Everybody loves monitor lizards, or varanids. And there is so much to learn about, and to appreciate, in these remarkable, charismatic, complex, sophisticated lizards that scientists across many disciplines are being encouraged to study them and – lo – to make remarkable discoveries. In recent months we’ve seen the discovery of a mammal-like rate of blood flow to varanid limb bones (Seymour et al. 2011), further documentation of ‘tail-assisted foraging’ (Patanant 2012), the demonstration of rapid, sophisticated coordination between jaw gaping and neck and forelimb movement during prey-grabbing (Montuelle et al. 2012a, b), and the recognition that varanids possess a unidirectional system of breathing (Schachner et al. 2013). What seems to be the first documented human death caused by varanid oral toxins – the death was that of a 55 year old woman, bitten by a Bengalese monitor Varanus bengalensis – was also published just a few weeks ago (Vikrant & Verma 2013). Yet more amazing stuff on varanids is due to hit the presses soon.
Varanids have been covered a few times here at Tet Zoo, but never in appropriate depth (see the links below). Of course, what I’d really like to do is produce a huge series of articles that covers the entire radiation in all of its glory, but I’ve learnt from bitter experience that finishing long multi-part reviews of that sort is tremendously difficult, if not impossible. Dammit, if only there wasn’t all that non-blogging stuff I have to do. Anyway, with all of the above in mind, here’s another article in which I look at just a few species.
Monitors: more and more and more
For no special reason other than that I find them neat, I focus this time round on several fairly obscure monitors from Indonesia and Australasia. There’s a huge amount of diversity and complexity in the monitors of this region, with recent work showing that we’ve seriously underestimated the taxonomic and phylogenetic diversity of these lizards. New species are described on a regular basis, and morphological and molecular investigations have both shown that many ‘species’ are actually species complexes, sometimes containing lineages that are not especially closely related.
As I’ve said on Tet Zoo before, I can certainly remember a time (I was born in the 1970s) when people said that there were “about 25” living monitor species. Within recent years, we’ve seen that figure creep up to 50 and beyond, then to 60 and 70 and beyond. Current lists recognise about 72, but this will continue to escalate given that distinct lineages await naming in species complexes like the water monitors, mangrove monitors, Timor monitors and banded tree monitors. As many as “10 as yet undescribed new species” might exist in the current concept of the Australian spotted tree monitor V. scalaris alone, according to some workers (L. Smith, in Jennings & Pianka 2004), for example.
We start with the extremely poorly known but odd and perhaps highly specialised Dumeril’s monitor V. dumerilii, a widespread SE Asian varanid known from Thailand, Burma, the Malayan Peninsula, Borneo, Sumatra and some smaller islands. A single specimen has also been reported from Java and uncertainly remains over the distribution of the species towards the east and north (Bennett 2004). This animal is unmistakeable: creamish transverse bands separate dark brown, mottled areas on the dorsal surface and black temporal stripes extend from behind the eyes and along the neck. Hatchlings combine glossy black with bright orange or yellow crossbands and a vividly coloured head. Vague suggestions have been made that this patterning might represent cobra mimicry (Bennett 1994) but this hypothesis requires testing. It's a reasonably large monitor, reaching 1.25 m in total at most (1.5 m has been widely claimed but may be erroneous).
Observational data on wild Dumeril’s monitor is few and far between. The species is said to be sedentary, well able to climb, but also keen to take to water. In fact, an association with coastal mangroves and an apparent fondness of crabs suggest that it's well adapted for aquatic foraging: Krebs (1979) considered it a crab specialist. This idea is consistent with some aspects of this varanid’s anatomy. Its skull is relatively flat and broad (features generally consistent with a lifestyle that involves swimming, or foraging between rocks), its nostrils are located well away from the tip of the snout, its teeth are blunt and peg-like [UPDATE: apparently this is not true! More later], and it possesses nasal valves that allows it to close its nostrils when submerged. Its tail is laterally compressed, too. Some observers talk of Dumeril’s monitors emptying their lungs and walking along the bottoms of streams and even taking to the sea to escape harassment from dogs (Bennett 1994, Steel 1996).
Large, irregularly positioned scales cover the dorsal surface of its neck. [Image above by Katerina Zareva, EERC Sofia Zoo.] These superficially recall those present in the Rough-necked monitor V. rudicollis and both species have been confused on occasion (see Sprackland 1993). Both appear to be members of the same varanid clade (the one that also includes water monitors and mangrove monitors in most - but not all - phylogenies), but they’re not especially close (Ast 2001, Vidal et al. 2012, Pyron et al. 2013).
On the subject of phylogeny: for all their fame and attraction as research subjects, surprisingly little has been done on varanid phylogeny since Jennifer Ast’s paper of 2001. Most studies that look at phylogenetic patterns use her tree. Nothing wrong with that, but more studies would be good. You’ll note that I’m citing Pyron et al.’s (2013) gigantic study of lizard phylogeny, of course. Collar et al. (2011) also produced a novel varanid phylogeny but it doesn’t include the species discussed in this article. I should also make honorary mentions of Conrad et al. (2012), Vidal et al. (2012) and Welton et al. (2014).
Timor monitors: extralimital odatrians
Moving on, here’s a portrait of a Timor monitor V. timorensis, another very poorly known, small monitor (total length up to 60 cm), this time marked dorsally with transversely arranged ocelli arranged in bands. The tail is round in cross-section. It’s a climbing animal of tropical forests, threatened by the clearing for agriculture of its habitat. As goes distribution, the Timor monitor is probably endemic to Timor and a few of the tiny islands nearby (Savu, Roti and Semau). However, when named by John Gray in 1831 it was regarded as a tremendously widespread animal that occurred from the Lesser Sundas eastwards to New Guinea and then south to northern Australia. Gray thought that V. scalaris, now regarded as a separate species and sometimes called the Australian spotted tree monitor, was conspecific with the Timor monitor – a mess that wasn’t really sorted out until 2001 or so. Even with V. scalaris out of the mix, it still seems that ‘V. timorensis’ is a composite: the populations of Timor and the nearby islands are fairly variable and cryptic species are probably present. The population from Roti was named V. auffenbergi in 1999 (Sprackland 1999) and has variously been termed the Peacock monitor or Auffenberg’s monitor.
The Timor monitor has always been of special interest since it seems to be one of the so-called odatrians, a monitor group otherwise considered unique to Australia. DNA-based studies find V. timorensis to be especially close to Mitchell's water monitor V. mitchelli and the Rusty monitor V. semiremex, with V. scalaris, the Black-headed monitor V. tristis and others as more distant relatives (Ast 2001, Vidal et al. 2012, Pyron et al. 2013). Whatever, it’s deeply nested within the otherwise Australian Odatria.
If this is correct, it means that the Timor monitor belongs to a lineage that dispersed out of Australia and crossed Weber’s Line, but not Wallace’s Line [adjacent image by Maximilian Dörrbecker (Chumwa)]. This is not really a big deal given the strong Australian influence of the fauna in this region anyway, especially given that other monitors – the Komodo dragon V. komodoensis for one – have crossed Weber’s Line too. That’s right: remember that the Komodo dragon belongs to an ancestrally Australasian clade and must have dispersed out of Australia at some point in post-Miocene times (see links below for more on this) (Hocknull et al. 2009). Oh, yes, GIVE UP on the thoroughly wrong idea that Komodo dragons are anachronistic or in any way ancient. They’re a young evolutionary event, as are most living monitors, it seems.
Incidentally, large varanid vertebrae from Timor might demonstrate the former presence of the Komodo dragon on the island and show that it used Timor as a ‘stepping stone’ in migrating between Australia and the Lesser Sunda islands (Hooijer 1972, Arida & Böhme 2010). Alternatively, those vertebrae from Timor might be from something new and unnamed (Hocknull et al. 2009).
Anyway, the odatrian radiation seemingly occurred rapidly, and recently (as in, since the Oligocene: Jennings & Pianka 2004, Vidal et al. 2012), in which case members of the Timor monitor lineage presumably dispersed over-water during the Miocene, Pliocene or Pleistocene. So, question: were overwater dispersal events common in monitor history (and do they explain the occurrences of all those populations of prasinoids and so on on satellite islands around New Guinea and Australia?), or were they rare, with satellite island populations instead owing their distribution to the use of Pleistocene landbridges? It’s going to take a lot of work to figure any of this out...
Pointy head and peachy throat
Let’s move away from the odatrians for now. Below, we see a portrait of V. jobiensis, sometimes called the Peachthroat or Peach-throated monitor. Molecular studies indicate that the Peachthroat is a member of the water monitor clade (Ast 2001, Pyron et al. 2013) (Euprepiosaurus, if we use the varanid ‘subgenera’ as clade labels), perhaps closest to the recently recognised Black-backed mangrove monitor V. yuwonoi of the Moluccan Islands (named in 1998) and Blue-tailed monitor V. doreanus of New Guinea (named in 1874 but mostly ignored until 1994, when it was revalidated). Vidal et al. (2012) found V. jobiensis and V. doreanus to be part of a mangrove monitor clade that is separate from the water monitor clade.
The Peachthroat is a mid-sized, slender varanid (up to 1.2 m long in total) native to New Guinea and various of the surrounding islands, namely Yapen, Biak, Salawati and Weigeo. It was named in 1932 as a subspecies of Mangrove monitor V. indicus (as were so many other varanid taxa). In 1951, Robert Mertens – the ‘father’ of modern varanid taxonomy – described the new species V. karlschmidti (sometimes called Schmidt’s monitor, and named for Chicago-based herpetologist Karl Schmidt). It later turned out that the type specimen of V. jobiensis is a juvenile of the same taxon as V. karlschmidti, rendering the latter a junior synonym of the former (Böhme 1991).
V. jobiensis has a very angular-looking head; it’s mostly blackish dorsally, with numerous small, light spots arranged in bands, and bluish or turquoise vertical bands of spots are arranged on the tail. Most remarkable is its pinkish, reddish or orange throat, displayed prominently in wild animals and likely serving an important signalling function. It seems to be a forest-dwelling species that takes readily to trees but it’s also been reported foraging along stream edges, apparently for fish or crustaceans. Insects make up the bulk of its diet. The idea has been proposed that V. jobiensis might contain unrecognised diversity and that new taxa will eventually be named for some of the populations currently included within it (Philipp et al. 2004).
As usual, my aim in this article was to cover a whole lot more material – in particular I was really hoping to cover the prasinoids, the strongly arboreal, prehensile-tailed tree monitors of New Guinea and the surrounds, a group I’ve written about a long time ago (Naish 1998a, b) and really want to revisit. At least, however, the several species covered here belong to various of the varanid clades that inhabit tropical SE Asia, the Wallacean region and Australasia. It’s a tangled, complex but fascinating story of numerous lineages. More varanids (including those prasinoids) in future. Until then, heed the advice in the adjacent image (kindly provided by Tony Gamble).
For previous articles on varanids and other platynotan lizards, see...
- Hell yes: Komodo dragons!!!
- Of giant plated lizards and rough-necked monitors
- Dinosaurs come out to play (so do turtles, and crocodilians, and Komodo dragons)
- What I saw at the zoo yesterday... (more brief comments on Komodo dragons)
- Perentie tries to swallow echidna. Echidna too spiky, Perentie gets horribly injured. Dies.
- Monstersauria vs Goannasauria
- Goanna-eating goannas: an evolutionary story of intraguild predation, dwarfism, gigantism, copious walking and reckless thermoregulation
Refs - -
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Bennett, D. 1994. Dumeril’s monitor lizard (Varanus dumerilii). The Reptilian Magazine 3 (3), 35-37.
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Böhme, W. 1991. New findings on the hemipenial morphology of monitor lizards and their systematic implications. Mertensiella 2, 42-49.
Gould, G. C. & MacFadden, B. J. 2004. Gigantism, dwarfism, and Cope’s rule: “nothing in evolution makes sense without a phylogeny”. Bulletin of the American Museum of Natural History 285, 219-237.
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