It’s time once more to talk about monitor lizards…
Caption: a selection of monitor profiles.
Once upon a time, at TetZoo ver 3 (the ill-fated SciAm years), I went through a reasonably successful phase of writing about monitor lizards, the articles concerned covering the Komodo dragon Varanus komodoensis, the ‘prasinoid’ tree monitors, and assorted obscure Pacific monitors (links below). I meant to do a lot more but became distracted. But don’t worry… there’s still tons more on monitors – among the most charismatic and fascinating of living reptiles – that I plan to cover, stay tuned.
Caption: monitors are a diverse bunch and vary substantially in skull shape, dentition, overall size and proportions, and in ecology, behaviour and much else. We mostly stick with the idea that all 80-odd living species should be included in the single genus Varanus, but there’s a good case to be made for splitting this into 10 genera or more. Names for those subdivisions already exist and are traditionally recognised as ‘subgenera’. The images here show (left to right) Savannah or Bosc’s monitor V. exanthematicus, Blue tree monitor V. macraeai and Crocodile monitor V. salvadorii. Images: Darren Naish.
Today I want to cover another modern monitor group – namely, the set of taxa that have conventionally been grouped together within the species Varanus salvator (most popularly known as the Water monitor) but which are increasingly recognised as a species complex (Koch et al. 2007, Koch & Böhme 2010, Welton et al. 2014). That is, as a group of several species that have been artificially lumped together... the caveat here being, as always, that ideas about ‘how distinct’ a population needs to be before it should be considered a ‘species’ still retains an element of subjectivity. Shockingly little is known of certain of these lizards and fieldwork on the biology, ecology and conservation status is sorely needed.
Caption: a captive water monitor encountered at a natural history meeting in 2010, I think of the nominate form V. salvator salvator. Note the handlers robust gloves. Image: Darren Naish.
Water monitors as a whole have an incredibly wide distribution, stretching from eastern India and Sri Lanka in the west to the shores of the South China Sea in the north and islands close to New Guinea in the south-east. There are some early 20th century reports from northern Australia but these are generally thought to be erroneous, presumably the result of misidentification with the Mangrove monitor (Bennett 1995). As suggested by the name, water monitors are good swimmers that regularly forage in, or close to, water. They eat molluscs, crabs, frogs, turtles, fish and crocodile eggs in places such as mangroves and swamps. But they aren’t just found in or close to aquatic environments, since they can also be found in tropical forests, grasslands and even agricultural areas.
Caption: a map of water monitor distribution, the different colours showing the ranges of V. salvator subspecies as recognised by Koch et al. (2007). Yellow: V. s. salvator; green: V. s. macromaculatus; blue: V. s. bivittatus; red: V. s. andamanensis; brown: V. s. ziegleri; grey: undescribed Sulawesi subspecies. Image: Martin-rnr, CC BY 3.0 (original here; based on map in Koch et al. (2007)).
They’re also credited with making sea crossings: a factoid often mentioned in the varanid literature is that water monitors were among the first tetrapods to colonise the newly-forming Krakatoa islands after the eruption of 1883 (in those places, they were presumably relying on fish and crustaceans and prey, since terrestrial resources would have been absent). In fact, by the 1920s people were visiting these islands specifically to hunt the many monitors there (Rawlinson et al. 1992, Bennett 1995).
Caption: what a beautiful animal; a water monitor photographed in Indonesia. Image: Christian Gloor, CC BY 2.0 (original here).
In habitats like grasslands and forests, water monitors eat arthropods, other lizards (including other monitors), small rodents and carrion. They also scavenge on human waste and will eat the faeces of humans and other mammals when given the chance. Their consumption of pest animals like rats and of carrion and faeces means that they might serve a valuable role as ‘ecosystem service providers’, though (so far as I know) this role remains under-researched and we would benefit from knowing more about it. The economic and nutritional importance of water monitors to people across Asia is also a really interesting and (I think) under-researched subject. Given that these lizards are highly fecund (they can produce 40 eggs every year), have highly flexible breeding cycles, are relatively fast-growing, and are physically big and considered delicious by people, we’d really benefit by knowing more about their economic role and on how viable and sustainable current utilisation is. I don’t know as much about this as I’d like to and would appreciate learning more. Indeed, hunting is so intense that water monitors have, in some areas, been made scarce or even eliminated entirely.
Large individuals (belonging to the ‘typical’ form V. s. macromaculatus*) can exceed 2 m in total length and are among the largest monitors in the world: one exhibited at Kuala Lampur in 1958 was 2.7 m long (Bennett 1995).
* Recent revisions mean that the nominate form V. s. salvator is endemic to Sri Lanka (Koch et al. 2007, Welton et al. 2014)
Caption: a number of varanid populations long included in V. salvator look very different from classic populations of the species, and a good case has been built that they should be regarded as distinct species. Look at this fantastic yellow animal (on display at ZSL London Zoo): it’s Cuming’s water monitor, aka Yellow-headed water monitor or Philippine water monitor V. cumingi. Image: Darren Naish.
There’s a lot more to say on these lizards: in particular I want to elaborate on the fact that V. salvator of tradition is a species complex, and actually includes a substantial amount of variation. But I’m out of time, and this’ll have to wait...
More on monitors soon, and be sure to check out the other varanid-themed articles in the TetZoo archives…
Perentie tries to swallow echidna. Echidna too spiky, Perentie gets horribly injured. Dies., December 2009
Monstersauria vs Goannasauria, May 2012
Lean, green and rarely seen: enthralling prasinoid tree monitors, February 2014
Obscure and attractive monitor lizards to know and love, February 2014
Hell yes: Komodo dragons!!! (again), March 2014
Refs - -
Bennett, D. 1995. The Water monitor Varanus salvator. Reptilian 3 (8), 15-21.
Koch, A. & Böhme, W. 2010. Heading East: a new subspecies of Varanus salvator from Obi Island, Maluku Province, Indonesia, with a discussion about the easternmost natural occurrence of southesast Asian Water Monitor Lizards. Russian Journal of Herpetology 17, 299-309.
Koch, A., Auliya, M., Schmitz, A., Kuch, U. & Böhme, W. 2007. Morphological studies on the systematics of the South East Asian water monitors (Varanus salvator Complex): nominotypic populations and taxonomic overview. In Horn, H-G., Böhme, W. & Krebs, U. (eds) Advances in Monitor Research III, Mertensiella 16, pp. 109-180.
Rawlinson, P. A., Zann, R. A., van Balen, S. & Thornton, I. W. B. 1992. Colonization of the Krakatau islands by vertebrates. GeoJournal 28, 225-231.
Welton, L. J., Travers, S. L., Siler, C. D. & Brown, R. M. 2014. Integrative taxonomy and phylogeny-based species delimitation of Philippine water monitor lizards (Varanus salvator Complex) with descriptions of two new cryptic species. Zootaxa 3881, 201-227.
