the forked tongue in this photo of a Malayan water monitor in Sungei Buloh. (Photo by Ria)

Malayan water monitor

Here in Singapore, we have 3 native species of monitor lizard; the species most commonly encountered is the Malayan water monitor (Varanus salvator).

This species can reach a huge size; the largest one ever measured was caught in Sri Lanka in 1932, and reached 3.21 metres in length - that's even longer than its giant cousin, the Komodo dragon! However, this is an exceptional record, and most individuals max out at around 2 metres or so.

As its name suggests, the water monitor loves water, and is most commonly found in areas close to water; these include our reservoirs, coastal areas including beaches and mangroves, and even drains and canals in urban areas. It is a strong swimmer and can also be found on many of our offshore islands. In fact, large monitor lizards seen swimming in the water are probably often misidentified as crocodiles. Like a crocodile, the Malayan water monitor uses its long and powerful tail to propel itself through the water.

Water monitor swimming in Sungei Buloh

One of the best places in Singapore to find water monitors is at the Sungei Buloh Wetland Reserve; one is almost guaranteed to find these massive reptiles lazily swimming in the water, foraging for food on the ground, or just basking and soaking up some sunshine.

The water monitors at Sungei Buloh are quite unafraid of people. (Photo by Jun)

Other places where water monitors may be spotted (although they are much more shy in these areas) include Chinese Garden, the mangroves at Pasir Ris Park, and Chek Jawa on Pulau Ubin.

Water monitor at Pasir Ris Park

If you're lucky, you may even get to see monitor lizards on Sentosa!

Water monitor on Sentosa (Photo by athewma)

Being such strong swimmers, water monitors are also found on many of our offshore islands.

Water monitor on Lazarus Island (Photo by James)

Apparently, after the 1883 eruption of Krakatoa in Indonesia wiped out all life on the island, Malayan water monitors were among the very first terrestrial vertebrates to recolonise the area, by swimming across from other neighbouring islands at least 12 kilometres away!

One notable feature about the Malayan water monitor is its versatility and adaptability. Not only is it a good swimmer, it is also an excellent climber; even large individuals will climb trees to bask or raid birds' nests.

Water monitor basking in tree (Photo by Daniel Koh)

Like most species of monitor lizard, this species is carnivorous. It is capable of feeding on a wide variety of prey, from insects, snails and crabs to fish, frogs, and smaller reptiles, to rodents and birds. One important source of food is carrion, and the scavenging activities of monitor lizards probably play an important role in recycling of nutrients in the ecosystems they live in. Monitor lizards have been seen consuming well-decomposed carcasses, and one can only wonder at how they can eat such putrid meat without suffering any visible ill effects. Elsewhere in South-east Asia, Malayan water monitors living close to human settlements may depend heavily on leftovers and scraps, as well as raiding poultry.

Water monitor feeding on decomposed carcass of smooth otter in Sungei Buloh (Photo by Marcus)

Water monitor swallowing dead fish in Sengkang (Photo by Daniel Koh)

Monitor lizards, like snakes, swallow their food whole. Their flexible jaws enable them to widen their gape and consume large items. If prey is too large, the lizard uses its powerful forelimbs to pin down its food while ripping off strips of meat with its sharp teeth.

Monitor lizards also love eggs of all sorts, and in other countries, they are known to dig up the nests of sea turtles and crocodiles (presumably when mama croc is not around!). Locally, predation from the large number of water monitors in Sungei Buloh is suspected to possibly pose a threat to the many birds, both residents and migrants, that live in the reserve.

Young Malayan water monitor seen at Chek Jawa (Photo by Liana)

Baby monitor lizards are very colourful and have very attractive patterns. However, as they grow older, these markings fade. Many adults bear scars on their backs, the result of fights that break out between males as they compete for females. They will actually rear up on their hind legs and wrestle in a contest of strength, as shown in this video:

When fully grown, Malayan water monitors are pretty much top of the food chain, and have few if any predators besides feral dogs, large snakes such as reticulated pythons and king cobras, and crocodiles.

There is also a record of a smooth otter attacking and killing a large water monitor in Malaysia, as seen in this series of photos.

You can find more information about the Malayan water monitor at the following websites:

Wild Fact Sheets
Mangrove and wetland wildlife at Sungei Buloh Wetlands Reserve
Ecology Asia
Guide to the Mangroves of Singapore
Wildlife Singapore
Mampam Conservation
Reptilian (posted on Mampam Conservation site)

Coming up, tackling the myths and misinformation about the Malayan water monitor.

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